Digital PDFs

EC-QD2KC-TE

October 1998

371 pages

Original

5.1MB

Document:	Alpha Architecture Handbook
Order Number:	EC-QD2KC-TE
Revision:	0
Pages:	371
Original Filename:

OCR Text

Alpha Architecture Handbook
Order Number: EC–QD2KC–TE

Revision/Update Information:

Compaq Computer Corporation

This is Version 4 of the Alpha
Architecture Handbook.

October 1998
The information in this publication is subject to change without notice.
COMPAQ COMPUTER CORPORATION SHALL NOT BE LIABLE FOR TECHNICAL OR EDITORIAL
ERRORS OR OMISSIONS CONTAINED HEREIN, NOR FOR INCIDENTAL OR CONSEQUENTIAL DAMAGES RESULTING FROM THE FURNISHING, PERFORMANCE, OR USE OF THIS MATERIAL. THIS
INFORMATION IS PROVIDED “AS IS” AND COMPAQ COMPUTER CORPORATION DISCLAIMS ANY
WARRANTIES, EXPRESS, IMPLIED OR STATUTORY AND EXPRESSLY DISCLAIMS THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR PARTICULAR PURPOSE, GOOD TITLE AND AGAINST
INFRINGEMENT.
This publication contains information protected by copyright. No part of this publication may be photocopied or
reproduced in any form without prior written consent from Compaq Computer Corporation.
© Compaq Computer Corporation 1998.
All rights reserved. Printed in the U.S.A.
The following are trademarks of Comaq Computer Corporation: Alpha AXP, AXP, DEC, DIGITAL, DIGITAL
UNIX, OpenVMS, PDP–11, VAX, VAX DOCUMENT, and the DIGITAL logo.
Cray is a registered trademark of Cray Research, Inc. IBM is a registered trademark of International Business
Machines Corporation. UNIX is a registered trademark in the United States and other countries licensed exclusively
through X/Open Company Ltd. Windows NT is a trademark of Microsoft Corporation.
All other trademarks and registered trademarks are the property of their respective owners.

Table of Contents

Introduction
1.1
1.2
1.3
1.4
1.5
1.6
1.6.1
1.6.2
1.6.3
1.6.4
1.6.5
1.6.6
1.6.7
1.6.8
1.6.9
1.6.10
1.6.11
1.6.12

The Alpha Approach to RISC Architecture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Data Format Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Instruction Format Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Instruction Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Instruction Set Characteristics. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Terminology and Conventions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Numbering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Security Holes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
UNPREDICTABLE and UNDEFINED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ranges and Extents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ALIGNED and UNALIGNED . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Must Be Zero (MBZ). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Read As Zero (RAZ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Should Be Zero (SBZ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Ignore (IGN) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Implementation Dependent (IMP) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Illustration Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Macro Code Example Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1–1
1–3
1–4
1–4
1–6
1–6
1–7
1–7
1–7
1–8
1–8
1–9
1–9
1–9
1–9
1–9
1–9
1–9

Basic Architecture
2.1
Addressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2
Data Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1
Byte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2
Word. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3
Longword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.4
Quadword. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.5
VAX Floating-Point Formats. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.5.1
F_floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.5.2
G_floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.5.3
D_floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.6
IEEE Floating-Point Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.6.1
S_Floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.6.2
T_floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.6.3
X_Floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.7
Longword Integer Format in Floating-Point Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.8
Quadword Integer Format in Floating-Point Unit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.9
Data Types with No Hardware Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2–1
2–1
2–1
2–1
2–2
2–2
2–3
2–3
2–4
2–5
2–6
2–7
2–8
2–9
2–11
2–12
2–12

iii

2.3

Big-Endian Addressing Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Instruction Formats
3.1
Alpha Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.1
Program Counter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.2
Integer Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.3
Floating-Point Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.4
Lock Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.5
Processor Cycle Counter (PCC) Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.6
Optional Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.6.1
Memory Prefetch Registers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1.6.2
VAX Compatibility Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2
Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1
Operand Notation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2
Instruction Operand Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2.1
Operand Name Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2.2
Operand Access Type Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2.3
Operand Data Type Notation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.3
Operators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.4
Notation Conventions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3
Instruction Formats . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1
Memory Instruction Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1.1
Memory Format Instructions with a Function Code . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1.2
Memory Format Jump Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.2
Branch Instruction Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.3
Operate Instruction Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.4
Floating-Point Operate Instruction Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.4.1
Floating-Point Convert Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.4.2
Floating-Point/Integer Register Moves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.5
PALcode Instruction Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3–1
3–1
3–1
3–2
3–2
3–3
3–3
3–3
3–3
3–3
3–4
3–5
3–5
3–5
3–6
3–6
3–10
3–10
3–11
3–11
3–12
3–12
3–12
3–13
3–14
3–14
3–14

Instruction Descriptions
4.1
4.1.1
4.1.2
4.1.3
4.1.4
4.2
4.2.1
4.2.2
4.2.3
4.2.4
4.2.5
4.2.6
4.2.7
4.3
4.3.1
4.3.2
4.3.3
4.4
4.4.1
4.4.2
4.4.3
4.4.4

2–13

Instruction Set Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Subsetting Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Floating-Point Subsets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Software Emulation Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Opcode Qualifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Memory Integer Load/Store Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Load Address . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Load Memory Data into Integer Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Load Unaligned Memory Data into Integer Register . . . . . . . . . . . . . . . . . . . . . . . . . . .
Load Memory Data into Integer Register Locked . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Store Integer Register Data into Memory Conditional . . . . . . . . . . . . . . . . . . . . . . . . . .
Store Integer Register Data into Memory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Store Unaligned Integer Register Data into Memory . . . . . . . . . . . . . . . . . . . . . . . . . . .
Control Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Conditional Branch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Unconditional Branch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Jumps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Integer Arithmetic Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Longword Add . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Scaled Longword Add . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Quadword Add . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Scaled Quadword Add . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4–1
4–2
4–2
4–3
4–3
4–4
4–5
4–6
4–8
4–9
4–12
4–15
4–17
4–18
4–20
4–21
4–22
4–24
4–25
4–26
4–27
4–28

4.4.5
Integer Signed Compare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.6
Integer Unsigned Compare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.7
Count Leading Zero . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.8
Count Population . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.9
Count Trailing Zero . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.10
Longword Multiply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.11
Quadword Multiply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.12
Unsigned Quadword Multiply High . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.13
Longword Subtract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.14
Scaled Longword Subtract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.15
Quadword Subtract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.16
Scaled Quadword Subtract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5
Logical and Shift Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.1
Logical Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.2
Conditional Move Integer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.3
Shift Logical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.4
Shift Arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6
Byte Manipulation Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.1
Compare Byte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.2
Extract Byte . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.3
Byte Insert . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.4
Byte Mask . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.5
Sign Extend . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.6.6
Zero Bytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7
Floating-Point Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.1
Single-Precision Operations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.2
Subsets and Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.3
Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.4
Encodings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.5
Rounding Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.6
Computational Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.6.1
VAX-Format Arithmetic with Precise Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.6.2
High-Performance VAX-Format Arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.6.3
IEEE-Compliant Arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.6.4
IEEE-Compliant Arithmetic Without Inexact Exception . . . . . . . . . . . . . . . . . . . . . .
4.7.6.5
High-Performance IEEE-Format Arithmetic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.7
Trapping Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.7.1
VAX Trapping Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.7.2
IEEE Trapping Modes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.7.3
Arithmetic Trap Completion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.7.3.1
Trap Shadow Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.7.3.2
Trap Shadow Length Rules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.7.4
Invalid Operation (INV) Arithmetic Trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.7.5
Division by Zero (DZE) Arithmetic Trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.7.6
Overflow (OVF) Arithmetic Trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.7.7
Underflow (UNF) Arithmetic Trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.7.8
Inexact Result (INE) Arithmetic Trap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.7.9
Integer Overflow (IOV) Arithmetic Trap. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.7.10
IEEE Floating-Point Trap Disable Bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.7.11
IEEE Denormal Control Bits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.8
Floating-Point Control Register (FPCR). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.8.1
Accessing the FPCR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.8.2
Default Values of the FPCR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.8.3
Saving and Restoring the FPCR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.9
Floating-Point Instruction Function Field Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.10
IEEE Standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.10.1
Conversion of NaN and Infinity Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.10.2
Copying NaN Values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.7.10.3
Generating NaN Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4–29
4–30
4–31
4–32
4–33
4–34
4–35
4–36
4–37
4–38
4–39
4–40
4–41
4–42
4–43
4–45
4–46
4–47
4–49
4–51
4–55
4–57
4–60
4–61
4–62
4–62
4–62
4–63
4–65
4–66
4–67
4–67
4–68
4–68
4–68
4–69
4–69
4–69
4–71
4–73
4–73
4–74
4–76
4–77
4–77
4–78
4–78
4–78
4–78
4–79
4–79
4–82
4–83
4–83
4–84
4–88
4–88
4–89
4–89
v

4.7.10.4
Propagating NaN Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.8
Memory Format Floating-Point Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.8.1
Load F_floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.8.2
Load G_floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.8.3
Load S_floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.8.4
Load T_floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.8.5
Store F_floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.8.6
Store G_floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.8.7
Store S_floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.8.8
Store T_floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.9
Branch Format Floating-Point Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.9.1
Conditional Branch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10
Floating-Point Operate Format Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.1
Copy Sign . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.2
Convert Integer to Integer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.3
Floating-Point Conditional Move . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.4
Move from/to Floating-Point Control Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.5
VAX Floating Add . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.6
IEEE Floating Add . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.7
VAX Floating Compare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.8
IEEE Floating Compare . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.9
Convert VAX Floating to Integer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.10
Convert Integer to VAX Floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.11
Convert VAX Floating to VAX Floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.12
Convert IEEE Floating to Integer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.13
Convert Integer to IEEE Floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.14
Convert IEEE S_Floating to IEEE T_Floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.15
Convert IEEE T_Floating to IEEE S_Floating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.16
VAX Floating Divide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.17
IEEE Floating Divide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.18
Floating-Point Register to Integer Register Move . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.19
Integer Register to Floating-Point Register Move . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.20
VAX Floating Multiply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.21
IEEE Floating Multiply . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.22
VAX Floating Square Root . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.23
IEEE Floating Square Root . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.24
VAX Floating Subtract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.10.25
IEEE Floating Subtract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.11
Miscellaneous Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.11.1
Architecture Mask . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.11.2
Call Privileged Architecture Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.11.3
Evict Data Cache Block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.11.4
Exception Barrier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.11.5
Prefetch Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.11.6
Implementation Version . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.11.7
Memory Barrier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.11.8
Read Processor Cycle Counter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.11.9
Trap Barrier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.11.10
Write Hint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.11.11
Write Memory Barrier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.12
VAX Compatibility Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.12.1
VAX Compatibility Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.13
Multimedia (Graphics and Video) Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.13.1
Byte and Word Minimum and Maximum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.13.2
Pixel Error . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.13.3
Pack Bytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.13.4
Unpack Bytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4–89
4–90
4–91
4–92
4–93
4–94
4–95
4–96
4–97
4–98
4–99
4–100
4–102
4–105
4–106
4–107
4–109
4–110
4–111
4–112
4–113
4–114
4–115
4–116
4–117
4–118
4–119
4–120
4–121
4–122
4–123
4–124
4–126
4–127
4–128
4–129
4–130
4–131
4–132
4–133
4–135
4–136
4–138
4–139
4–141
4–142
4–143
4–144
4–145
4–147
4–149
4–150
4–151
4–152
4–154
4–155
4–156

System Architecture and Programming Implications
5.1
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2
Physical Address Space Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.1
Coherency of Memory Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.2
Granularity of Memory Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.3
Width of Memory Access . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.2.4
Memory-Like and Non-Memory-Like Behavior. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3
Translation Buffers and Virtual Caches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4
Caches and Write Buffers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5
Data Sharing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.1
Atomic Change of a Single Datum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.2
Atomic Update of a Single Datum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.3
Atomic Update of Data Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5.4
Ordering Considerations for Shared Data Structures . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6
Read/Write Ordering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.1
Alpha Shared Memory Model. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.1.1
Architectural Definition of Processor Issue Sequence . . . . . . . . . . . . . . . . . . . . . .
5.6.1.2
Definition of Before and After . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.1.3
Definition of Processor Issue Constraints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.1.4
Definition of Location Access Constraints. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.1.5
Definition of Visibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.1.6
Definition of Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.1.7
Definition of Dependence Constraint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.1.8
Definition of Load-Locked and Store-Conditional . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.1.9
Timeliness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.2
Litmus Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.2.1
Litmus Test 1 (Impossible Sequence). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.2.2
Litmus Test 2 (Impossible Sequence). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.2.3
Litmus Test 3 (Impossible Sequence). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.2.4
Litmus Test 4 (Sequence Okay) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.2.5
Litmus Test 5 (Sequence Okay) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.2.6
Litmus Test 6 (Sequence Okay) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.2.7
Litmus Test 7 (Impossible Sequence). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.2.8
Litmus Test 8 (Impossible Sequence). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.2.9
Litmus Test 9 (Impossible Sequence). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.2.10
Litmus Test 10 (Sequence Okay) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.2.11
Litmus Test 11 (Impossible Sequence). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.3
Implied Barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.4
Implications for Software . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.4.1
Single Processor Data Stream . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.4.2
Single Processor Instruction Stream. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.4.3
Multiprocessor Data Stream (Including Single Processor with DMA I/O) . . . . . . . .
5.6.4.4
Multiprocessor Instruction Stream (Including Single Processor with DMA I/O) . . .
5.6.4.5
Multiprocessor Context Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.4.6
Multiprocessor Send/Receive Interrupt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.4.7
Implications for Memory Mapped I/O . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.6.4.8
Multiple Processors Writing to a Single I/O Device. . . . . . . . . . . . . . . . . . . . . . . . .
5.6.5
Implications for Hardware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.7
Arithmetic Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5–1
5–1
5–1
5–2
5–3
5–3
5–4
5–4
5–6
5–6
5–6
5–7
5–9
5–10
5–10
5–12
5–12
5–12
5–14
5–14
5–14
5–15
5–16
5–17
5–17
5–17
5–18
5–18
5–19
5–19
5–19
5–20
5–20
5–21
5–21
5–21
5–22
5–22
5–22
5–22
5–22
5–23
5–24
5–26
5–27
5–28
5–29
5–30

Common PALcode Architecture
6.1
6.2
6.3
6.4

PALcode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PALcode Instructions and Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PALcode Environment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Special Functions Required for PALcode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6–1
6–1
6–2
6–2

vii

6.5
6.6
6.7
6.7.1
6.7.2
6.7.3

PALcode Effects on System Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PALcode Replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Required PALcode Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Drain Aborts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Halt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Instruction Memory Barrier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Console Subsystem Overview

Input/Output Overview

OpenVMS Alpha
9.1
9.2

Unprivileged Digital UNIX PALcode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Privileged Digital UNIX PALcode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Unprivileged Windows NT Alpha PALcode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Privileged Windows NT Alpha PALcode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11–1
11–2

Software Considerations
A.1
Hardware-Software Compact . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.2
Instruction-Stream Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.2.1
Instruction Alignment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.2.2
Branch Prediction and Minimizing Branch-Taken — Factor of 3 . . . . . . . . . . . . . . . . . .
A.2.3
Improving I-Stream Density — Factor of 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.2.4
Instruction Scheduling — Factor of 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.3
Data-Stream Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.3.1
Data Alignment — Factor of 10 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.3.2
Shared Data in Multiple Processors — Factor of 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.3.3
Avoiding Cache/TB Conflicts — Factor of 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.3.4
Sequential Read/Write — Factor of 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.3.5
Prefetching — Factor of 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4
Code Sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4.1
Aligned Byte/Word (Within Register) Memory Accesses . . . . . . . . . . . . . . . . . . . . . . . .
A.4.2
Division . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4.3
Byte Swap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4.4
Stylized Code Forms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4.4.1
NOP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4.4.2
Clear a Register . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4.4.3
Load Literal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4.4.4
Register-to-Register Move . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4.4.5
Negate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

viii

10–1
10–2

Windows NT Alpha
11.1
11.2

9–1
9–8

Digital UNIX
10.1
10.2

Unprivileged OpenVMS Alpha PALcode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Privileged OpenVMS Alpha Palcode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6–3
6–3
6–4
6–6
6–7
6–8

A–1
A–2
A–2
A–2
A–4
A–4
A–4
A–4
A–5
A–6
A–8
A–8
A–9
A–9
A–10
A–11
A–11
A–11
A–12
A–12
A–13
A–13

A.4.4.6
NOT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4.4.7
Booleans . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4.5
Exceptions and Trap Barriers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.4.6
Pseudo-Operations (Stylized Code Forms) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A.5
Timing Considerations: Atomic Sequences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

IEEE Floating-Point Conformance
B.1
B.2
B.2.1
B.3

B–1
B–3
B–4
B–6

Common Architecture Instruction Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IEEE Floating-Point Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
VAX Floating-Point Instructions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Independent Floating-Point Instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Opcode Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Common Architecture Opcodes in Numerical Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
OpenVMS Alpha PALcode Instruction Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DIGITAL UNIX PALcode Instruction Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Windows NT Alpha Instruction Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PALcode Opcodes in Numerical Order . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Required PALcode Opcodes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Opcodes Reserved to PALcode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Opcodes Reserved to Compaq . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Unused Function Code Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ASCII Character Set . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

C–1
C–6
C–7
C–8
C–8
C–10
C–14
C–16
C–17
C–18
C–20
C–20
C–21
C–21
C–22

Registered System and Processor Identifiers
D.1
D.2
D.3

Alpha Choices for IEEE Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Alpha Support for OS Completion Handlers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IEEE Floating-Point Control (FP_C) Quadword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mapping to IEEE Standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Instruction Summary
C.1
C.2
C.3
C.4
C.5
C.6
C.7
C.8
C.9
C.10
C.11
C.12
C.13
C.14
C.15

A–13
A–13
A–14
A–14
A–16

Processor Type Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PALcode Variation Assignments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Architecture Mask and Implementation Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

D–1
D–2
D–3

Waivers and Implementation-Dependent Functionality
E.1
Waivers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.1.1
DECchip 21064, DECchip 21066, and DECchip 21068 IEEE Divide Instruction Violation
E.1.2
DECchip 21064, DECchip 21066, and DECchip 21068 Write Buffer Violation . . . . . . .
E.1.3
DECchip 21264 LDx_L/STx_C with WH64 Violation . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.2
Implementation-Specific Functionality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.2.1
DECchip 21064/21066/21068 Performance Monitoring . . . . . . . . . . . . . . . . . . . . . . . .
E.2.1.1
DECchip 21064/21066/21068 Performance Monitor Interrupt Mechanism . . . . . .
E.2.1.2
Functions and Arguments for the DECchip 21064/21066/21068 . . . . . . . . . . . . . .
E.2.2
DECchip 21164/21164PC Performance Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . .
E.2.2.1
Performance Monitor Interrupt Mechanism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

E–1
E–1
E–2
E–2
E–3
E–3
E–4
E–5
E–9
E–9

E.2.2.2
E.2.2.3
E.2.3
E.2.3.1
E.2.3.2
E.2.3.3

Index

Windows NT Alpha Functions and Argument . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
OpenVMS Alpha and DIGITAL UNIX Functions and Arguments . . . . . . . . . . . . . .
21264 Performance Monitoring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Performance Monitor Interrupt Mechanism. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Windows NT Alpha Functions and Argument . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
OpenVMS Alpha and DIGITAL UNIX Functions and Arguments . . . . . . . . . . . . . .

E–10
E–12
E–23
E–23
E–24
E–25

Figures
1–1
2–1
2–2
2–3
2–4
2–5
2–6
2–7
2–8
2–9
2–10
2–11
2–12
2–13
2–14
2–15
2–16
2–17
2–18
2–19
2–20
2–21
2–22
2–23
2–24
3–1
3–2
3–3
3–4
3–5
3–6
4–1
4–2
8–1
A–1
A–2
A–3
A–4
A–5
B–1
B–2

Instruction Format Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Byte Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Word Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Longword Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Quadword Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
F_floating Datum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
F_floating Register Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G_floating Datum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
G_floating Register Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D_floating Datum. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
D_floating Register Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
S_floating Datum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
S_floating Register Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
T_floating Datum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
T_floating Register Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
X_floating Datum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
X_floating Register Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
X_floating Big-Endian Datum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
X_floating Big-Endian Register Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Longword Integer Datum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Longword Integer Floating-Register Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Quadword Integer Datum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Quadword Integer Floating-Register Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Little-Endian Byte Addressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Big-Endian Byte Addressing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Memory Instruction Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Memory Instruction with Function Code Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Branch Instruction Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Operate Instruction Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Floating-Point Operate Instruction Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
PALcode Instruction Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Floating-Point Control Register (FPCR) Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Floating-Point Instruction Function Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Alpha System Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Branch-Format BSR and BR Opcodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Memory-Format JSR Instruction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Bad Allocation in Cache . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Better Allocation in Cache . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Best Allocation in Cache . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IEEE Floating-Point Control (FP_C) Quadword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
IEEE Trap Handling Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1–4
2–1
2–2
2–2
2–2
2–3
2–3
2–4
2–5
2–5
2–5
2–7
2–7
2–8
2–9
2–10
2–10
2–11
2–11
2–11
2–11
2–12
2–12
2–13
2–13
3–11
3–11
3–12
3–12
3–13
3–15
4–80
4–84
8–1
A–3
A–3
A–7
A–7
A–7
B–4
B–7

Tables
2–1
2–2
3–1
3–2
3–3
3–4
3–5
3–6
3–7
4–1
4–2
4–3
4–4
4–5
4–6
4–7
4–8
4–9
4–10
4–11
4–12
4–13
4–14
4–15
4–16
4–17
4–18
5–1
6–1
6–2
9–1
9–2
10–1
10–2
11–1
11–2
A–1
A–2
B–1
B–2
B–3
C–1
C–2
C–3
C–4
C–5
C–6
C–7
C–8
C–9
C–10
C–11
C–12
C–13
C–14

xii

F_floating Load Exponent Mapping (MAP_F) ................................................................ 2–4
S_floating Load Exponent Mapping (MAP_S) ................................................................ 2–7
Operand Notation ........................................................................................................... 3–4
Operand Value Notation ................................................................................................. 3–4
Expression Operand Notation ........................................................................................ 3–4
Operand Name Notation ................................................................................................ 3–5
Operand Access Type Notation .................................................................................... 3–5
Operand Data Type Notation ......................................................................................... 3–6
Operators ....................................................................................................................... 3–6
Opcode Qualifiers ........................................................................................................... 4–3
Memory Integer Load/Store Instructions ......................................................................... 4–4
Control Instructions Summary ...................................................................................... 4–18
Jump Instructions Branch Prediction ............................................................................ 4–23
Integer Arithmetic Instructions Summary ...................................................................... 4–24
Logical and Shift Instructions Summary ........................................................................ 4–41
Byte-Within-Register Manipulation Instructions Summary ........................................... 4–47
VAX Trapping Modes Summary ................................................................................... 4–71
Summary of IEEE Trapping Modes .............................................................................. 4–72
Trap Shadow Length Rules .......................................................................................... 4–75
Floating-Point Control Register (FPCR) Bit Descriptions ............................................. 4–80
IEEE Floating-Point Function Field Bit Summary ......................................................... 4–85
VAX Floating-Point Function Field Bit Summary .......................................................... 4–87
Memory Format Floating-Point Instructions Summary .................................................. 4–90
Floating-Point Branch Instructions Summary ................................................................ 4–99
Floating-Point Operate Instructions Summary ........................................................... 4–102
Miscellaneous Instructions Summary .......................................................................... 4–132
VAX Compatibility Instructions Summary .................................................................... 4–149
Processor Issue Constraints ....................................................................................... 5–13
PALcode Instructions that Require Recognition .............................................................. 6–4
Required PALcode Instructions ....................................................................................... 6–5
Unprivileged OpenVMS Alpha PALcode Instruction Summary ..................................... 9–1
Privileged OpenVMS Alpha PALcode Instructions Summary ........................................ 9–8
Unprivileged Digital UNIX PALcode Instruction Summary .......................................... 10–1
Privileged Digital UNIX PALcode Instruction Summary ............................................... 10–2
Unprivileged Windows NT Alpha PALcode Instruction Summary ................................ 11–1
Privileged Windows NT Alpha PALcode Instruction Summary ..................................... 11–2
Cache Block Prefetching ................................................................................................ A–8
Decodable Pseudo-Operations (Stylized Code Forms) ............................................... A–14
Floating-Point Control (FP_C) Quadword Bit Summary ................................................ B–5
IEEE Floating-Point Trap Handling ............................................................................... B–8
IEEE Standard Charts ................................................................................................. B–12
Instruction Format and Opcode Notation ....................................................................... C–1
Common Architecture Instructions ................................................................................ C–2
IEEE Floating-Point Instruction Function Codes ........................................................... C–6
VAX Floating-Point Instruction Function Codes ............................................................ C–7
Independent Floating-Point Instruction Function Codes ............................................... C–8
Opcode Summary ......................................................................................................... C–9
Key to Opcode Summary ............................................................................................... C–9
Common Architecture Opcodes in Numerical Order ................................................... C–10
OpenVMS Alpha Unprivileged PALcode Instructions .................................................. C–14
OpenVMS Alpha Privileged PALcode Instructions ...................................................... C–15
DIGITAL UNIX Unprivileged PALcode Instructions ..................................................... C–16
DIGITAL UNIX Privileged PALcode Instructions ......................................................... C–16
Windows NT Alpha Unprivileged PALcode Instructions ............................................. C–17
Windows NT Alpha Privileged PALcode instructions .................................................. C–17

C–15
C–16
C–17
C–18
C–19
D–1
D–2
D–3
D–4
E–1
E–2
E–3
E–4
E–5
E–6
E–7
E–8
E–9
E–10
E–11
E–12
E–13
E–14
E–15
E–16
E–17
E–18
E–19
E–20
E–21
E–22
E–23
E–24
E–25
E–26

PALcode Opcodes in Numerical Order ....................................................................... C–18
Required PALcode Opcodes........................................................................................ C–20
Opcodes Reserved for PALcode .................................................................................. C–20
Opcodes Reserved for Compaq ................................................................................... C–21
ASCII Character Set ..................................................................................................... C–22
Processor Type Assignments ........................................................................................ D–1
PALcode Variation Assignments .................................................................................... D–2
AMASK Bit Assignments ............................................................................................... D–3
IMPLVER Value Assignments ....................................................................................... D–3
DECchip 21064/21066/21068 Performance Monitoring Functions ............................ E–5
DECchip 21064/21066/21068 MUX Control Fields in ICCSR Register ......................... E–7
Bit Summary of PMCTR Register for Windows NT Alpha .......................................... E–11
OpenVMS Alpha and DIGITAL UNIX Performance Monitoring Functions .................. E–12
21164/21164PC Enable Counters for OpenVMS Alpha and DIGITAL UNIX ............... E–15
21164/21164PC Disable Counters for OpenVMS Alpha and DIGITAL UNIX ............. E–15
21164 Select Desired Events for OpenVMS Alpha and DIGITAL UNIX ..................... E–16
21164PC Select Desired Events for OpenVMS Alpha and DIGITAL UNIX ............. E–16
21164/21164PC Select Special Options for OpenVMS Alpha and DIGITAL UNIX...... E–17
21164/21164PC Select Desired Frequencies for OpenVMS Alpha and DIGITAL UNIX E–18
21164/21164PC Read Counters for OpenVMS Alpha and DIGITAL UNIX ................. E–19
21164/21164PC Write Counters for OpenVMS Alpha and DIGITAL UNIX ................. E–19
21164/21164PC Counter 1 (PCSEL1) Event Selection .............................................. E–19
21164/21164PC Counter 2 (PCSEL2) Event Selection .............................................. E–20
21164 CBOX1 Event Selection ................................................................................... E–21
21164 CBOX2 Event Selection ................................................................................... E–21
21164PC PM0_MUX Event Selection ......................................................................... E–22
21164PC PM1_MUX Event Selection ......................................................................... E–22
Bit Summary of PCTR_CTL Register for Windows NT Alpha .................................... E–24
OpenVMS Alpha and DIGITAL UNIX Performance Monitoring Functions ................... E–25
21264 Enable Counters for OpenVMS Alpha and DIGITAL UNIX ............................... E–27
21264 Disable Counters for OpenVMS Alpha and DIGITAL UNIX ............................. E–27
21264 Select Desired Events for OpenVMS Alpha and DIGITAL UNIX ..................... E–28
21264 Read Counters for OpenVMS Alpha and DIGITAL UNIX ................................. E–28
21264 Write Counters for OpenVMS Alpha and DIGITAL UNIX ................................. E–28
21264 Enable and Write Counters for OpenVMS Alpha and DIGITAL UNIX............... E–29

xiii

xiv

Preface
Chapters 1 through 8 and appendixes A through E of this book are directly derived from the Alpha System Reference Manual, Version 7 and passed engineering change orders (ECOs) that have been
applied. It is an accurate representation of the described parts of the Alpha architecture.
References in this handbook to the Alpha Architecture Reference Manual are to the Third Edition of
that manual, EY-W938E-DP.

Chapter 1

Introduction

Alpha is a 64-bit load/store RISC architecture that is designed with particular emphasis on the
three elements that most affect performance: clock speed, multiple instruction issue, and multiple processors.
The Alpha architects examined and analyzed current and theoretical RISC architecture design
elements and developed high-performance alternatives for the Alpha architecture. The architects adopted only those design elements that appeared valuable for a projected 25-year design
horizon. Thus, Alpha becomes the first 21st century computer architecture.
The Alpha architecture is designed to avoid bias toward any particular operating system or programming language. Alpha supports the OpenVMS Alpha, DIGITAL UNIX, and Windows NT
Alpha operating systems and supports simple software migration for applications that run on
those operating systems.
This manual describes in detail how Alpha is designed to be the leadership 64-bit architecture
of the computer industry.

1.1 The Alpha Approach to RISC Architecture
Alpha Is a True 64-Bit Architecture
Alpha was designed as a 64-bit architecture. All registers are 64 bits in length and all operations are performed between 64-bit registers. It is not a 32-bit architecture that was later
expanded to 64 bits.

Alpha Is Designed for Very High-Speed Implementations
The instructions are very simple. All instructions are 32 bits in length. Memory operations are
either loads or stores. All data manipulation is done between registers.
The Alpha architecture facilitates pipelining multiple instances of the same operations because
there are no special registers and no condition codes.
The instructions interact with each other only by one instruction writing a register or memory
and another instruction reading from the same place. That makes it particularly easy to build
implementations that issue multiple instructions every CPU cycle.

Introduction 1–1

Alpha makes it easy to maintain binary compatibility across multiple implementations and easy
to maintain full speed on multiple-issue implementations. For example, there are no implementation-specific pipeline timing hazards, no load-delay slots, and no branch-delay slots.

The Alpha Approach to Byte Manipulation
The Alpha architecture reads and writes bytes between registers and memory with the LDBU
and STB instructions. (Alpha also supports word read/writes with the LDWU and STW
instructions.)
Byte shifting and masking is performed with normal 64-bit register-to-register instructions,
crafted to keep instruction sequences short.

The Alpha Approach to Multiprocessor Shared Memory
As viewed from a second processor (including an I/O device), a sequence of reads and writes
issued by one processor may be arbitrarily reordered by an implementation. This allows implementations to use multibank caches, bypassed write buffers, write merging, pipelined writes
with retry on error, and so forth. If strict ordering between two accesses must be maintained,
explicit memory barrier instructions can be inserted in the program.
The basic multiprocessor interlocking primitive is a RISC-style load_locked, modify,
store_conditional sequence. If the sequence runs without interrupt, exception, or an interfering
write from another processor, then the conditional store succeeds. Otherwise, the store fails and
the program eventually must branch back and retry the sequence. This style of interlocking
scales well with very fast caches and makes Alpha an especially attractive architecture for
building multiple-processor systems.

Alpha Instructions Include Hints for Achieving Higher Speed
A number of Alpha instructions include hints for implementations, all aimed at achieving
higher speed.

•

Calculated jump instructions have a target hint that can allow much faster subroutine
calls and returns.

•

There are prefetching hints for the memory system that can allow much higher cache hit
rates.

•

There are granularity hints for the virtual-address mapping that can allow much more
effective use of translation lookaside buffers for large contiguous structures.

PALcode – Alpha’s Very Flexible Privileged Software Library
A Privileged Architecture Library (PALcode) is a set of subroutines that are specific to a particular Alpha operating system implementation. These subroutines provide operating-system
primitives for context switching, interrupts, exceptions, and memory management. PALcode is
similar to the BIOS libraries that are provided in personal computers.
PALcode subroutines are invoked by implementation hardware or by software CALL_PAL
instructions.

1–2 Alpha Architecture Handbook

PALcode is written in standard machine code with some implementation-specific extensions to
provide access to low-level hardware.
PALcode lets Alpha implementations run the full OpenVMS Alpha, DIGITAL UNIX, and
Windows NT Alpha operating systems. PALcode can provide this functionality with little
overhead. For example, the OpenVMS Alpha PALcode instructions let Alpha run OpenVMS
with little more hardware than that found on a conventional RISC machine: the PAL mode bit
itself, plus four extra protection bits in each translation buffer entry.
Other versions of PALcode can be developed for real-time, teaching, and other applications.
PALcode makes Alpha an especially attractive architecture for multiple operating systems.

Alpha and Programming Languages
Alpha is an attractive architecture for compiling a large variety of programming languages.
Alpha has been carefully designed to avoid bias toward one or two programming languages.
For example:

•

Alpha does not contain a subroutine call instruction that moves a register window by a
fixed amount. Thus, Alpha is a good match for programming languages with many
parameters and programming languages with no parameters.

•

Alpha does not contain a global integer overflow enable bit. Such a bit would need to
be changed at every subroutine boundary when a FORTRAN program calls a C program.

1.2 Data Format Overview
Alpha is a load/store RISC architecture with the following data characteristics:

•

All operations are done between 64-bit registers.

•

Memory is accessed via 64-bit virtual byte addresses, using the little-endian or, optionally, the big-endian byte numbering convention.

•

There are 32 integer registers and 32 floating-point registers.

•

Longword (32-bit) and quadword (64-bit) integers are supported.

•

Five floating-point data types are supported:
–

VAX F_floating (32-bit)

–

VAX G_floating (64-bit)

–

IEEE single (32-bit)

–

IEEE double (64-bit)

–

IEEE extended (128-bit)

Introduction 1–3

1.3 Instruction Format Overview
As shown in Figure 1–1, Alpha instructions are all 32 bits in length. There are four major
instruction format classes that contain 0, 1, 2, or 3 register fields. All formats have a 6-bit
opcode.

Figure 1–1: Instruction Format Overview
31

26 25

21 20

16 15

Opcode

5 4

PALcode Format

Number
Disp

Opcode

Branch Format
Disp

Function

Memory Format
RC

Operate Format

•

PALcode instructions specify, in the function code field, one of a few dozen complex
operations to be performed.

•

Conditional branch instructions test register Ra and specify a signed 21-bit PC-relative longword target displacement. Subroutine calls put the return address in register
Ra.

•

Load and store instructions move bytes, words, longwords, or quadwords between
register Ra and memory, using Rb plus a signed 16-bit displacement as the memory
address.

•

Operate instructions for floating-point and integer operations are both represented in
Figure 1–1 by the operate format illustration and are as follows:
–

Word and byte sign-extension operators.

–

Floating-point operations use Ra and Rb as source registers and write the result in
register Rc. There is an 11-bit extended opcode in the function field.

–

Integer operations use Ra and Rb or an 8-bit literal as the source operand, and write
the result in register Rc.

–

Integer operate instructions can use the Rb field and part of the function field to
specify an 8-bit literal. There is a 7-bit extended opcode in the function field.

1.4 Instruction Overview
PALcode Instructions
As described in Section 1.1, a Privileged Architecture Library (PALcode) is a set of subroutines that is specific to a particular Alpha operating-system implementation. These subroutines
can be invoked by hardware or by software CALL_PAL instructions, which use the function
field to vector to the specified subroutine.

1–4 Alpha Architecture Handbook

Branch Instructions
Conditional branch instructions can test a register for positive/negative or for zero/nonzero,
and they can test integer registers for even/odd. Unconditional branch instructions can write a
return address into a register.
There is also a calculated jump instruction that branches to an arbitrary 64-bit address in a
register.

Load/Store Instructions
Load and store instructions move 8-bit, 16-bit, 32-bit, or 64-bit aligned quantities from and to
memory. Memory addresses are flat 64-bit virtual addresses with no segmentation.
The VAX floating-point load/store instructions swap words to give a consistent register format
for floating-point operations.
A 32-bit integer datum is placed in a register in a canonical form that makes 33 copies of the
high bit of the datum. A 32-bit floating-point datum is placed in a register in a canonical form
that extends the exponent by 3 bits and extends the fraction with 29 low-order zeros. The 32bit operates preserve these canonical forms.
Compilers, as directed by user declarations, can generate any mixture of 32-bit and 64-bit operations. The Alpha architecture has no 32/64 mode bit.

Integer Operate Instructions
The integer operate instructions manipulate full 64-bit values and include the usual assortment
of arithmetic, compare, logical, and shift instructions.
There are just three 32-bit integer operates: add, subtract, and multiply. They differ from their
64-bit counterparts only in overflow detection and in producing 32-bit canonical results.
There is no integer divide instruction.
The Alpha architecture also supports the following additional operations:

•

Scaled add/subtract instructions for quick subscript calculation

•

128-bit multiply for division by a constant, and multiprecision arithmetic

•

Conditional move instructions for avoiding branch instructions

•

An extensive set of in-register byte and word manipulation instructions

•

A set of multimedia instructions that support graphics and video

Integer overflow trap enable is encoded in the function field of each instruction, rather than
kept in a global state bit. Thus, for example, both ADDQ/V and ADDQ opcodes exist for specifying 64-bit ADD with and without overflow checking. That makes it easier to pipeline
implementations.

Introduction 1–5

Floating-Point Operate Instructions
The floating-point operate instructions include four complete sets of VAX and IEEE arithmetic instructions, plus instructions for performing conversions between floating-point and
integer quantities.
In addition to the operations found in conventional RISC architectures, Alpha includes conditional move instructions for avoiding branches and merge sign/exponent instructions for simple
field manipulation.
The arithmetic trap enables and rounding mode are encoded in the function field of each
in stru cti on , rath er t han kep t in g lo bal state bits. Th at mak es it easier to pi pel in e
implementations.

1.5 Instruction Set Characteristics
Alpha instruction set characteristics are as follows:

•

All instructions are 32 bits long and have a regular format.

•

There are 32 integer registers (R0 through R31), each 64 bits wide. R31 reads as zero,
and writes to R31 are ignored.

•

All integer data manipulation is between integer registers, with up to two variable register source operands (one may be an 8-bit literal) and one register destination operand.

•

There are 32 floating-point registers (F0 through F31), each 64 bits wide. F31 reads as
zero, and writes to F31 are ignored.

•

All floating-point data manipulation is between floating-point registers, with up to two
register source operands and one register destination operand.

•

Instructions can move data in an integer register file to a floating-point register file, and
data in a floating-point register file to an integer register file. The instructions do not
interpret bits in the register files and do not access memory.

•

All memory reference instructions are of the load/store type that moves data between
registers and memory.

•

There are no branch condition codes. Branch instructions test an integer or floatingpoint register value, which may be the result of a previous compare.

•

Integer and logical instructions operate on quadwords.

•

Floating-point instructions operate on G_floating, F_floating, and IEEE extended, double, and single operands. D_floating "format compatibility," in which binary files of
D_floating numbers may be processed, but without the last 3 bits of fraction precision,
is also provided.

•

A minimal number of VAX compatibility instructions are included.

1.6 Terminology and Conventions
The following sections describe the terminology and conventions used in this book.

1–6 Alpha Architecture Handbook

1.6.1 Numbering
All numbers are decimal unless otherwise indicated. Where there is ambiguity, numbers other
than decimal are indicated with the name of the base in subscript form, for example, 1016.

1.6.2 Security Holes
A security hole is an error of commission, omission, or oversight in a system that allows protection mechanisms to be bypassed.
Security holes exist when unprivileged software (software running outside of kernel mode)
can:

•

Affect the operation of another process without authorization from the operating system;

•

Amplify its privilege without authorization from the operating system; or

•

Communicate with another process, either overtly or covertly, without authorization
from the operating system.

The Alpha architecture has been designed to contain no architectural security holes. Hardware
(processors, buses, controllers, and so on) and software should likewise be designed to avoid
security holes.

1.6.3 UNPREDICTABLE and UNDEFINED
The terms UNPREDICTABLE and UNDEFINED are used throughout this book. Their meanings are quite different and must be carefully distinguished.
In particular, only privileged software (software running in kernel mode) can trigger UNDEFINED operations. Unprivileged software cannot trigger UNDEFINED operations. However,
either privileged or unprivileged software can trigger UNPREDICTABLE results or
occurrences.
UNPREDICTABLE results or occurrences do not disrupt the basic operation of the processor;
it continues to execute instructions in its normal manner. In contrast, UNDEFINED operation
can halt the processor or cause it to lose information.
The terms UNPREDICTABLE and UNDEFINED can be further described as follows:

UNPREDICTABLE
•

Results or occurrences specified as UNPREDICTABLE may vary from moment to
moment, implementation to implementation, and instruction to instruction within
implementations. Software can never depend on results specified as UNPREDICTABLE.

•

An UNPREDICTABLE result may acquire an arbitrary value subject to a few constraints. Such a result may be an arbitrary function of the input operands or of any state
information that is accessible to the process in its current access mode. UNPREDICTABLE results may be unchanged from their previous values.

Introduction 1–7

Operations that produce UNPREDICTABLE results may also produce exceptions.

•

An occurrence specified as UNPREDICTABLE may happen or not based on an arbitrary choice function. The choice function is subject to the same constraints as are
UNPREDICTABLE results and, in particular, must not constitute a security hole.
Specifically, UNPREDICTABLE results must not depend upon, or be a function of,
the contents of memory locations or registers that are inaccessible to the current
process in the current access mode.
Also, operations that may produce UNPREDICTABLE results must not:
–

Write or modify the contents of memory locations or registers to which the current
process in the current access mode does not have access, or

–

Halt or hang the system or any of its components.

For example, a security hole would exist if some UNPREDICTABLE result depended
on the value of a register in another process, on the contents of processor temporary
registers left behind by some previously running process, or on a sequence of actions
of different processes.

UNDEFINED
•

Operations specified as UNDEFINED may vary from moment to moment, implementation to implementation, and instruction to instruction within implementations. The
operation may vary in effect from nothing to stopping system operation.

•

UNDEFINED operations may halt the processor or cause it to lose information. However, UNDEFINED operations must not cause the processor to hang, that is, reach an
unhalted state from which there is no transition to a normal state in which the machine
executes instructions.

1.6.4 Ranges and Extents
Ranges are specified by a pair of numbers separated by two periods and are inclusive. For
example, a range of integers 0..4 includes the integers 0, 1, 2, 3, and 4.
Extents are specified by a pair of numbers in angle brackets separated by a colon and are inclusive. For example, bits <7:3> specify an extent of bits including bits 7, 6, 5, 4, and 3.

1.6.5 ALIGNED and UNALIGNED
In this document the terms ALIGNED and NATURALLY ALIGNED are used interchangeably to refer to data objects that are powers of two in size. An aligned datum of size 2**N is
stored in memory at a byte address that is a multiple of 2**N, that is, one that has N low-order
zeros. Thus, an aligned 64-byte stack frame has a memory address that is a multiple of 64.
If a datum of size 2**N is stored at a byte address that is not a multiple of 2**N, it is called
UNALIGNED.

1–8 Alpha Architecture Handbook

1.6.6 Must Be Zero (MBZ)
Fields specified as Must be Zero (MBZ) must never be filled by software with a non-zero
value. These fields may be used at some future time. If the processor encounters a non-zero
value in a field specified as MBZ, an Illegal Operand exception occurs.

1.6.7 Read As Zero (RAZ)
Fields specified as Read as Zero (RAZ) return a zero when read.

1.6.8 Should Be Zero (SBZ)
Fields specified as Should be Zero (SBZ) should be filled by software with a zero value. Nonzero values in SBZ fields produce UNPREDICTABLE results and may produce extraneous
instruction-issue delays.

1.6.9 Ignore (IGN)
Fields specified as Ignore (IGN) are ignored when written.

1.6.10 Implementation Dependent (IMP)
Fields specified as Implementation Dependent (IMP) may be used for implementation-specific
purposes. Each implementation must document fully the behavior of all fields marked as IMP
by the Alpha specification.

1.6.11 Illustration Conventions
Illustrations that depict registers or memory follow the convention that increasing addresses
run right to left and top to bottom.

1.6.12 Macro Code Example Conventions
All instructions in macro code examples are either listed in Chapter 4 or are stylized code
forms found in Section A.4.6.

Introduction 1–9

Chapter 2

Basic Architecture

2.1 Addressing
The basic addressable unit in the Alpha architecture is the 8-bit byte. Virtual addresses are 64
bits long. An implementation may support a smaller virtual address space. The minimum virtual address size is 43 bits.
Virtual addresses as seen by the program are translated into physical memory addresses by the
memory management mechanism.
Although the data types in Section 2.2 are described in terms of little-endian byte addressing,
implementations may also include big-endian addressing support, as described in Section 2.3.
All current implementations have some big-endian support.

2.2 Data Types
Following are descriptions of the Alpha architecture data types.

2.2.1 Byte
A byte is 8 contiguous bits starting on an addressable byte boundary. The bits are numbered
from right to left, 0 through 7, as shown in Figure 2–1.

Figure 2–1: Byte Format
7

A byte is specified by its address A. A byte is an 8-bit value. The byte is only supported in
Alpha by the load, store, sign-extend, extract, mask, insert, and zap instructions.

2.2.2 Word
A word is 2 contiguous bytes starting on an arbitrary byte boundary. The bits are numbered
from right to left, 0 through 15, as shown in Figure 2–2.

Basic Architecture 2–1

Figure 2–2: Word Format
15

A word is specified by its address, the address of the byte containing bit 0.
A word is a 16-bit value. The word is only supported in Alpha by the load, store, sign-extend,
extract, mask, and insert instructions.

2.2.3 Longword
A longword is 4 contiguous bytes starting on an arbitrary byte boundary. The bits are numbered from right to left, 0 through 31, as shown in Figure 2–3.

Figure 2–3: Longword Format
31

A longword is specified by its address A, the address of the byte containing bit 0. A longword
is a 32-bit value.
When interpreted arithmetically, a longword is a two’s-complement integer with bits of
increasing significance from 0 through 30. Bit 31 is the sign bit. The longword is only supported in Alpha by sign-extended load and store instructions and by longword arithmetic
instructions.

Note:
Alpha implementations will impose a significant performance penalty when accessing
longword operands that are not naturally aligned. (A naturally aligned longword has zero
as the low-order two bits of its address.)

2.2.4 Quadword
A quadword is 8 contiguous bytes starting on an arbitrary byte boundary. The bits are numbered from right to left, 0 through 63, as shown in Figure 2–4.

Figure 2–4: Quadword Format
63

2–2 Alpha Architecture Handbook

A quadword is specified by its address A, the address of the byte containing bit 0. A quadword
is a 64-bit value. When interpreted arithmetically, a quadword is either a two’s-complement
integer with bits of increasing significance from 0 through 62 and bit 63 as the sign bit, or an
unsigned integer with bits of increasing significance from 0 through 63.

Note:
Alpha implementations will impose a significant performance penalty when accessing
quadword operands that are not naturally aligned. (A naturally aligned quadword has zero
as the low-order three bits of its address.)

2.2.5 VAX Floating-Point Formats
VAX floating-point numbers are stored in one set of formats in memory and in a second set of
formats in registers. The floating-point load and store instructions convert between these formats purely by rearranging bits; no rounding or range-checking is done by the load and store
instructions.

2.2.5.1 F_floating
An F_floating datum is 4 contiguous bytes in memory starting on an arbitrary byte boundary.
The bits are labeled from right to left, 0 through 31, as shown in Figure 2–5 .

Figure 2–5: F_floating Datum
16 15 14

Fraction Lo

7 6

Exp.

Frac. Hi

An F_floating operand occupies 64 bits in a floating register, left-justified in the 64-bit register, as shown in Figure 2–6.

Figure 2–6: F_floating Register Format
63 62

52 51

Exp.

29 28

Fraction

:Fx

The F_floating load instruction reorders bits on the way in from memory, expands the exponent from 8 to 11 bits, and sets the low-order fraction bits to zero. This produces in the register
an equivalent G_floating number suitable for either F_floating or G_floating operations. The
mapping from 8-bit memory-format exponents to 11-bit register-format exponents is shown in
Table 2–1. This mapping preserves both normal values and exceptional values.

Basic Architecture 2–3

Table 2–1: F_floating Load Exponent Mapping (MAP_F)
Memory <14:7>

1 1111111

1 000 1111111

1 xxxxxxx

1 000 xxxxxxx

(xxxxxxx not all 1’s)

0 xxxxxxx

0 111 xxxxxxx

(xxxxxxx not all 0’s)

0 0000000

0 000 0000000

The F_floating store instruction reorders register bits on the way to memory and does no
checking of the low-order fraction bits. Register bits <61:59> and <28:0> are ignored by the
store instruction.
An F_floating datum is specified by its address A, the address of the byte containing bit 0. The
memory form of an F_floating datum is sign magnitude with bit 15 the sign bit, bits <14:7> an
excess-128 binary exponent, and bits <6:0> and <31:16> a normalized 24-bit fraction with the
redundant most significant fraction bit not represented. Within the fraction, bits of increasing
significance are from 16 through 31 and 0 through 6. The 8-bit exponent field encodes the values 0 through 255. An exponent value of 0, together with a sign bit of 0, is taken to indicate
that the F_floating datum has a value of 0.
If the result of a VAX floating-point format instruction has a value of zero, the instruction
always produces a datum with a sign bit of 0, an exponent of 0, and all fraction bits of 0. Exponent values of 1..255 indicate true binary exponents of –127..127. An exponent value of 0,
together with a sign bit of 1, is taken as a reserved operand. Floating-point instructions processing a reserved operand take an arithmetic exception. The value of an F_floating datum is in
the approximate range 0.29*10**–38 through 1.7*10**38. The precision of an F_floating
datum is approximately one part in 2**23, typically 7 decimal digits. See Section 4.7.

Note:
Alpha implementations will impose a significant performance penalty when accessing
F_floating operands that are not naturally aligned. (A naturally aligned F_floating datum
has zero as the low-order two bits of its address.)

2.2.5.2 G_floating
A G_floating datum in memory is 8 contiguous bytes starting on an arbitrary byte boundary.
The bits are labeled from right to left, 0 through 63, as shown in Figure 2–7.

Figure 2–7: G_floating Datum
4 3

16 15 14

Fraction Midh

Fraction Lo

2–4 Alpha Architecture Handbook

Exp.

Frac.Hi :A

Fraction Midl

:A+4

A G_floating operand occupies 64 bits in a floating register, arranged as shown in Figure 2–8.

Figure 2–8: G_floating Register Format
63 62

52 51

Exp.

32 31

Fraction Hi

Fraction Lo

:Fx

A G_floating datum is specified by its address A, the address of the byte containing bit 0. The
form of a G_floating datum is sign magnitude with bit 15 the sign bit, bits <14:4> an excess1024 binary exponent, and bits <3:0> and <63:16> a normalized 53-bit fraction with the redundant most significant fraction bit not represented. Within the fraction, bits of increasing
significance are from 48 through 63, 32 through 47, 16 through 31, and 0 through 3. The 11-bit
exponent field encodes the values 0 through 2047. An exponent value of 0, together with a sign
bit of 0, is taken to indicate that the G_floating datum has a value of 0.
If the result of a floating-point instruction has a value of zero, the instruction always produces
a datum with a sign bit of 0, an exponent of 0, and all fraction bits of 0. Exponent values of
1..2047 indicate true binary exponents of –1023..1023. An exponent value of 0, together with a
sign bit of 1, is taken as a reserved operand. Floating-point instructions processing a reserved
operand take a user-visible arithmetic exception. The value of a G_floating datum is in the
approximate range 0.56*1 0**–308 through 0.9*10**308. The precision of a G_floating datum
is approximately one part in 2**52, typically 15 decimal digits. See Section 4.7.

Note:
Alpha implementations will impose a significant performance penalty when accessing
G_floating operands that are not naturally aligned. (A naturally aligned G_floating datum
has zero as the low-order three bits of its address.)

2.2.5.3 D_floating
A D_floating datum in memory is 8 contiguous bytes starting on an arbitrary byte boundary.
The bits are labeled from right to left, 0 through 63, as shown in Figure 2–9.

Figure 2–9: D_floating Datum
16 15 14

Fraction Midh
Fraction Lo

7 6

Exp.

Frac.Hi

Fraction Midl

:A
:A+4

A D_floating operand occupies 64 bits in a floating register, arranged as shown in Figure 2–10.

Figure 2–10: D_floating Register Format
63 62

55 54

Exp.

48 47

Frac. Hi

32 31

Fraction Midh

16 15

Fraction Midl

Fraction Lo

:Fx

Basic Architecture 2–5

The reordering of bits required for a D_floating load or store is identical to that required for a
G_floating load or store. The G_floating load and store instructions are therefore used for loading or storing D_floating data.
A D_floating datum is specified by its address A, the address of the byte containing bit 0. The
memory form of a D_floating datum is identical to an F_floating datum except for 32 additional low significance fraction bits. Within the fraction, bits of increasing significance are
from 48 through 63, 32 through 47, 16 through 31, and 0 through 6. The exponent conventions
and approximate range of values is the same for D_floating as F_floating. The precision of a
D_floating datum is approximately one part in 2**55, typically 16 decimal digits.

Notes:
D_floating is not a fully supported data type; no D_floating arithmetic operations are
provided in the architecture. For backward compatibility, exact D_floating arithmetic may
be provided via software emulation. D_floating "format compatibility"in which binary files
of D_floating numbers may be processed, but without the last three bits of fraction
precision, can be obtained via conversions to G_floating, G arithmetic operations, then
conversion back to D_floating.
Alpha implementations will impose a significant performance penalty on access to
D_floating operands that are not naturally aligned. (A naturally aligned D_floating datum
has zero as the low-order three bits of its address.)

2.2.6 IEEE Floating-Point Formats
The IEEE standard for binary floating-point arithmetic, ANSI/IEEE 754-1985, defines four
floating-point formats in two groups, basic and extended, each having two widths, single and
double. The Alpha architecture supports the basic single and double formats, with the basic
double format serving as the extended single format. The values representable within a format
are specified by using three integer parameters:

•

P – the number of fraction bits

•

Emax – the maximum exponent

•

Emin – the minimum exponent

Within each format, only the following entities are permitted:

•

Numbers of the form (–1)**S x 2**E x b(0).b(1)b(2)..b(P–1) where:
–

S = 0 or 1

–

E = any integer between Emin and Emax, inclusive

–

b(n) = 0 or 1

•

Two infinities – positive and negative

•

At least one Signaling NaN

•

At least one Quiet NaN

NaN is an acronym for Not-a-Number. A NaN is an IEEE floating-point bit pattern that represents something other than a number. NaNs come in two forms: Signaling NaNs and Quiet

2–6 Alpha Architecture Handbook

NaNs. Signaling NaNs are used to provide values for uninitialized variables and for arithmetic
enhancements. Quiet NaNs provide retrospective diagnostic information regarding previous
invalid or unavailable data and results. Signaling NaNs signal an invalid operation when they
are an operand to an arithmetic instruction, and may generate an arithmetic exception. Quiet
NaNs propagate through almost every operation without generating an arithmetic exception.
Arithmetic with the infinities is handled as if the operands were of arbitrarily large magnitude.
Negative infinity is less than every finite number; positive infinity is greater than every finite
number.

2.2.6.1 S_Floating
An IEEE single-precision, or S_floating, datum occupies 4 contiguous bytes in memory starting on an arbitrary byte boundary. The bits are labeled from right to left, 0 through 31, as
shown in Figure 2–11.

Figure 2–11: S_floating Datum
23 22

31 30

Exp.

Fraction

An S_floating operand occupies 64 bits in a floating register, left-justified in the 64-bit register, as shown in Figure 2–12.

Figure 2–12: S_floating Register Format
63 62

52 51

Exp.

29 28

Fraction

:Fx

The S_floating load instruction reorders bits on the way in from memory, expanding the exponent from 8 to 11 bits, and sets the low-order fraction bits to zero. This produces in the register
an equivalent T_floating number, suitable for either S_floating or T_floating operations. The
mapping from 8-bit memory-format exponents to 11-bit register-format exponents is shown in
Table 2–2.

Table 2–2: S_floating Load Exponent Mapping (MAP_S)
Memory <30:23>

1 1111111

1 111 1111111

1 xxxxxxx

1 000 xxxxxxx

(xxxxxxx not all 1’s)

0 xxxxxxx

0 111 xxxxxxx

(xxxxxxx not all 0’s)

0 0000000

0 000 0000000

Basic Architecture 2–7

This mapping preserves both normal values and exceptional values. Note that the mapping for
all 1’s differs from that of F_floating load, since for S_floating all 1’s is an exceptional value
and for F_floating all 1’s is a normal value.
The S_floating store instruction reorders register bits on the way to memory and does no
checking of the low-order fraction bits. Register bits <61:59> and <28:0> are ignored by the
store instruction. The S_floating load instruction does no checking of the input.
The S_floating store instruction does no checking of the data; the preceding operation should
have specified an S_floating result.
An S_floating datum is specified by its address A, the address of the byte containing bit 0. The
memory form of an S_floating datum is sign magnitude with bit 31 the sign bit, bits <30:23>
an excess-127 binary exponent, and bits <22:0> a 23-bit fraction.
The value (V) of an S_floating number is inferred from its constituent sign (S), exponent (E),
and fraction (F) fields as follows:

•

If E=255 and F<>0, then V is NaN, regardless of S.

•

If E=255 and F=0, then V = (–1)**S x Infinity.

•

If 0 < E < 255, then V = (–1)**S x 2**(E–127) x (1.F).

•

If E=0 and F<>0, then V = (–1)**S x 2**(–126) x (0.F).

•

If E=0 and F=0, then V = (–1)**S x 0 (zero).

Floating-point operations on S_floating numbers may take an arithmetic exception for a variety of reasons, including invalid operations, overflow, underflow, division by zero, and inexact
results.

Note:
Alpha implementations will impose a significant performance penalty when accessing
S_floating operands that are not naturally aligned. (A naturally aligned S_floating datum
has zero as the low-order two bits of its address.)

2.2.6.2 T_floating
An IEEE double-precision, or T_floating, datum occupies 8 contiguous bytes in memory starting on an arbitrary byte boundary. The bits are labeled from right to left, 0 through 63, as
shown in Figure 2–13.

Figure 2–13: T_floating Datum
20 19

31 30

Fraction Lo
S

Exponent

Fraction Hi

2–8 Alpha Architecture Handbook

:A
:A+4

A T_floating operand occupies 64 bits in a floating register, arranged as shown in Figure 2–14.

Figure 2–14: T_floating Register Format
63 62

52 51

Exp.

32 31

Fraction Hi

Fraction Lo

:Fx

The T_floating load instruction performs no bit reordering on input, nor does it perform checking of the input data.
The T_floating store instruction performs no bit reordering on output. This instruction does no
checking of the data; the preceding operation should have specified a T_floating result.
A T_floating datum is specified by its address A, the address of the byte containing bit 0. The
form of a T_floating datum is sign magnitude with bit 63 the sign bit, bits <62:52> an excess1023 binary exponent, and bits <51:0> a 52-bit fraction.
The value (V) of a T_floating number is inferred from its constituent sign (S), exponent (E),
and fraction (F) fields as follows:

•

If E=2047 and F<>0, then V is NaN, regardless of S.

•

If E=2047 and F=0, then V = (–1)**S x Infinity.

•

If 0 < E < 2047, then V = (–1)**S x 2**(E–1023) x (1.F).

•

If E=0 and F<>0, then V = (–1)**S x 2**(–1022) x (0.F).

•

If E=0 and F=0, then V = (–1)**S x 0 (zero).

Floating-point operations on T_floating numbers may take an arithmetic exception for a variety of reasons, including invalid operations, overflow, underflow, division by zero, and inexact
results.

Note:
Alpha implementations will impose a significant performance penalty when accessing
T_floating operands that are not naturally aligned. (A naturally aligned T_floating datum
has zero as the low-order three bits of its address.)

2.2.6.3 X_Floating
Support for 128-bit IEEE extended-precision (X_float) floating-point is initially provided
entirely through software. This section is included to preserve the intended consistency of
implementation with other IEEE floating-point data types, should the X_float data type be supported in future hardware.
An IEEE extended-precision, or X_floating, datum occupies 16 contiguous bytes in memory,
starting on an arbitrary byte boundary. The bits are labeled from right to left, 0 through 127, as
shown in Figure 2–15.

Basic Architecture 2–9

Figure 2–15: X_floating Datum
63 62

48 47

Fraction_low
S

Exponent

:A+8

Fraction_high

An X_floating datum occupies two consecutive even/odd floating-point registers (such as
F4/F5), as shown in Figure 2–16.

Figure 2–16: X_floating Register Format
127 126

112 111

Exponent

64 63

Fraction_high

Fraction_low

Fn OR 1

An X_floating datum is specified by its address A, the address of the byte containing bit 0. The
form of an X_floating datum is sign magnitude with bit 127 the sign bit, bits <126:112> an
excess–16383 binary exponent, and bits <111:0> a 112-bit fraction.
The value (V) of an X_floating number is inferred from its constituent sign (S), exponent (E),
and fraction (F) fields as follows:

•

If E=32767 and F<>0, then V is a NaN, regardless of S.

•

If E=32767 and F=0, then V = (–1)**S x Infinity.

•

If 0 < E < 32767, then V = (–1)**S x 2**(E–16383) x (1.F).

•

If E=0 and F<> 0, then V = (–1)**S x 2**(–16382) x (0.F).

•

If E = 0 and F = 0, then V = (–1)**S x 0 (zero).

Note:
Alpha implementations will impose a significant performance penalty when accessing
X_floating operands that are not naturally aligned. (A naturally aligned X_floating datum
has zero as the low-order four bits of its address.)

X_Floating Big-Endian Formats
Section 2.3 describes Alpha support for big-endian data types. It is intended that software or
hardware implementation for a big-endian X_float data type comply with that support and have
the following formats.

2–10 Alpha Architecture Handbook

Figure 2–17: X_floating Big-Endian Datum
Byte
0

Exponent

Fraction_high
Byte
15

A+8:

Fraction_low

Figure 2–18: X_floating Big-Endian Register Format
Byte

Byte

Exponent

Fraction_high

Fraction_low

Fn OR 1

2.2.7 Longword Integer Format in Floating-Point Unit
A longword integer operand occupies 32 bits in memory, arranged as shown in Figure 2–19.

Figure 2–19: Longword Integer Datum
31 30

Integer

A longword integer operand occupies 64 bits in a floating register, arranged as shown in Figure 2–20.

Figure 2–20: Longword Integer Floating-Register Format
63 62 61 59 58

S I

xxx

29 28

Integer

:Fx

There is no explicit longword load or store instruction; the S_floating load/store instructions
are used to move longword data into or out of the floating registers. The register bits <61:59>
are set by the S_floating load exponent mapping. They are ignored by S_floating store. They
are also ignored in operands of a longword integer operate instruction, and they are set to 000
in the result of a longword operate instruction.
The register format bit <62> "I" in Figure 2–20 is part of the Integer field in Figure 2–19 and
represents the high-order bit of that field.

Basic Architecture 2–11

Note:
Alpha implementations will impose a significant performance penalty when accessing
longwords that are not naturally aligned. (A naturally aligned longword datum has zero as
the low-order two bits of its address.)

2.2.8 Quadword Integer Format in Floating-Point Unit
A quadword integer operand occupies 64 bits in memory, arranged as shown in Figure 2–21.

Figure 2–21: Quadword Integer Datum
31 30

Integer Lo

Integer Hi

:A+4

A quadword integer operand occupies 64 bits in a floating register, arranged as shown in Figure 2–22.

Figure 2–22: Quadword Integer Floating-Register Format
63 62

32 31

Integer Hi

Integer Lo

:Fx

There is no explicit quadword load or store instruction; the T_floating load/store instructions
are used to move quadword data between memory and the floating registers. (The ITOFT and
FTOIT are used to move quadword data between integer and floating registers.)
The T_floating load instruction performs no bit reordering on input. The T_floating store
instruction performs no bit reordering on output. This instruction does no checking of the data;
when used to store quadwords, the preceding operation should have specified a quadword
result.

Note:
Alpha implementations will impose a significant performance penalty when accessing
quadwords that are not naturally aligned. (A naturally aligned quadword datum has zero as
the low-order three bits of its address.)

2.2.9 Data Types with No Hardware Support
•

The following VAX data types are not directly supported in Alpha hardware. Octaword

•

H_floating

•

D_floating (except load/store and convert to/from G_floating)

•

Variable-Length Bit Field

•

Character String

2–12 Alpha Architecture Handbook

•

Trailing Numeric String

•

Leading Separate Numeric String

•

Packed Decimal String

2.3 Big-Endian Addressing Support
Alpha implementations may include optional big-endian addressing support.
In a little-endian machine, the bytes within a quadword are numbered right to left:

Figure 2–23: Little-Endian Byte Addressing

In a big-endian machine, they are numbered left to right:

Figure 2–24: Big-Endian Byte Addressing

Bit numbering within bytes is not affected by the byte numbering convention (big-endian or little-endian).
The format for the X_floating big-endian data type is shown in Section 2.2.6.3.
The byte numbering convention does not matter when accessing complete aligned quadwords
in memory. However, the numbering convention does matter when accessing smaller or
unaligned quantities, or when manipulating data in registers, as follows:

•

A quadword load or store of data at location 0 moves the same eight bytes under both
numbering conventions. However, a longword load or store of data at location 4 must
move the leftmost half of a quadword under the little-endian convention, and the rightmost half under the big-endian convention. Thus, to support both conventions, the convention being used must be known and it must affect longword load/store operations.

•

A byte extract of byte 5 from a quadword of data into the low byte of a register requires
a right shift of 5 bytes under the little-endian convention, but a right shift of 2 bytes
under the big-endian convention.

•

Manipulation of data in a register is almost the same for both conventions. In both, integer and floating-point data have their sign bits in the leftmost byte and their least significant bit in the rightmost byte, so the same integer and floating-point instructions are

Basic Architecture 2–13

used unchanged for both conventions. Big-endian character strings have their most significant character on the left, while little-endian strings have their most significant character on the right.

•

The compare byte (CMPBGE) instruction is neutral about direction, doing eight byte
compares in parallel. However, following the CMPBGE instruction, the code is different that examines the byte mask to determine which string is larger, depending on
whether the rightmost or leftmost unequal byte is used. Thus, compilers must be
instructed to generate somewhat different code sequences for the two conventions.

Implementations that include big-endian support must supply all of the following features:

•

A means at boot time to choose the byte numbering convention. The implementation is
not required to support dynamically changing the convention during program execution. The chosen convention applies to all code executed, both operating-system and
user.

•

If the big-endian convention is chosen, the longword-length load/store instructions
(LDF, LDL, LDL_L, LDS, STF, STL, STL_C, STS) invert bit va<2> (bit 2 of the virtual address). This has the effect of accessing the half of a quadword other than the half
that would be accessed under the little-endian convention.

•

If the big-endian convention is chosen, the word-length load instruction, LDWU,
inverts bits va<1:2> (bits 1 and 2 of the virtual address). This has the effect of accessing
the half of the longword that would be accessed under the little-endian convention.

•

If the big-endian convention is chosen, the byte-length load instruction, LDBU, inverts
bits va<0:2> (bits 0 through 2 of the virtual address). This has the effect of accessing
the half of the word that would be accessed under the little-endian convention.

•

If the big-endian convention is chosen, the byte manipulation instructions (EXTxx,
INSxx, MSKxx) invert bits Rbv<2:0>. This has the effect of changing a shift of 5 bytes
into a shift of 2 bytes, for example.

The instruction stream is always considered to be little-endian, and is independent of the chosen byte numbering convention. Compilers, linkers, and debuggers must be aware of this when
accessing an instruction stream using data-stream load/store instructions. Thus, the rightmost
instruction in a quadword is always executed first and always has the instruction-stream
address 0 MOD 8. The same bytes accessed by a longword load/store instruction have datastream address 0 MOD 8 under the little-endian convention, and 4 MOD 8 under the bigendian convention.
Using either byte numbering convention, it is sometimes necessary to access data that originated on a machine that used the other convention. When this occurs, it is often necessary to
swap the bytes within a datum. See Section A.4.3 for a suggested code sequence.

2–14 Alpha Architecture Handbook

Chapter 3

Instruction Formats

3.1 Alpha Registers
Each Alpha processor has a set of registers that hold the current processor state. If an Alpha
system contains multiple Alpha processors, there are multiple per-processor sets of these
registers.

3.1.1 Program Counter
The Program Counter (PC) is a special register that addresses the instruction stream. As each
instruction is decoded, the PC is advanced to the next sequential instruction. This is referred to
as the updated PC. Any instruction that uses the value of the PC will use the updated PC. The
PC includes only bits <63:2> with bits <1:0> treated as RAZ/IGN. This quantity is a longword-aligned byte address. The PC is an implied operand on conditional branch and subroutine
jump instructions. The PC is not accessible as an integer register.

3.1.2 Integer Registers
There are 32 integer registers (R0 through R31), each 64 bits wide.
Register R31 is assigned special meaning by the Alpha architecture. When R31 is specified as
a register source operand, a zero-valued operand is supplied.
For all cases except the Unconditional Branch and Jump instructions, results of an instruction
that specifies R31 as a destination operand are discarded. Also, it is UNPREDICTABLE
whether the other destination operands (implicit and explicit) are changed by the instruction. It
is implementation dependent to what extent the instruction is actually executed once it has
been fetched. An exception is never signaled for a load that specifies R31 as a destination operation. For all other operations, it is UNPREDICTABLE whether exceptions are signaled during
the execution of such an instruction. Note, however, that exceptions associated with the
instruction fetch of such an instruction are always signaled.

Implementation note:
As described in Section A.3.5, certain load instructions to an R31 destination are the
preferred method for performing a cache block prefetch.

Instruction Formats 3–1

There are some interesting cases involving R31 as a destination:

•

STx_C R31,disp(Rb)
Although this might seem like a good way to zero out a shared location and reset the
lock_flag, this instruction causes the lock_flag and virtual location {Rbv +
SEXT(disp)} to become UNPREDICTABLE.

•

LDx_L R31,disp(Rb)
This instruction produces no useful result since it causes both lock_flag and
locked_physical_address to become UNPREDICTABLE.

Unconditional Branch (BR and BSR) and Jump (JMP, JSR, RET, and JSR_COROUTINE)
instructions, when R31 is specified as the Ra operand, execute normally and update the PC
with the target virtual address. Of course, no PC value can be saved in R31.

3.1.3 Floating-Point Registers
There are 32 floating-point registers (F0 through F31), each 64 bits wide.
When F31 is specified as a register source operand, a true zero-valued operand is supplied. See
Section 4.7.3 for a definition of true zero.
Results of an instruction that specifies F31 as a destination operand are discarded and it is
UNPREDICTABLE whether the other destination operands (implicit and explicit) are changed
by the instruction. In this case, it is implementation-dependent to what extent the instruction is
actually executed once it has been fetched. An exception is never signaled for a load that specifies F31 as a destination operation. For all other operations, it is UNPREDICTABLE whether
exceptions are signaled during the execution of such an instruction. Note, however, that exceptions associated with the instruction fetch of such an instruction are always signaled.

Implementation note:
As described in Section A.3.5, certain load instructions to an F31 destination are the
preferred method for signalling a cache block prefetch.
A floating-point instruction that operates on single-precision data reads all bits <63:0> of the
source floating-point register. A floating-point instruction that produces a single-precision
result writes all bits <63:0> of the destination floating-point register.

3.1.4 Lock Registers
There are two per-processor registers associated with the LDx_L and STx_C instructions, the
lock_flag and the locked_physical_address register. The use of these registers is described in
Section 4.2.

3–2 Alpha Architecture Handbook

3.1.5 Processor Cycle Counter (PCC) Register
The PCC register consists of two 32-bit fields. The low-order 32 bits (PCC<31:0>) are an
unsigned wrapping counter, PCC_CNT. The high-order 32 bits (PCC<63:32>), PCC_OFF, are
operating system dependent in their implementation.
PCC_CNT is the base clock register for measuring time intervals and is suitable for timing
intervals on the order of nanoseconds.
PCC_CNT increments once per N CPU cycles, where N is an implementation-specific integer
in the range 1..16. The cycle counter frequency is the number of times the processor cycle
counter gets incremented per second. The integer count wraps to 0 from a count of FFFF
FFFF 16. The counter wraps no more frequently than 1.5 times the implementation’s interval
clock interrupt period (which is two thirds of the interval clock interrupt frequency), which
guarantees that an interrupt occurs before PCC _CNT overflows twice.
PCC_OFF need not contain a value related to time and could contain all zeros in a simple
implementation. However, if PCC_OFF is used to calculate a per-process or per-thread cycle
count, it must contain a value that, when added to PCC_CNT, returns the total PCC register
count for that process or thread, modulo 2**32.

Implementation Note:
OpenVMS Alpha and DIGITAL UNIX supply a per-process value in PCC_OFF.
PCC is required on all implementations. It is required for every processor, and each processor
on a multiprocessor system has its own private, independent PCC.
The PCC is read by the RPCC instruction. See Section 4.11.8.

3.1.6 Optional Registers
Some Alpha implementations may include optional memory prefetch or VAX compatibility
processor registers.

3.1.6.1 Memory Prefetch Registers
If the prefetch instructions FETCH and FETCH_M are implemented, an implementation will
include two sets of state prefetch registers used by those instructions. The use of these registers is described in Section 4.11. These registers are not directly accessible by software and are
listed for completeness.

3.1.6.2 VAX Compatibility Register
The VAX compatibility instructions RC and RS include the intr_flag register, as described in
Section 4.12.

3.2 Notation
The notation used to describe the operation of each instruction is given as a sequence of control and assignment statements in an ALGOL-like syntax.
Instruction Formats 3–3

3.2.1 Operand Notation
Tables 3–1, 3–2, and 3–3 list the notation for the operands, the operand values, and the other
expression operands.

Table 3–1: Operand Notation
Notation

Meaning

An integer register operand in the Ra field of the instruction

An integer register operand in the Rb field of the instruction

An integer literal operand in the Rb field of the instruction

An integer register operand in the Rc field of the instruction

A floating-point register operand in the Ra field of the instruction

A floating-point register operand in the Rb field of the instruction

A floating-point register operand in the Rc field of the instruction

Table 3–2: Operand Value Notation
Notation

Meaning

Rav

The value of the Ra operand. This is the contents of register Ra.

Rbv

The value of the Rb operand. This could be the contents of register Rb, or
a zero-extended 8-bit literal in the case of an Operate format instruction.

Fav

The value of the floating point Fa operand. This is the contents of register
Fa.

Fbv

The value of the floating point Fb operand. This is the contents of register
Fb.

Table 3–3: Expression Operand Notation
Notation

Meaning

IPR_x

Contents of Internal Processor Register x)

IPR_SP[mode]

Contents of the per-mode stack pointer selected by mode

Updated PC value

Contents of integer register n

Contents of floating-point register n

X[m]

Element m of array X

3–4 Alpha Architecture Handbook

3.2.2 Instruction Operand Notation
The notation used to describe instruction operands follows from the operand specifier notation
used in the VAX Architecture Standard. Instruction operands are described as follows:

<name>.<access type><data type>
3.2.2.1 Operand Name Notation
Specifies the instruction field (Ra, Rb, Rc, or disp) and register type of the operand (integer or
floating). It can be one of the following:

Table 3–4: Operand Name Notation
Name

Meaning

disp

The displacement field of the instruction

fnc

The PALcode function field of the instruction

An integer register operand in the Ra field of the instruction

An integer register operand in the Rb field of the instruction

An integer literal operand in the Rb field of the instruction

An integer register operand in the Rc field of the instruction

A floating-point register operand in the Ra field of the instruction

A floating-point register operand in the Rb field of the instruction

A floating-point register operand in the Rc field of the instruction

3.2.2.2 Operand Access Type Notation
A letter that denotes the operand access type:

Table 3–5: Operand Access Type Notation
Access Type

Meaning

The operand is used in an address calculation to form an effective
address. The data type code that follows indicates the units of addressability (or scale factor) applied to this operand when the instruction is
decoded.
For example:
".al" means scale by 4 (longwords) to get byte units (used in branch displacements); ".ab" means the operand is already in byte units (used in
load/store instructions).

The operand is an immediate literal in the instruction.
Instruction Formats 3–5

Table 3–5: Operand Access Type Notation (Continued)
Access Type

Meaning

The operand is read only.

The operand is both read and written.

The operand is write only.

3.2.2.3 Operand Data Type Notation
A letter that denotes the data type of the operand:

Table 3–6: Operand Data Type Notation
Data Type

Meaning

Byte

F_floating

G_floating

Longword

Quadword

IEEE single floating (S_floating)

IEEE double floating (T_floating)

Word

The data type is specified by the instruction

3.2.3 Operators
Table 3–7 describes the operators:

Table 3–7: Operators
Operator

Meaning

Comment delimiter

Addition

Subtraction

Signed multiplication

Unsigned multiplication

Exponentiation (left argument raised to right argument)

Division

←

Replacement

3–6 Alpha Architecture Handbook

Table 3–7: Operators (Continued)
Operator

Meaning

Bit concatenation

{}

Indicates explicit operator precedence

(x)

Contents of memory location whose address is x

x <m:n>

Contents of bit field of x defined by bits n through m

x <m>

M’th bit of x

ACCESS(x,y)

Accessibility of the location whose address is x using the
access mode y. Returns a Boolean value TRUE if the
address is accessible, else FALSE.

AND

Logical product

ARITH_RIGHT_SHIFT(x,y)

Arithmetic right shift of first operand by the second operand. Y is an unsigned shift value. Bit 63, the sign bit, is
copied into vacated bit positions and shifted out bits are
discarded.

BYTE_ZAP(x,y)

X is a quadword, y is an 8-bit vector in which each bit
corresponds to a byte of the result. The y bit to x byte correspondence is y <n> ↔ x <8n+7:8n>. This correspondence also exists between y and the result.
For each bit of y from n = 0 to 7, if y <n> is 0 then byte
<n> of x is copied to byte <n> of result, and if y <n> is 1
then byte <n> of result is forced to all zeros.

Instruction Formats 3–7

Table 3–7: Operators (Continued)
Operator

Meaning

CASE

The CASE construct selects one of several actions based
on the value of its argument. The form of a case is:
CASE argument OF
argvalue1: action_1
argvalue2: action_2
...
argvaluen:action_n
[otherwise: default_action]
ENDCASE

If the value of argument is argvalue1 then action_1 is executed; if argument = argvalue2, then action_2 is executed,
and so forth.
Once a single action is executed, the code stream breaks
to the ENDCASE (there is an implicit break as in Pascal).
Each action may nonetheless be a sequence of
pseudocode operations, one operation per line.
Optionally, the last argvalue may be the atom ‘otherwise’.
The associated default action will be taken if none of the
other argvalues match the argument.
DIV

Integer division (truncates)

LEFT_SHIFT(x,y)

Logical left shift of first operand by the second operand.Y
is an unsigned shift value. Zeros are moved into the
vacated bit positions, and shifted out bits are discarded.

LOAD_LOCKED

The processor records the target physical address in a perprocessor locked_physical_address register and sets the
per-processor lock_flag.

Log to the base 2.

MAP_x

F_float or S_float memory-to-register exponent mapping
function.

MAXS(x,y)

Returns the larger of x and y, with x and y interpreted as
signed integers.

MAXU(x,y)

Returns the larger of x and y, with x and y interpreted as
unsigned integers.

MINS(x,y)

Returns the smaller of x and y, with x and y interpreted as
signed integers.

MINU(x,y)

Returns the smaller of x and y, with x and y interpreted as
unsigned integers.

x MOD y

x modulo y

3–8 Alpha Architecture Handbook

Table 3–7: Operators (Continued)
Operator

Meaning

NOT

Logical (ones) complement

Logical sum

PHYSICAL_ADDRESS

Translation of a virtual address

PRIORITY_ENCODE

Returns the bit position of most significant set bit, interpreting its argument as a positive integer (=int(lg(x))). For
example:
priority_encode( 255 ) = 7

Relational Operators:
Operator

Meaning

Less than signed

LTU

Less than unsigned

Less or equal signed

LEU

Less or equal unsigned

Equal signed and unsigned

Not equal signed and unsigned

Greater or equal signed

GEU

Greater or equal unsigned

Greater signed

GTU

Greater unsigned

LBC

Low bit clear

LBS

Low bit signed

RIGHT_SHIFT(x,y)

Logical right shift of first operand by the second operand.
Y is an unsigned shift value. Zeros are moved into
vacated bit positions, and shifted out bits are discarded.

SEXT(x)

X is sign-extended to the required size.

STORE_CONDITIONAL

If the lock_flag is set, then do the indicated store and clear
the lock_flag.

Instruction Formats 3–9

Table 3–7: Operators (Continued)
Operator

Meaning

TEST(x,cond)

The contents of register x are tested for branch condition
(cond) true. TEST returns a Boolean value TRUE if x
bears the specified relation to 0, else FALSE is returned.
Integer and floating test conditions are drawn from the
preceding list of relational operators.

XOR

Logical difference

ZEXT(x)

X is zero-extended to the required size.

3.2.4 Notation Conventions
The following conventions are used:

•

Only operands that appear on the left side of a replacement operator are modified.

•

No operator precedence is assumed other than that replacement (←) has the lowest precedence. Explicit precedence is indicated by the use of "{}".

•

All arithmetic, logical, and relational operators are defined in the context of their operands. For example, "+" applied to G_floating operands means a G_floating add,
whereas "+" applied to quadword operands is an integer add. Similarly, "LT" is a
G_floating comparison when applied to G_floating operands and an integer comparison
when applied to quadword operands.

3.3 Instruction Formats
There are five basic Alpha instruction formats:

•

Memory

•

Branch

•

Operate

•

Floating-point Operate

•

PALcode

All instruction formats are 32 bits long with a 6-bit major opcode field in bits <31:26> of the
instruction.
Any unused register field (Ra, Rb, Fa, Fb) of an instruction must be set to a value of 31.

Software Note:
There are several instructions, each formatted as a memory instruction, that do not use the
Ra and/or Rb fields. These instructions are: Memory Barrier, Fetch, Fetch_M, Read
Process Cycle Counter, Read and Clear, Read and Set, and Trap Barrier.

3–10 Alpha Architecture Handbook

3.3.1 Memory Instruction Format
The Memory format is used to transfer data between registers and memory, to load an effective address, and for subroutine jumps. It has the format shown in Figure 3–1.

Figure 3–1: Memory Instruction Format
31

26 25

Opcode

21 20

16 15

Memory_disp

A Memory format instruction contains a 6-bit opcode field, two 5-bit register address fields, Ra
and Rb, and a 16-bit signed displacement field.
The displacement field is a byte offset. It is sign-extended and added to the contents of register
Rb to form a virtual address. Overflow is ignored in this calculation.
The virtual address is used as a memory load/store address or a result value, depending on the
specific instruction. The virtual address (va) is computed as follows for all memory format
instructions except the load address high (LDAH):
va ← {Rbv + SEXT(Memory_disp)}

For LDAH the virtual address (va) is computed as follows:
va ← {Rbv + SEXT(Memory_disp*65536)}

3.3.1.1 Memory Format Instructions with a Function Code
Memory format instructions with a function code replace the memory displacement field in the
memory instruction format with a function code that designates a set of miscellaneous instructions. The format is shown in Figure 3–2.

Figure 3–2: Memory Instruction with Function Code Format
31

26 25

Opcode

21 20

16 15

Function

The memory instruction with function code format contains a 6-bit opcode field and a 16-bit
function field. Unused function codes produce UNPREDICTABLE but not UNDEFINED
results; they are not security holes.
There are two fields, Ra and Rb. The usage of those fields depends on the instruction. See Section 4.11.

Instruction Formats 3–11

3.3.1.2 Memory Format Jump Instructions
For computed branch instructions (CALL, RET, JMP, JSR_COROUTINE) the displacement
field is used to provide branch-prediction hints as described in Section 4.3.

3.3.2 Branch Instruction Format
The Branch format is used for conditional branch instructions and for PC-relative subroutine
jumps. It has the format shown in Figure 3–3.

Figure 3–3: Branch Instruction Format
31

26 25

Opcode

21 20

Branch_disp

A Branch format instruction contains a 6-bit opcode field, one 5-bit register address field (Ra),
and a 21-bit signed displacement field.
The displacement is treated as a longword offset. This means it is shifted left two bits (to
address a longword boundary), sign-extended to 64 bits, and added to the updated PC to form
the target virtual address. Overflow is ignored in this calculation. The target virtual address
(va) is computed as follows:
va ← PC + {4*SEXT(Branch_disp)}

3.3.3 Operate Instruction Format
The Operate format is used for instructions that perform integer register to integer register
operations. The Operate format allows the specification of one destination operand and two
source operands. One of the source operands can be a literal constant. The Operate format in
Figure 3–4 shows the two cases when bit <12> of the instruction is 0 and 1.

Figure 3–4: Operate Instruction Format
31

26 25

Opcode

21 20

26 25

Opcode

16 15 13 12 11

21 20

5 4

SBZ 0 Function

13 12 11

LIT

3–12 Alpha Architecture Handbook

Function

5 4

An Operate format instruction contains a 6-bit opcode field and a 7-bit function code field.
Unused function codes for opcodes defined as reserved in the Version 5 Alpha architecture
specification (May 1992) produce an illegal instruction trap. Those opcodes are 01, 02, 03, 04,
05, 06, 07, 0A, 0C, 0D, 0E, 14, 19, 1B, 1D, 1E, and 1F. For other opcodes, unused function
codes produce UNPREDICTABLE but not UNDEFINED results; they are not security holes.
There are three operand fields, Ra, Rb, and Rc.
The Ra field specifies a source operand. Symbolically, the integer Rav operand is formed as
follows:
IF inst<25:21> EQ 31 THEN
Rav ← 0
ELSE
Rav ← Ra
END

The Rb field specifies a source operand. Integer operands can specify a literal or an integer
register using bit <12> of the instruction.
If bit <12> of the instruction is 0, the Rb field specifies a source register operand.
If bit <12> of the instruction is 1, an 8-bit zero-extended literal constant is formed by bits
<20:13> of the instruction. The literal is interpreted as a positive integer between 0 and 255
and is zero-extended to 64 bits. Symbolically, the integer Rbv operand is formed as follows:
IF inst <12> EQ 1 THEN
Rbv ← ZEXT(inst<20:13>)
ELSE
IF inst <20:16> EQ 31 THEN
Rbv ← 0
ELSE
Rbv ← Rb
END
END

The Rc field specifies a destination operand.

3.3.4 Floating-Point Operate Instruction Format
The Floating-point Operate format is used for instructions that perform floating-point register
to floating-point register operations. The Floating-point Operate format allows the specification of one destination operand and two source operands. The Floating-point Operate format is
shown in Figure 3–5.

Figure 3–5: Floating-Point Operate Instruction Format
31

26 25

Opcode

21 20

16 15

5 4

Function

Instruction Formats 3–13

A Floating-point Operate format instruction contains a 6-bit opcode field and an 11-bit function field. Unused function codes for those opcodes defined as reserved in the Version 5 Alpha
architecture specification (May 1992) produce an illegal instruction trap. Those opcodes are
01, 02, 03, 04, 05, 06, 07, 14, 19, 1B, 1D, 1E, and 1F. For other opcodes, unused function
codes produce UNPREDICTABLE but not UNDEFINED results; they are not security holes.
There are three operand fields, Fa, Fb, and Fc. Each operand field specifies either an integer or
floating-point operand as defined by the instruction.
The Fa field specifies a source operand. Symbolically, the Fav operand is formed as follows:
IF inst<25:21> EQ 31 THEN
Fav ← 0
ELSE
Fav ← Fa
END

The Fb field specifies a source operand. Symbolically, the Fbv operand is formed as follows:
IF inst<20:16> EQ 31 THEN
Fbv ← 0
ELSE
Fbv ← Fb
END

Note:
Neither Fa nor Fb can be a literal in Floating-point Operate instructions.
The Fc field specifies a destination operand.

3.3.4.1 Floating-Point Convert Instructions
Floating-point Convert instructions use a subset of the Floating-point Operate format and perform register-to-register conversion operations. The Fb operand specifies the source; the Fa
field must be F31.

3.3.4.2 Floating-Point/Integer Register Moves
Instructions that move data between a floating-point register file and an integer register file are
a subset of of the Floating-point Operate format. The unused source field must be 31.

3.3.5 PALcode Instruction Format
The Privileged Architecture Library (PALcode) format is used to specify extended processor
functions. It has the format shown in Figure 3–6.

3–14 Alpha Architecture Handbook

Figure 3–6: PALcode Instruction Format
31

26 25

Opcode

PALcode Function

The 26-bit PALcode function field specifies the operation. The source and destination operands for PALcode instructions are supplied in fixed registers that are specified in the individual
instruction descriptions.
An opcode of zero and a PALcode function of zero specify the HALT instruction.

Instruction Formats 3–15

Chapter 4

Instruction Descriptions

4.1 Instruction Set Overview
This chapter describes the instructions implemented by the Alpha architecture. The instruction
set is divided into the following sections:

Instruction Type

Section

Integer load and store

4.2

Integer control

4.3

Integer arithmetic

4.4

Logical and shift

4.5

Byte manipulation

4.6

Floating-point load and store

4.7

Floating-point control

4.8

Floating-point branch

4.9

Floating-point operate

4.10

Miscellaneous

4.11

VAX compatibility

4.12

Multimedia (graphics and video)

4.13

Within each major section, closely related instructions are combined into groups and described
together.
The instruction group description is composed of the following:

•

The group name

•

The format of each instruction in the group, which includes the name, access type, and
data type of each instruction operand

•

The operation of the instruction

•

Exceptions specific to the instruction

•

The instruction mnemonic and name of each instruction in the group

Instruction Descriptions 4–1

•

Qualifiers specific to the instructions in the group

•

A description of the instruction operation

•

Optional programming examples and optional notes on the instruction

4.1.1 Subsetting Rules
An instruction that is omitted in a subset implementation of the Alpha architecture is not performed in either hardware or PALcode. System software may provide emulation routines for
subsetted instructions.

4.1.2 Floating-Point Subsets
Floating-point support is optional on an Alpha processor. An implementation that supports
floating-point must implement the following:

•

The 32 floating-point registers

•

The Floating-point Control Register (FPCR) and the instructions to access it

•

The floating-point branch instructions

•

The floating-point copy sign (CPYSx) instructions

•

The floating-point convert instructions

•

The floating-point conditional move instruction (FCMOV)

•

The S_floating and T_floating memory operations

Software Note:
A system that will not support floating-point operations is still required to provide the 32
floating-point registers, the Floating-point Control Register (FPCR) and the instructions to
access it, and the T_floating memory operations if the system intends to support the
OpenVMS Alpha operating system. This requirement facilitates the implementation of a
floating-point emulator and simplifies context-switching.
In addition, floating-point support requires at least one of the following subset groups:
1. VAX Floating-point Operate and Memory instructions (F_ and G_floating).
2. IEEE Floating-point Operate instructions (S_ and T_floating). Within this group, an
implementation can choose to include or omit separately the ability to perform IEEE
rounding to plus infinity and minus infinity.

Note:
If one instruction in a group is provided, all other instructions in that group must be
provided. An implementation with full floating-point support includes both groups; a
subset floating-point implementation supports only one of these groups. The individual
instruction descriptions indicate whether an instruction can be subsetted.

4–2 Alpha Architecture Handbook

4.1.3 Software Emulation Rules
General-purpose layered and application software that executes in User mode may assume that
certain loads (LDL, LDQ, LDF, LDG, LDS, and LDT) and certain stores (STL, STQ, STF,
STG, STL, and STT) of unaligned data are emulated by system software. General-purpose layered and application software that executes in User mode may assume that subsetted
instructions are emulated by system software. Frequent use of emulation may be significantly
slower than using alternative code sequences.
Emulation of loads and stores of unaligned data and subsetted instructions need not be provided in privileged access modes. System software that supports special-purpose dedicated
applications need not provide emulation in User mode if emulation is not needed for correct
execution of the special-purpose applications.

4.1.4 Opcode Qualifiers
Some Operate format and Floating-point Operate format instructions have several variants. For
example, for the VAX formats, Add F_floating (ADDF) is supported with and without floating underflow enabled and with either chopped or VAX rounding. For IEEE formats, IEEE
unbiased rounding, chopped, round toward plus infinity, and round toward minus infinity can
be selected.
The different variants of such instructions are denoted by opcode qualifiers, which consist of a
slash (/) followed by a string of selected qualifiers. Each qualifier is denoted by a single character as shown in Table 4–1. The opcodes for each qualifier are listed in Appendix C.

Table 4–1: Opcode Qualifiers
Qualifier

Meaning

Chopped rounding

Rounding mode dynamic

Round toward minus infinity

Inexact result enable

Exception completion enable

Floating underflow enable

Integer overflow enable

The default values are normal rounding, exception completion disabled, inexact result disabled, floating underflow disabled, and integer overflow disabled.

Instruction Descriptions 4–3

4.2 Memory Integer Load/Store Instructions
The instructions in this section move data between the integer registers and memory.
They use the Memory instruction format. The instructions are summarized in Table 4–2.

Table 4–2: Memory Integer Load/Store Instructions
Mnemonic

Operation

LDA

Load Address

LDAH

Load Address High

LDBU

Load Zero-Extended Byte from Memory to Register

LDL

Load Sign-Extended Longword

LDL_L

Load Sign-Extended Longword Locked

LDQ

Load Quadword

LDQ_L

Load Quadword Locked

LDQ_U

Load Quadword Unaligned

LDWU

Load Zero-Extended Word from Memory to Register

STB

Store Byte

STL

Store Longword

STL_C

Store Longword Conditional

STQ

Store Quadword

STQ_C

Store Quadword Conditional

STQ_U

Store Quadword Unaligned

STW

Store Word

4–4 Alpha Architecture Handbook

4.2.1 Load Address
Format:
LDAx

!Memory format

Ra.wq,disp.ab(Rb.ab)

Operation:
Ra ← Rbv + SEXT(disp)
Ra ← Rbv + SEXT(disp*65536)

!LDA
!LDAH

Exceptions:
None

Instruction mnemonics:
LDA

Load Address

LDAH

Load Address High

Qualifiers:
None

Description:
The virtual address is computed by adding register Rb to the sign-extended 16-bit displacement for LDA, and 65536 times the sign-extended 16-bit displacement for LDAH. The 64-bit
result is written to register Ra.

Instruction Descriptions 4–5

4.2.2 Load Memory Data into Integer Register
Format:
LDx

!Memory format

Ra.wq,disp.ab(Rb.ab)

Operation:
va ← {Rbv + SEXT(disp)}
CASE
big_endian_data: va’ ← va XOR 0002
big_endian_data: va’ ← va XOR 1002
big_endian_data: va’ ← va XOR 1102
big_endian_data: va’ ← va XOR 1112
little_endian_data: va’ ← va
ENDCASE
Ra ← (va’)<63:0>
Ra ← SEXT((va’)<31:0>)
Ra ← ZEXT((va’)<15:0>)
Ra ← ZEXT((va’)<07:0>)

!LDQ
!LDL
!LDWU
!LDBU

Exceptions:
Access Violation
Alignment
Fault on Read
Translation Not Valid

Instruction mnemonics:
LDBU

Load Zero-Extended Byte from Memory to Register

LDL

Load Sign-Extended Longword from Memory to Register

LDQ

Load Quadword from Memory to Register

LDWU

Load Zero-Extended Word from Memory to Register

Qualifiers:
None

Description:
The virtual address is computed by adding register Rb to the sign-extended 16-bit displacement. For a big-endian access, the indicated bits are inverted, and any memory management
fault is reported for va (not va’).

4–6 Alpha Architecture Handbook

In the case of LDQ and LDL, the source operand is fetched from memory, sign-extended, and
written to register Ra.
In the case of LDWU and LDBU, the source operand is fetched from memory, zero-extended,
and written to register Ra.
In all cases, if the data is not naturally aligned, an alignment exception is generated.

Notes:
•

The word or byte that the LDWU or LDBU instruction fetches from memory is placed
in the low (rightmost) word or byte of Ra, with the remaining 6 or 7 bytes set to zero.

•

Accesses have byte granularity.

•

For big-endian access with LDWU or LDBU, the word/byte remains in the rightmost
part of Ra, but the va sent to memory has the indicated bits inverted. See Operation section, above.

•

No sparse address space mechanisms are allowed with the LDWU and LDBU instructions.

Implementation Notes:
•

The LDWU and LDBU instructions are supported in hardware on Alpha implementations for which the AMASK instruction returns bit 0 set. LDWU and LDBU are supported with software emulation in Alpha implementations for which AMASK does not
return bit 0 set. Software emulation of LDWU and LDBU is significantly slower than
hardware support.

•

Depending on an address space region’s caching policy, implementations may read a
(partial) cache block in order to do word/byte stores. This may only be done in regions
that have memory-like behavior.

•

Implementations are expected to provide sufficient low-order address bits and
length-of-access information to devices on I/O buses. But, strictly speaking, this is outside the scope of architecture.

Instruction Descriptions 4–7

4.2.3 Load Unaligned Memory Data into Integer Register
Format:
LDQ_U

Ra.wq,disp.ab(Rb.ab)

!Memory format

Operation:
va ← {{Rbv + SEXT(disp)} AND NOT 7}
Ra ← (va)<63:0>

Exceptions:
Access Violation
Fault on Read
Translation Not Valid

Instruction mnemonics:
LDQ_U

Load Unaligned Quadword from Memory to Register

Qualifiers:
None

Description:
The virtual address is computed by adding register Rb to the sign-extended 16-bit displacement, then the low-order three bits are cleared. The source operand is fetched from memory
and written to register Ra.

4–8 Alpha Architecture Handbook

4.2.4 Load Memory Data into Integer Register Locked
Format:
LDx_L

!Memory format

Ra.wq,disp.ab(Rb.ab)

Operation:
va ←

{Rbv + SEXT(disp)}

CASE
big_endian_data: va’ ← va XOR 0002
big_endian_data: va’ ← va XOR 1002
little_endian_data: va’ ← va
ENDCASE

! LDQ_L
! LDL_L
! LDL_L

lock_flag ← 1
locked_physical_address ← PHYSICAL_ADDRESS(va)
Ra ← SEXT((va’)<31:0>)
Ra ← (va)<63:0>

! LDL_L
! LDQ_L

Exceptions:
Access Violation
Alignment
Fault on Read
Translation Not Valid

Instruction mnemonics:
LDL_L

Load Sign-Extended Longword from Memory to Register
Locked

LDQ_L

Load Quadword from Memory to Register Locked

Qualifiers:
None

Instruction Descriptions 4–9

When a LDx_L instruction is executed without faulting, the processor records the target physical address in a per-processor locked_physical_address register and sets the per-processor
lock_flag.
If the per-processor lock_flag is (still) set when a STx_C instruction is executed (accessing
within the same 16-byte naturally aligned block as the LDx_L), the store occurs; otherwise, it
does not occur, as described for the STx_C instructions. The behavior of an STx_C instruction
is UNPREDICTABLE, as described in Section 4.2.5, when it does not access the same 16-byte
naturally aligned block as the LDx_L.
Processor A causes the clearing of a set lock_flag in processor B by doing any of the following
in B’s locked range of physical addresses: a successful store, a successful store_conditional, or
executing a WH64 instruction that modifies data on processor B. A processor’s locked range is
the aligned block of 2**N bytes that includes the locked_physical_address. The 2**N value is
implementation dependent. It is at least 16 (minimum lock range is an aligned 16-byte block)
and is at most the page size for that implementation (maximum lock range is one physical
page).
A processor’s lock_flag is also cleared if that processor encounters a CALL_PAL REI,
CALL_PAL rti, or CALL_PAL rfe instruction. It is UNPREDICTABLE whether or not a processor’s lock_flag is cleared on any other CALL_PAL instruction. It is UNPREDICTABLE
whether a processor’s lock_flag is cleared by that processor executing a normal load or store
instruction. It is UNPREDICTABLE whether a processor’s lock_flag is cleared by that processor executing a taken branch (including BR, BSR, and Jumps); conditional branches that fall
through do not clear the lock_flag. It is UNPREDICTABLE whether a processor’s lock_flag is
cleared by that processor executing a WH64 or ECB instruction.
The sequence:
LDx_L
Modify
STx_C
BEQ xxx
when executed on a given processor, does an atomic read-modify-write of a datum in shared
memory if the branch falls through. If the branch is taken, the store did not modify memory
and the sequence may be repeated until it succeeds.

Notes:
•

LDx_L instructions do not check for write access; hence a matching STx_C may take
an access-violation or fault-on-write exception.
Executing a LDx_L instruction on one processor does not affect any architecturally
visible state on another processor, and in particular cannot cause an STx_C on another
processor to fail.
LDx_L and STx_C instructions need not be paired. In particular, an LDx_L may be
followed by a conditional branch: on the fall-through path an STx_C is executed,
whereas on the taken path no matching STx_C is executed.

4–10 Alpha Architecture Handbook

If two LDx_L instructions execute with no intervening STx_C, the second one
overwrites the state of the first one. If two STx_C instructions execute with no
intervening LDx_L, the second one always fails because the first clears lock_flag.

•

Software will not emulate unaligned LDx_L instructions.

•

If the virtual and physical addresses for a LDx_L and STx_C sequence are not within
the same naturally aligned 16-byte sections of virtual and physical memory, that
sequence may always fail, or may succeed despite another processor’s store to the lock
range; hence, no useful program should do this.

•

If any other memory access (ECB, LDx, LDQ_U, STx, STQ_U, WH64) is executed on
the given processor between the LDx_L and the STx_C, the sequence above may
always fail on some implementations; hence, no useful program should do this.

•

If a branch is taken between the LDx_L and the STx_C, the sequence above may
always fail on some implementations; hence, no useful program should do this.
(CMOVxx may be used to avoid branching.)

•

If a subsetted instruction (for example, floating-point) is executed between the LDx_L
and the STx_C, the sequence above may always fail on some implementations because
of the Illegal Instruction Trap; hence, no useful program should do this.

•

If an instruction with an unused function code is executed between the LDx_L and the
STx_C, the sequence above may always fail on some implementations because an
instruction with an unused function code is UNPREDICTABLE.

•

If a large number of instructions are executed between the LDx_L and the STx_C, the
sequence above may always fail on some implementations because of a timer interrupt
always clearing the lock_flag before the sequence completes; hence, no useful program
should do this.

•

Hardware implementations are encouraged to lock no more than 128 bytes. Software
implementations are encouraged to separate locked locations by at least 128 bytes from
other locations that could potentially be written by another processor while the first
location is locked.

•

Execution of a WH64 instruction on processor A to a region within the lock range of
processor B, where the execution of the WH64 changes the contents of memory, causes
the lock_flag on processor B to be cleared. If the WH64 does not change the contents of
memory on processor B, it need not clear the lock_flag.

Implementation Notes:
Implementations that impede the mobility of a cache block on LDx_L, such as that which
may occur in a Read for Ownership cache coherency protocol, may release the cache block
and make the subsequent STx_C fail if a branch-taken or memory instruction is executed
on that processor.
All implementations should guarantee that at least 40 non-subsetted operate instructions
can be executed between timer interrupts.

Instruction Descriptions 4–11

4.2.5 Store Integer Register Data into Memory Conditional
Format:
STx_C

!Memory format

Ra.mx,disp.ab(Rb.ab)

Operation:
va ← {Rbv + SEXT(disp)}
CASE
big_endian_data: va’ ← va XOR 0002
big_endian_data: va’ ← va XOR 1002
little_endian_data: va’ ← va
ENDCASE
IF lock_flag EQ 1 THEN
(va’)<31:0> ← Rav<31:0>
(va’)
← Rav
Ra ← lock_flag
lock_flag ← 0

! STQ_C
! STL_C
! STL_C

! STL_C
! STQ_C

Exceptions:
Access Violation
Fault on Write
Alignment
Translation Not Valid

Instruction mnemonics:
STL_C

Store Longword from Register to Memory Conditional

STQ_C

Store Quadword from Register to Memory Conditional

Qualifiers:
None

Description:
The virtual address is computed by adding register Rb to the sign-extended 16-bit displacement. For a big-endian longword access, va<2> (bit 2 of the virtual address) is inverted, and
any memory management fault is reported for va (not va’).
If the lock_flag is set and the address meets the following constraints relative to the address
specified by the preceding LDx_L instruction, the Ra operand is written to memory at this
address. If the address meets the following constraints but the lock_flag is not set, a zero is
returned in Ra and no write to memory occurs. The constraints are:

4–12 Alpha Architecture Handbook

•

The computed virtual address must specify a location within the naturally aligned
16-byte block in virtual memory accessed by the preceding LDx_L instruction.

•

The resultant physical address must specify a location within the naturally aligned
16-byte block in physical memory accessed by the preceding LDx_L instruction.

If those addressing constraints are not met, it is UNPREDICTABLE whether the STx_C
instruction succeeds or fails, regardless of the state of the lock_flag, unless the lock_flag is
cleared as described in the next paragraph.
Whether or not the addressing constraints are met, a zero is returned and no write to memory
occurs if the lock_flag was cleared by execution on a processor of a CALL_PAL REI,
CALL_PAL rti, CALL_PAL rfe, or STx_C, after the most recent execution on that processor
of a LDx_L instruction (in processor issue sequence).
In all cases, the lock_flag is set to zero at the end of the operation.

Notes:
•

Software will not emulate unaligned STx_C instructions.

•

Each implementation must do the test and store atomically, as illustrated in the following two examples. (See Section 5.6.1 for complete information.)
–

If two processors attempt STx_C instructions to the same lock range and that lock
range was accessed by both processors’ preceding LDx_L instructions, exactly one
of the stores succeeds.

–

A processor executes a LDx_L/STx_C sequence and includes an MB between the
LDx_L to a particular address and the successful STx_C to a different address (one
that meets the constraints required for predictable behavior). That instruction
sequence establishes an access order under which a store operation by another processor to that lock range occurs before the LDx_L or after the STx_C.

•

The following sequence should not be used:
try_again: LDQ_L
R1, x
<modify R1>
STQ_C
R1, x
BEQ
R1, try_again

That sequence penalizes performance when the STQ_C succeeds, because the
sequence contains a backward branch, which is predicted to be taken in the Alpha
architecture. In the case where the STQ_C succeeds and the branch will actually fall
through, that sequence incurs unnecessary delay due to a mispredicted backward
branch. Instead, a forward branch should be used to handle the failure case, as shown
in Section 5.5.2.

Instruction Descriptions 4–13

Software Note:
If the address specified by a STx_C instruction does not match the one given in the
preceding LDx_L instruction, an MB is required to guarantee ordering between the two
instructions.

Hardware/Software Implementation Note:
STQ_C is used in the first Alpha implementations to access the MailBox Pointer Register
(MBPR). In this special case, the effect of the STQ_C is well defined (that is, not
UNPREDICTABLE) even though the preceding LDx_L did not specify the address of the
MBPR. The effect of STx_C in this special case may vary from implementation to
implementation.

Implementation Notes:
A STx_C must propagate to the point of coherency, where it is guaranteed to prevent any
other store from changing the state of the lock bit, before its outcome can be determined.
If an implementation could encounter a TB or cache miss on the data reference of the
STx_C in the sequence above (as might occur in some shared I- and D-stream
direct-mapped TBs/caches), it must be able to resolve the miss and complete the store
without always failing.

4–14 Alpha Architecture Handbook

4.2.6 Store Integer Register Data into Memory
Format:
STx

!Memory format

Ra.rx,disp.ab(Rb.ab)

Operation:
va ← {Rbv + SEXT(disp)}
CASE
big_endian_data: va’ ← va XOR 0002
big_endian_data: va’ ← va XOR 1002
big_endian_data: va’ ← va XOR 1102
big_endian_data: va’ ← va XOR 1112
little_endian_data: va’ ← va
ENDCASE
(va’) ← Rav
(va’)<31:00> ← Rav<31:0>
(va’)<15:00> ← Rav<15:0>
(va’)<07:00> ← Rav<07:0>

!STQ
!STL
!STW
!STB

Exceptions:
Access Violation
Alignment
Fault on Write
Translation Not Valid

Instruction mnemonics:
STB

Store Byte from Register to Memory

STL

Store Longword from Register to Memory

STQ

Store Quadword from Register to Memory

STW

Store Word from Register to Memory

Qualifiers:
None

Instruction Descriptions 4–15

The Ra operand is written to memory at this address. If the data is not naturally aligned, an
alignment exception is generated.

Notes:
•

The word or byte that the STB or STW instruction stores to memory comes from the
low (rightmost) byte or word of Ra.

•

Accesses have byte granularity.

•

For big-endian access with STB or STW, the byte/word remains in the rightmost part of
Ra, but the va sent to memory has the indicated bits inverted. See Operation section,
above.

•

No sparse address space mechanisms are allowed with the STB and STW instructions.

Implementation Notes:
•

The STB and STW instructions are supported in hardware on Alpha implementations
for which the AMASK instruction returns bit 0 set. STB and STW are supported with
software emulation in Alpha implementations for which AMASK does not return bit 0
set. Software emulation of STB and STW is significantly slower than hardware support.

•

Depending on an address space region’s caching policy, implementations may read a
(partial) cache block in order to do byte/word stores. This may only be done in regions
that have memory-like behavior.

•

Implementations are expected to provide sufficient low-order address bits and
length-of-access information to devices on I/O buses. But, strictly speaking, this is outside the scope of architecture.

4–16 Alpha Architecture Handbook

4.2.7 Store Unaligned Integer Register Data into Memory
Format:
STQ_U

Ra.rq,disp.ab(Rb.ab)

!Memory format

Operation:
va ← {{Rbv + SEXT(disp)} AND NOT 7}
(va)<63:0> ← Rav<63:0>

Exceptions:
Access Violation
Fault on Write
Translation Not Valid

Instruction mnemonics:
STQ_U

Store Unaligned Quadword from Register to Memory

Qualifiers:
None

Description:
The virtual address is computed by adding register Rb to the sign-extended 16-bit displacement, then clearing the low order three bits. The Ra operand is written to memory at this
address.

Instruction Descriptions 4–17

4.3 Control Instructions
Alpha provides integer conditional branch, unconditional branch, branch to subroutine, and
jump instructions. The PC used in these instructions is the updated PC, as described in Section
3.1.1.
To allow implementations to achieve high performance, the Alpha architecture includes
explicit hints based on a branch-prediction model:

•

For many implementations of computed branches (JSR/RET/JMP), there is a substantial performance gain in forming a good guess of the expected target I-cache address
before register Rb is accessed.

•

For many implementations, the first-level (or only) I-cache is no bigger than a page (8
KB to 64 KB).

•

Correctly predicting subroutine returns is important for good performance. Some
implementations will therefore keep a small stack of predicted subroutine return
I-cache addresses.

The Alpha architecture provides three kinds of branch-prediction hints: likely target address,
return-address stack action, and conditional branch-taken.
For computed branches, the otherwise unused displacement field contains a function code
(JMP/JSR/RET/JSR_COROUTINE), and, for JSR and JMP, a field that statically specifies the
16 low bits of the most likely target address. The PC-relative calculation using these bits can
be exactly the PC-relative calculation used in unconditional branches. The low 16 bits are
enough to specify an I-cache block within the largest possible Alpha page and hence are
expected to be enough for branch-prediction logic to start an early I-cache access for the most
likely target.
For all branches, hint or opcode bits are used to distinguish simple branches, subroutine calls,
subroutine returns, and coroutine links. These distinctions allow branch-predict logic to maintain an accurate stack of predicted return addresses.
For conditional branches, the sign of the target displacement is used as a taken/fall-through
hint. The instructions are summarized in Table 4–3.

Table 4–3: Control Instructions Summary
Mnemonic

Operation

BEQ

Branch if Register Equal to Zero

BGE

Branch if Register Greater Than or Equal to Zero

BGT

Branch if Register Greater Than Zero

BLBC

Branch if Register Low Bit Is Clear

BLBS

Branch if Register Low Bit Is Set

BLE

Branch if Register Less Than or Equal to Zero

BLT

Branch if Register Less Than Zero

4–18 Alpha Architecture Handbook

Table 4–3: Control Instructions Summary (Continued)
Mnemonic

Operation

BNE

Branch if Register Not Equal to Zero

Unconditional Branch

BSR

Branch to Subroutine

JMP

Jump

JSR

Jump to Subroutine

RET

Return from Subroutine

JSR_COROUTINE

Jump to Subroutine Return

Instruction Descriptions 4–19

4.3.1 Conditional Branch
Format:
Bxx

Ra.rq,disp.al

!Branch format

Operation:
{update PC}
va ← PC + {4*SEXT(disp)}
IF TEST(Rav, Condition_based_on_Opcode) THEN
PC ← va

Exceptions:
None

Instruction mnemonics:
BEQ

Branch if Register Equal to Zero

BGE

Branch if Register Greater Than or Equal to Zero

BGT

Branch if Register Greater Than Zero

BLBC

Branch if Register Low Bit Is Clear

BLBS

Branch if Register Low Bit Is Set

BLE

Branch if Register Less Than or Equal to Zero

BLT

Branch if Register Less Than Zero

BNE

Branch if Register Not Equal to Zero

Qualifiers:
None

Description:
Register Ra is tested. If the specified relationship is true, the PC is loaded with the target virtual address; otherwise, execution continues with the next sequential instruction.
The displacement is treated as a signed longword offset. This means it is shifted left two bits
(to address a longword boundary), sign-extended to 64 bits, and added to the updated PC to
form the target virtual address.
The conditional branch instructions are PC-relative only. The 21-bit signed displacement gives
a forward/backward branch distance of +/– 1M instructions.
The test is on the signed quadword integer interpretation of the register contents; all 64 bits are
tested.

4–20 Alpha Architecture Handbook

4.3.2 Unconditional Branch
Format:
BxR

Ra.wq,disp.al

!Branch format

Operation:
{update PC}
Ra ← PC
PC ← PC + {4*SEXT(disp)}

Exceptions:
None

Instruction mnemonics:
BR

Unconditional Branch

BSR

Branch to Subroutine

Qualifiers:
None

Description:
The PC of the following instruction (the updated PC) is written to register Ra and then the PC
is loaded with the target address.
The displacement is treated as a signed longword offset. This means it is shifted left two bits
(to address a longword boundary), sign-extended to 64 bits, and added to the updated PC to
form the target virtual address.
The unconditional branch instructions are PC-relative. The 21-bit signed displacement gives a
forward/backward branch distance of +/– 1M instructions.
PC-relative addressability can be established by:
BR Rx,L1
L1:

Notes:
•

BR and BSR do identical operations. They only differ in hints to possible branch-prediction logic. BSR is predicted as a subroutine call (pushes the return address on a
branch-prediction stack), whereas BR is predicted as a branch (no push).

Instruction Descriptions 4–21

4.3.3 Jumps
Format:
mnemonic

Ra.wq,(Rb.ab),hint

!Memory format

Operation:
{update PC}
va ← Rbv AND {NOT 3}
Ra ← PC
PC ← va

Exceptions:
None

Instruction mnemonics:
JMP

Jump

JSR

Jump to Subroutine

RET

Return from Subroutine

JSR_COROUTINE

Jump to Subroutine Return

Qualifiers:
None

Description:
The PC of the instruction following the Jump instruction (the updated PC) is written to register
Ra and then the PC is loaded with the target virtual address.
The new PC is supplied from register Rb. The low two bits of Rb are ignored. Ra and Rb may
specify the same register; the target calculation using the old value is done before the new
value is assigned.
All Jump instructions do identical operations. They only differ in hints to possible branch-prediction logic. The displacement field of the instruction is used to pass this information. The
four different "opcodes" set different bit patterns in disp<15:14>, and the hint operand sets
disp<13:0>.
These bits are intended to be used as shown in Table 4–4.

4–22 Alpha Architecture Handbook

Table 4–4: Jump Instructions Branch Prediction
disp<15:14> Meaning

Predicted
Target<15:0>

Prediction
Stack Action

JMP

PC + {4*disp<13:0>}

–

JSR

PC + {4*disp<13:0>}

Push PC

RET

Prediction stack

Pop

JSR_COROUTINE

Prediction stack

Pop, push PC

The design in Table 4–4 allows specification of the low 16 bits of a likely longword target
address (enough bits to start a useful I-cache access early), and also allows distinguishing call
from return (and from the other two less frequent operations).
Note that the above information is used only as a hint; correct setting of these bits can improve
performance but is not needed for correct operation. See Section A.2.2 for more information on
branch prediction.
An unconditional long jump can be performed by:
JMP R31,(Rb),hint

Coroutine linkage can be performed by specifying the same register in both the Ra and Rb
operands. When disp<15:14> equals ‘10’ (RET) or ‘11’ (JSR_COROUTINE) (that is, the target address prediction, if any, would come from a predictor implementation stack), then bits
<13:0> are reserved for software and must be ignored by all implementations. All encodings
for bits <13:0> are used by Compaq software or Reserved to Compaq, as follows:

Encoding

Meaning

0000 16

Indicates non-procedure return

0001 16

Indicates procedure return
All other encodings are reserved to Compaq.

Instruction Descriptions 4–23

4.4 Integer Arithmetic Instructions
The integer arithmetic instructions perform add, subtract, multiply, signed and unsigned compare, and bit count operations.

Count instruction (CIX) extension implementation note:
The CIX extension to the architecture provides the CTLZ, CTPOP, and CTTZ instructions.
Alpha processors for which the AMASK instruction returns bit 2 set implement these
instructions. Those processors for which AMASK does not return bit 2 set can take an
Illegal Instruction trap, and software can emulate their function, if required. AMASK is
described in Sections 4.11.1 and D.3.
The integer instructions are summarized in Table 4–5

Table 4–5: Integer Arithmetic Instructions Summary
Mnemonic

Operation

ADD

Add Quadword/Longword

S4ADD

Scaled Add by 4

S8ADD

Scaled Add by 8

CMPEQ

Compare Signed Quadword Equal

CMPLT

Compare Signed Quadword Less Than

CMPLE

Compare Signed Quadword Less Than or Equal

CTLZ

Count leading zero

CTPOP

Count population

CTTZ

Count trailing zero

CMPULT

Compare Unsigned Quadword Less Than

CMPULE

Compare Unsigned Quadword Less Than or Equal

MUL

Multiply Quadword/Longword

UMULH

Multiply Quadword Unsigned High

SUB

Subtract Quadword/Longword

S4SUB

Scaled Subtract by 4

S8SUB

Scaled Subtract by 8

There is no integer divide instruction. Division by a constant can be done by using UMULH;
division by a variable can be done by using a subroutine. See Section A.4.2.
4–24 Alpha Architecture Handbook

4.4.1 Longword Add
Format:
ADDL

Ra.rl,Rb.rl,Rc.wq

!Operate format

ADDL

Ra.rl,#b.ib,Rc.wq

!Operate format

Operation:
Rc ←

SEXT( (Rav + Rbv)<31:0>)

Exceptions:
Integer Overflow

Instruction mnemonics:
ADDL

Add Longword

Qualifiers:
Integer Overflow Enable (/V)

Description:
Register Ra is added to register Rb or a literal and the sign-extended 32-bit sum is written to
Rc.
The high order 32 bits of Ra and Rb are ignored. Rc is a proper sign extension of the truncated
32-bit sum. Overflow detection is based on the longword sum Rav<31:0> + Rbv<31:0>.

Instruction Descriptions 4–25

4.4.2 Scaled Longword Add
Format:
SxADDL

Ra.rl,Rb.rq,Rc.wq

!Operate format

SxADDL

Ra.rl,#b.ib,Rc.wq

!Operate format

Operation:
CASE
S4ADDL: Rc ← SEXT (((LEFT_SHIFT(Rav,2)) + Rbv)<31:0>)
S8ADDL: Rc ← SEXT (((LEFT_SHIFT(Rav,3)) + Rbv)<31:0>)
ENDCASE

Exceptions:
None

Instruction mnemonics:
S4ADDL

Scaled Add Longword by 4

S8ADDL

Scaled Add Longword by 8

Qualifiers:
None

Description:
Register Ra is scaled by 4 (for S4ADDL) or 8 (for S8ADDL) and is added to register Rb or a
literal, and the sign-extended 32-bit sum is written to Rc.
The high 32 bits of Ra and Rb are ignored. Rc is a proper sign extension of the truncated 32-bit
sum.

4–26 Alpha Architecture Handbook

4.4.3 Quadword Add
Format:
ADDQ

Ra.rq,Rb.rq,Rc.wq

!Operate format

ADDQ

Ra.rq,#b.ib,Rc.wq

!Operate format

Operation:
Rc ←

Rav + Rbv

Exceptions:
Integer Overflow

Instruction mnemonics:
ADDQ

Add Quadword

Qualifiers:
Integer Overflow Enable (/V)

Description:
Register Ra is added to register Rb or a literal and the 64-bit sum is written to Rc.
On overflow, the least significant 64 bits of the true result are written to the destination
register.
The unsigned compare instructions can be used to generate carry. After adding two values, if
the sum is less unsigned than either one of the inputs, there was a carry out of the most significant bit.

Instruction Descriptions 4–27

4.4.4 Scaled Quadword Add
Format:
SxADDQ

Ra.rq,Rb.rq,Rc.wq

!Operate format

SxADDQ

Ra.rq,#b.ib,Rc.wq

!Operate format

Operation:
CASE
S4ADDQ: Rc ← LEFT_SHIFT(Rav,2) + Rbv
S8ADDQ: Rc ← LEFT_SHIFT(Rav,3) + Rbv
ENDCASE

Exceptions:
None

Instruction mnemonics:
S4ADDQ
S8ADDQ

Scaled Add Quadword by 4
Scaled Add Quadword by 8

Qualifiers:
None

Description:
Register Ra is scaled by 4 (for S4ADDQ) or 8 (for S8ADDQ) and is added to register Rb or a
literal, and the 64-bit sum is written to Rc.
On overflow, the least significant 64 bits of the true result are written to the destination
register.

4–28 Alpha Architecture Handbook

4.4.5 Integer Signed Compare
Format:
CMPxx

Ra.rq,Rb.rq,Rc.wq

!Operate format

CMPxx

Ra.rq,#b.ib,Rc.wq

!Operate format

Operation:
IF Rav SIGNED_RELATION Rbv THEN
Rc ← 1
ELSE
Rc ← 0

Exceptions:
None

Instruction mnemonics:
CMPEQ
CMPLE
CMPLT

Compare Signed Quadword Equal
Compare Signed Quadword Less Than or Equal
Compare Signed Quadword Less Than

Qualifiers:
None

Description:
Register Ra is compared to Register Rb or a literal. If the specified relationship is true, the
value one is written to register Rc; otherwise, zero is written to Rc.

Notes:
•

Compare Less Than A,B is the same as Compare Greater Than B,A; Compare Less
Than or Equal A,B is the same as Compare Greater Than or Equal B,A. Therefore, only
the less-than operations are included.

Instruction Descriptions 4–29

4.4.6 Integer Unsigned Compare
Format:
CMPUxx

Ra.rq,Rb.rq,Rc.wq

!Operate format

CMPUxx

Ra.rq,#b.ib,Rc.wq

!Operate format

Operation:
IF Rav UNSIGNED_RELATION Rbv THEN
Rc ← 1
ELSE
Rc ← 0

Exceptions:
None

Instruction mnemonics:
CMPULE
CMPULT

Compare Unsigned Quadword Less Than or Equal
Compare Unsigned Quadword Less Than

Qualifiers:
None

Description:
Register Ra is compared to Register Rb or a literal. If the specified relationship is true, the
value one is written to register Rc; otherwise, zero is written to Rc.

4–30 Alpha Architecture Handbook

4.4.7 Count Leading Zero
Format:
CTLZ

Rb.rq,Rc.wq

! Operate format

Operation:
temp = 0
FOR i FROM 63 DOWN TO 0
IF { Rbv EQ 1 } THEN BREAK
temp = temp + 1
END
Rc<6:0> ← temp<6:0>
Rc<63:7> ← 0

Exceptions:
None

Instruction mnemonics:
CTLZ

Count Leading Zero

Qualifiers:
None

Description:
The number of leading zeros in Rb, starting at the most significant bit position, is written to Rc.
Ra must be R31.

Instruction Descriptions 4–31

4.4.8 Count Population
Format:
CTPOP

Rb.rq,Rc.wq

Operation:
temp = 0
FOR i FROM 0 TO 63
IF { Rbv EQ 1 } THEN temp = temp + 1
END
Rc<6:0> ← temp<6:0>
Rc<63:7> ← 0

Exceptions:
None

Instruction mnemonics:
CTPOP

Count Population

Qualifiers:
None

Description:
The number of ones in Rb is written to Rc. Ra must be R31.

4–32 Alpha Architecture Handbook

! Operate format

4.4.9 Count Trailing Zero
Format:
CTTZ

Rb.rq,Rc.wq

! Operate format

Operation:
temp = 0
FOR i FROM 0 TO 63
IF { Rbv EQ 1 } THEN BREAK
temp = temp + 1
END
Rc<6:0> ← temp<6:0>
Rc<63:7> ← 0

Exceptions:
None

Instruction mnemonics:
CTTZ

Count Trailing Zero

Qualifiers:
None

Description:
The number of trailing zeros in Rb, starting at the least significant bit position, is written to Rc.
Ra must be R31.

Instruction Descriptions 4–33

4.4.10 Longword Multiply
Format:
MULL

Ra.rl,Rb.rl,Rc.wq

!Operate format

MULL

Ra.rl,#b.ib,Rc.wq

!Operate format

Operation:
Rc ←

SEXT ((Rav * Rbv)<31:0>)

Exceptions:
Integer Overflow

Instruction mnemonics:
MULL

Multiply Longword

Qualifiers:
Integer Overflow Enable (/V)

Description:
Register Ra is multiplied by register Rb or a literal and the sign-extended 32-bit product is
written to Rc.
The high 32 bits of Ra and Rb are ignored. Rc is a proper sign extension of the truncated 32-bit
product. Overflow detection is based on the longword product Rav<31:0> * Rbv<31:0>. On
overflow, the proper sign extension of the least significant 32 bits of the true result is written to
the destination register.
The MULQ instruction can be used to return the full 64-bit product.

4–34 Alpha Architecture Handbook

4.4.11 Quadword Multiply
Format:
MULQ

Ra.rq,Rb.rq,Rc.wq

!Operate format

MULQ

Ra.Rq,#b.ib,Rc.wq

!Operate format

Operation:
Rc ←

Rav * Rbv

Exceptions:
Integer Overflow

Instruction mnemonics:
MULQ

Multiply Quadword

Qualifiers:
Integer Overflow Enable (/V)

Description:
Register Ra is multiplied by register Rb or a literal and the 64-bit product is written to register
Rc. Overflow detection is based on considering the operands and the result as signed quantities. On overflow, the least significant 64 bits of the true result are written to the destination
register.
The UMULH instruction can be used to generate the upper 64 bits of the 128-bit result when
an overflow occurs.

Instruction Descriptions 4–35

4.4.12 Unsigned Quadword Multiply High
Format:
UMULH

Ra.rq,Rb.rq,Rc.wq

!Operate format

UMULH

Ra.rq,#b.ib,Rc.wq

!Operate format

Operation:
Rc ← {Rav * U Rbv}<127:64>

Exceptions:
None

Instruction mnemonics:
UMULH

Unsigned Multiply Quadword High

Qualifiers:
None

Description:
Register Ra and Rb or a literal are multiplied as unsigned numbers to produce a 128-bit result.
The high-order 64-bits are written to register Rc.
The UMULH instruction can be used to generate the upper 64 bits of a 128-bit result as
follows:
Ra and Rb are unsigned: result of UMULH
Ra and Rb are signed: (result of UMULH) – Ra<63>*Rb – Rb<63>*Ra
The MULQ instruction gives the low 64 bits of the result in either case.

4–36 Alpha Architecture Handbook

4.4.13 Longword Subtract
Format:
SUBL

Ra.rl,Rb.rl,Rc.wq

!Operate format

SUBL

Ra.rl,#b.ib,Rc.wq

!Operate format

Operation:
Rc ←

SEXT ((Rav - Rbv)<31:0>)

Exceptions:
Integer Overflow

Instruction mnemonics:
SUBL

Subtract Longword

Qualifiers:
Integer Overflow Enable (/V)

Description:
Register Rb or a literal is subtracted from register Ra and the sign-extended 32-bit difference is
written to Rc.
The high 32 bits of Ra and Rb are ignored. Rc is a proper sign extension of the truncated 32-bit
difference. Overflow detection is based on the longword difference Rav<31:0> – Rbv<31:0>.

Instruction Descriptions 4–37

4.4.14 Scaled Longword Subtract
Format:
SxSUBL

Ra.rl,Rb.rl,Rc.wq

!Operate format

SxSUBL

Ra.rl,#b.ib,Rc.wq

!Operate format

Operation:
CASE
S4SUBL: Rc ← SEXT (((LEFT_SHIFT(Rav,2)) - Rbv)<31:0>)
S8SUBL: Rc ← SEXT (((LEFT_SHIFT(Rav,3)) - Rbv)<31:0>)
ENDCASE

Exceptions:
None

Instruction mnemonics:
S4SUBL

Scaled Subtract Longword by 4

S8SUBL

Scaled Subtract Longword by 8

Qualifiers:
None

Description:
Register Rb or a literal is subtracted from the scaled value of register Ra, which is scaled by 4
(for S4SUBL) or 8 (for S8SUBL), and the sign-extended 32-bit difference is written to Rc.
The high 32 bits of Ra and Rb are ignored. Rc is a proper sign extension of the truncated 32-bit
difference.

4–38 Alpha Architecture Handbook

4.4.15 Quadword Subtract
Format:
SUBQ

Ra.rq,Rb.rq,Rc.wq

!Operate format

SUBQ

Ra.rq,#b.ib,Rc.wq

!Operate format

Operation:
Rc ←

Rav - Rbv

Exceptions:
Integer Overflow

Instruction mnemonics:
SUBQ

Subtract Quadword

Qualifiers:
Integer Overflow Enable (/V)

Description:
Register Rb or a literal is subtracted from register Ra and the 64-bit difference is written to register Rc. On overflow, the least significant 64 bits of the true result are written to the
destination register.
The unsigned compare instructions can be used to generate borrow. If the minuend (Rav) is
less unsigned than the subtrahend (Rbv), a borrow will occur.

Instruction Descriptions 4–39

4.4.16 Scaled Quadword Subtract
Format:
SxSUBQ

Ra.rq,Rb.rq,Rc.wq

!Operate format

SxSUBQ

Ra.rq,#b.ib,Rc.wq

!Operate format

Operation:
CASE
S4SUBQ: Rc ← LEFT_SHIFT(Rav,2) - Rbv
S8SUBQ: Rc ← LEFT_SHIFT(Rav,3) - Rbv
ENDCASE

Exceptions:
None

Instruction mnemonics:
S4SUBQ

Scaled Subtract Quadword by 4

S8SUBQ

Scaled Subtract Quadword by 8

Qualifiers:
None

Description:
Register Rb or a literal is subtracted from the scaled value of register Ra, which is scaled by 4
(for S4SUBQ) or 8 (for S8SUBQ), and the 64-bit difference is written to Rc.

4–40 Alpha Architecture Handbook

4.5 Logical and Shift Instructions
The logical instructions perform quadword Boolean operations. The conditional move integer
instructions perform conditionals without a branch. The shift instructions perform left and right
logical shift and right arithmetic shift. These are summarized in Table 4–6.

Table 4–6: Logical and Shift Instructions Summary
Mnemonic

Operation

AND

Logical Product

BIC

Logical Product with Complement

BIS

Logical Sum (OR)

EQV

Logical Equivalence (XORNOT)

ORNOT

Logical Sum with Complement

XOR

Logical Difference

CMOVxx

Conditional Move Integer

SLL

Shift Left Logical

SRA

Shift Right Arithmetic

SRL

Shift Right Logical

Software Note:
There is no arithmetic left shift instruction. Where an arithmetic left shift would be used, a
logical shift will do. For multiplying by a small power of two in address computations,
logical left shift is acceptable.
Integer multiply should be used to perform an arithmetic left shift with overflow checking.
Bit field extracts can be done with two logical shifts. Sign extension can be done with a left
logical shift and a right arithmetic shift.

Instruction Descriptions 4–41

4.5.1 Logical Functions
Format:
mnemonic

Ra.rq,Rb.rq,Rc.wq

!Operate format

mnemonic

Ra.rq,#b.ib,Rc.wq

!Operate format

Operation:
Rc ← Rav AND Rbv
Rc ← Rav OR Rbv
Rc ← Rav XOR Rbv
Rc ← Rav AND {NOT Rbv}
Rc ← Rav OR {NOT Rbv}
Rc ← Rav XOR {NOT Rbv}

!AND
!BIS
!XOR
!BIC
!ORNOT
!EQV

Exceptions:
None

Instruction mnemonics:
AND
BIC
BIS
EQV
ORNOT
XOR

Logical Product
Logical Product with Complement
Logical Sum (OR)
Logical Equivalence (XORNOT)
Logical Sum with Complement
Logical Difference

Qualifiers:
None

Description:
These instructions perform the designated Boolean function between register Ra and register
Rb or a literal. The result is written to register Rc.
The NOT function can be performed by doing an ORNOT with zero (Ra = R31).

4–42 Alpha Architecture Handbook

4.5.2 Conditional Move Integer
Format:
CMOVxx

Ra.rq,Rb.rq,Rc.wq

!Operate format

CMOVxx

Ra.rq,#b.ib,Rc.wq

!Operate format

Operation:
IF TEST(Rav, Condition_based_on_Opcode) THEN
Rc ←

Rbv

Exceptions:
None

Instruction mnemonics:
CMOVEQ
CMOVGE
CMOVGT
CMOVLBC
CMOVLBS
CMOVLE
CMOVLT
CMOVNE

CMOVE if Register Equal to Zero
CMOVE if Register Greater Than or Equal to Zero
CMOVE if Register Greater Than Zero
CMOVE if Register Low Bit Clear
CMOVE if Register Low Bit Set
CMOVE if Register Less Than or Equal to Zero
CMOVE if Register Less Than Zero
CMOVE if Register Not Equal to Zero

Qualifiers:
None

Description:
Register Ra is tested. If the specified relationship is true, the value Rbv is written to register
Rc.

Instruction Descriptions 4–43

Notes:
Except that it is likely in many implementations to be substantially faster, the instruction:
CMOVEQ Ra,Rb,Rc

is exactly equivalent to:
BNE Ra,label
OR Rb,Rb,Rc
label: ...

For example, a branchless sequence for:
R1=MAX(R1,R2)

is:
CMPLT R1,R2,R3
CMOVNE R3,R2,R1

4–44 Alpha Architecture Handbook

! R3=1 if R1<R2
! Move R2 to R1 if R1<R2

4.5.3 Shift Logical
Format:
SxL

Ra.rq,Rb.rq,Rc.wq

!Operate format

SxL

Ra.rq,#b.ib,Rc.wq

!Operate format

Operation:
Rc ←
Rc ←

LEFT_SHIFT(Rav, Rbv<5:0>)
RIGHT_SHIFT(Rav, Rbv<5:0>)

!SLL
!SRL

Exceptions:
None

Instruction mnemonics:
SLL
SRL

Shift Left Logical
Shift Right Logical

Qualifiers:
None

Description:
Register Ra is shifted logically left or right 0 to 63 bits by the count in register Rb or a literal.
The result is written to register Rc. Zero bits are propagated into the vacated bit positions.

Instruction Descriptions 4–45

4.5.4 Shift Arithmetic
Format:
SRA

Ra.rq,Rb.rq,Rc.wq

!Operate format

SRA

Ra.rq,#b.ib,Rc.wq

!Operate format

Operation:
Rc ← ARITH_RIGHT_SHIFT(Rav, Rbv<5:0>)

Exceptions:
None

Instruction mnemonics:
SRA

Shift Right Arithmetic

Qualifiers:
None

Description:
Register Ra is right shifted arithmetically 0 to 63 bits by the count in register Rb or a literal.
The result is written to register Rc. The sign bit (Rav<63>) is propagated into the vacated bit
positions.

4–46 Alpha Architecture Handbook

4.6 Byte Manipulation Instructions
Alpha implementations that support the BWX extension provide the following instructions for
loading, sign-extending, and storing bytes and words between a register and memory:

Instruction

Meaning

Described in Section

LDBU/LDWU

Load byte/word unaligned

4.2.2

SEXTB/SEXTW

Sign-extend byte/word

4.6.5

STB/STW

Store byte/word

4.2.6

The AMASK instruction reports whether a particular Alpha implementation supports the BWX
extension. AMASK is described in Sections 4.11.1 and D.3.
LDBU and STB are the recommended way to perform byte load and store operations on Alpha
implementations that support them; use them rather than the extract, insert, and mask byte
instructions described in this section. In particular, the implementation examples in this section that illustrate byte operations are not appropriate for Alpha implementations that support
the BWX extension – instead use the recommendations in Section A.4.1.
In addition to LDBU and STB, Alpha provides the instructions in Table 4–7 for operating on
byte operands within registers.

Table 4–7: Byte-Within-Register Manipulation Instructions Summary

Mnemonic

Operation

CMPBGE

Compare Byte

EXTBL

Extract Byte Low

EXTWL

Extract Word Low

EXTLL

Extract Longword Low

EXTQL

Extract Quadword Low

EXTWH

Extract Word High

EXTLH

Extract Longword High

EXTQH

Extract Quadword High

INSBL

Insert Byte Low

INSWL

Insert Word Low

INSLL

Insert Longword Low

INSQL

Insert Quadword Low
Instruction Descriptions 4–47

Table 4–7: Byte-Within-Register Manipulation Instructions Summary
(Continued)
Mnemonic

Operation

INSWH

Insert Word High

INSLH

Insert Longword High

INSQH

Insert Quadword High

MSKBL

Mask Byte Low

MSKWL

Mask Word Low

MSKLL

Mask Longword Low

MSKQL

Mask Quadword Low

MSKWH

Mask Word High

MSKLH

Mask Longword High

MSKQH

Mask Quadword High

SEXTB

Sign extend byte

SEXTW

Sign extend word

ZAP

Zero Bytes

ZAPNOT

Zero Bytes Not

4–48 Alpha Architecture Handbook

4.6.1 Compare Byte
Format:
CMPBGE

Ra.rq,Rb.rq,Rc.wq

!Operate format

CMPBGE

Ra.rq,#b.ib,Rc.wq

!Operate format

Operation:
FOR i FROM 0 TO 7
temp<8:0> ← 0 || Rav<i*8+7:i*8>} + {0 || NOT Rbv<i*8+7:i*8>} + 1
Rc ← temp<8>
END
Rc<63:8> ← 0

Exceptions:
None

Instruction mnemonics:
CMPBGE

Compare Byte

Qualifiers:
None

Description:
CMPBGE does eight parallel unsigned byte comparisons between corresponding bytes of Rav
and Rbv, storing the eight results in the low eight bits of Rc. The high 56 bits of Rc are set to
zero. Bit 0 of Rc corresponds to byte 0, bit 1 of Rc corresponds to byte 1, and so forth. A result
bit is set in Rc if the corresponding byte of Rav is greater than or equal to Rbv (unsigned).

Notes:
The result of CMPBGE can be used as an input to ZAP and ZAPNOT.
To scan for a byte of zeros in a character string:
<initialize R1 to aligned QW address of string>
LOOP:

LDQ
R2, 0(R1)
LDA
R1, 8(R1)
CMPBGE R31, R2,R3
BEQ
R3, LOOP
...

; Pick up 8 bytes
; Increment string pointer
; If NO bytes of zero, R3<7:0>=0
; Loop if no terminator byte found
; At this point, R3 can be used to
; determine which byte terminated

Instruction Descriptions 4–49

To compare two character strings for greater/equal/less:
<initialize R1 to aligned QW address of string1>
<initialize R2 to aligned QW address of string2>
LOOP:

LDQ
R3, 0(R1)
LDA
R1, 8(R1)
LDQ
R4, 0(R2)
LDA
R2, 8(R2)
CMPBGE R31, R3, R6
XOR
R3, R4, R5
BNE
R6, DONE
BEQ
R5, LOOP
DONE: CMPBGE R31, R5, R5

; Pick up 8 bytes of string1
; Increment string1 pointer
; Pick up 8 bytes of string2
; Increment string2 pointer
; Test for zeros in string1
; Test for all equal bytes
; Exit if a zero found
; Loop if all equal
;

...
; At this point, R5 can be used to determine the first not-equal
; byte position (if any), and R6 can be used to determine the
; position of the terminating zero in string1 (if any).

To range-check a string of characters in R1 for ‘0’…‘9’:
LDQ

R2, lit0s

LDQ

R3, lit9s

CMPBGE R2, R1, R4
CMPBGE R1, R3, R5
BNE
R4, ERROR
BNE
R5, ERROR

4–50 Alpha Architecture Handbook

; Pick up 8 bytes of the character
; BELOW ‘0’ ‘////////’
; Pick up 8 bytes of the character
; ABOVE ‘9’ ‘::::::::’
; Some R4=1 if character is LT ‘0’
; Some R5=1 if character is GT ‘9’
; Branch if some char too low
; Branch if some char too high

4.6.2 Extract Byte
Format:

EXTxx

Ra.rq,Rb.rq,Rc.wq

!Operate format

EXTxx

Ra.rq,#b.ib,Rc.wq

!Operate format

Operation:
CASE
big_endian_data: Rbv’ ← Rbv XOR 1112
little_endian_data: Rbv’ ← Rbv
ENDCASE
CASE
EXTBL: byte_mask ← 0000 00012
EXTWx: byte_mask ← 0000 00112
EXTLx: byte_mask ← 0000 11112
EXTQx: byte_mask ← 1111 11112
ENDCASE
CASE
EXTxL:
byte_loc ← Rbv’<2:0>*8
temp ← RIGHT_SHIFT(Rav, byte_loc<5:0>)
Rc ← BYTE_ZAP(temp, NOT(byte_mask) )
EXTxH:
byte_loc ← 64 - Rbv’<2:0>*8
temp ← LEFT_SHIFT(Rav, byte_loc<5:0>)
Rc ← BYTE_ZAP(temp, NOT(byte_mask) )
ENDCASE

Exceptions:
None

Instruction mnemonics:
EXTBL
EXTWL
EXTLL
EXTQL
EXTWH
EXTLH
EXTQH

Extract Byte Low
Extract Word Low
Extract Longword Low
Extract Quadword Low
Extract Word High
Extract Longword High
Extract Quadword High

Qualifiers:
None

Instruction Descriptions 4–51

Description:
EXTxL shifts register Ra right by 0 to 7 bytes, inserts zeros into vacated bit positions, and then
extracts 1, 2, 4, or 8 bytes into register Rc. EXTxH shifts register Ra left by 0 to 7 bytes,
inserts zeros into vacated bit positions, and then extracts 2, 4, or 8 bytes into register Rc. The
number of bytes to shift is specified by Rbv’<2:0>. The number of bytes to extract is specified in the function code. Remaining bytes are filled with zeros.

Notes:
The comments in the examples below assume that the effective address (ea) of X(R11) is such
that (ea mod 8) = 5), the value of the aligned quadword containing X(R11) is CBAx xxxx, and
the value of the aligned quadword containing X+7(R11) is yyyH GFED, and the datum is
little-endian.
The examples below are the most general case unless otherwise noted; if more information is
known about the value or intended alignment of X, shorter sequences can be used.
The intended sequence for loading a quadword from unaligned address X(R11) is:
LDQ_U
LDQ_U
LDA
EXTQL
EXTQH
OR

R1, X(R11)
R2, X+7(R11)
R3, X(R11)
R1, R3, R1
R2, R3, R2
R2, R1, R1

; Ignores va<2:0>, R1 = CBAx xxxx
; Ignores va<2:0>, R2 = yyyH GFED
; R3<2:0> = (X mod 8) = 5
; R1 = 0000 0CBA
; R2 = HGFE D000
; R1 = HGFE DCBA

The intended sequence for loading and zero-extending a longword from unaligned address X
is:
LDQ_U
LDQ_U
LDA
EXTLL
EXTLH
OR

R1, X(R11)
R2, X+3(R11)
R3, X(R11)
R1, R3, R1
R2, R3, R2
R2, R1, R1

; Ignores va<2:0>, R1 = CBAx xxxx
; Ignores va<2:0>, R2 = yyyy yyyD
; R3<2:0> = (X mod 8) = 5
; R1 = 0000 0CBA
; R2 = 0000 D000
; R1 = 0000 DCBA

The intended sequence for loading and sign-extending a longword from unaligned address X
is:
LDQ_U
LDQ_U
LDA
EXTLL
EXTLH
OR
ADDL

R1, X(R11)
R2, X+3(R11)
R3, X(R11)
R1, R3, R1
R2, R3, R2
R2, R1, R1
R31, R1, R1

4–52 Alpha Architecture Handbook

; Ignores va<2:0>, R1 = CBAx xxxx
; Ignores va<2:0>, R2 = yyyy yyyD
; R3<2:0> = (X mod 8) = 5
; R1 = 0000 0CBA
; R2 = 0000 D000
; R1 = 0000 DCBA
; R1 = ssss DCBA

For software that is not designed to use the BWX extension, the intended sequence for loading
and zero-extending a word from unaligned address X is:
LDQ_U
LDQ_U
LDA
EXTWL
EXTWH
OR

R1, X(R11)
R2, X+1(R11)
R3, X(R11)
R1, R3, R1
R2, R3, R2
R2, R1, R1

; Ignores va<2:0>, R1 = yBAx xxxx
; Ignores va<2:0>, R2 = yBAx xxxx
; R3<2:0> = (X mod 8) = 5
; R1 = 0000 00BA
; R2 = 0000 0000
; R1 = 0000 00BA

For software that is not designed to use the BWX extension, the intended sequence for loading
and sign-extending a word from unaligned address X is:
LDQ_U
LDQ_U
LDA
EXTQL
EXTQH
OR
SRA

R1, X(R11)
R2, X+1(R11)
R3, X+1+1(R11)
R1, R3, R1
R2, R3, R2
R2, R1, R1
R1, #48, R1

; Ignores va<2:0>, R1 = yBAx xxxx
; Ignores va<2:0>, R2 = yBAx xxxx
; R3<2:0> = 5+1+1 = 7
; R1 = 0000 000y
; R2 = BAxx xxx0
; R1 = BAxx xxxy
; R1 = ssss ssBA

For software that is not designed to use the BWX extension, the intended sequence for loading
and zero-extending a byte from address X is:
LDQ_U
LDA
EXTBL

R1, X(R11)
R3, X(R11)
R1, R3, R1

; Ignores va<2:0>, R1 = yyAx xxxx
; R3<2:0> = (X mod 8) = 5
; R1 = 0000 000A

For software that is not designed to use the BWX extension, the intended sequence for loading
and sign-extending a byte from address X is:
LDQ_U
LDA

R1, X(R11)
R3, X+1(R11)

EXTQH

R1, R3,

SRA

R1, #56, R1

; Ignores va<2:0>, R1 = yyAx xxxx
; R3<2:0> = (X + 1) mod 8, i.e.,
; convert byte position within
; quadword to one-origin based
; Places the desired byte into byte 7
; of R1.final by left shifting
; R1.initial by ( 8 - R3<2:0> ) byte
; positions
; Arithmetic Shift of byte 7 down
; into byte 0,

Optimized examples:
Assume that a word fetch is needed from 10(R3), where R3 is intended to contain a longword-aligned address. The optimized sequences below take advantage of the known constant
offset, and the longword alignment (hence a single aligned longword contains the entire word).
The sequences generate a Data Alignment Fault if R3 does not contain a longword-aligned
address.

Instruction Descriptions 4–53

For software that is not designed to use the BWX extension, the intended sequence for loading
and zero-extending an aligned word from 10(R3) is:
LDL

R1, 8(R3)

EXTWL

R1, #2, R1

; R1 = ssss BAxx
; Faults if R3 is not longword aligned
; R1 = 0000 00BA

For software that is not designed to use the BWX extension, the intended sequence for loading
and sign-extending an aligned word from 10(R3) is:
LDL

R1, 8(R3)

SRA

R1, #16, R1

; R1 = ssss BAxx
; Faults if R3 is not longword aligned
; R1 = ssss ssBA

Big-endian examples:
For software that is not designed to use the BWX extension, the intended sequence for loading
and zero-extending a byte from address X is:
LDQ_U R1, X(R11)
LDA
R3, X(R11)
EXTBL R1, R3, R1

; Ignores va<2:0>, R1 = xxxx xAyy
; R3<2:0> = 5, shift will be 2 bytes
; R1 = 0000 000A

The intended sequence for loading a quadword from unaligned address X(R11) is:
LDQ_U
LDQ_U
LDA
EXTQH
EXTQL
OR

R1, X(R11)
R2, X+7(R11)
R3, X+7(R11)
R1, R3, R1
R2, R3, R2
R1, R2, R1

; Ignores va<2:0>, R1 = xxxxxABC
; Ignores va<2:0>, R2 = DEFGHyyy
; R3<2:0> = 4, shift will be 3 bytes
; R1 = ABC0 0000
; R2 = 000D EFGH
; R1 = ABCD EFGH

Note that the address in the LDA instruction for big-endian quadwords is X+7, for longwords
is X+3, and for words is X+1; for little-endian, these are all just X. Also note that the EXTQH
and EXTQL instructions are reversed with respect to the little-endian sequence.

4–54 Alpha Architecture Handbook

4.6.3 Byte Insert
Format:
INSxx

Ra.rq,Rb.rq,Rc.wq

!Operate format

INSxx

Ra.rq,#b.ib,Rc.wq

!Operate format

Operation:
CASE
big_endian_data: Rbv’ ← Rbv XOR 1112
little_endian_data: Rbv’ ← Rbv
ENDCASE
CASE
INSBL: byte_mask ← 0000 0000 0000 00012
INSWx: byte_mask ← 0000 0000 0000 00112
INSLx: byte_mask ← 0000 0000 0000 11112
INSQx: byte_mask ← 0000 0000 1111 11112
ENDCASE
byte_mask ← LEFT_SHIFT(byte_mask, Rbv’<2:0>)
CASE
INSxL:
byte_loc ← Rbv’<2:0>*8
temp ← LEFT_SHIFT(Rav, byte_loc<5:0>)
Rc ← BYTE_ZAP(temp, NOT(byte_mask<7:0>))
INSxH:
byte_loc ← 64 - Rbv’<2:0>*8
temp ← RIGHT_SHIFT(Rav, byte_loc<5:0>)
Rc ← BYTE_ZAP(temp, NOT(byte_mask<15:8>))
ENDCASE

Exceptions:
None

Instruction mnemonics:
INSBL
INSWL
INSLL
INSQL
INSWH
INSLH
INSQH

Insert Byte Low
Insert Word Low
Insert Longword Low
Insert Quadword Low
Insert Word High
Insert Longword High
Insert Quadword High

Instruction Descriptions 4–55

Qualifiers:
None

Description:
INSxL and INSxH shift bytes from register Ra and insert them into a field of zeros, storing the
result in register Rc. Register Rbv’<2:0> selects the shift amount, and the function code
selects the maximum field width: 1, 2, 4, or 8 bytes. The instructions can generate a byte,
word, longword, or quadword datum that is spread across two registers at an arbitrary byte
alignment.

4–56 Alpha Architecture Handbook

4.6.4 Byte Mask
Format:
MSKxx

Ra.rq,Rb.rq,Rc.wq

!Operate format

MSKxx

Ra.rq,#b.ib,Rc.wq

!Operate format

Operation:
CASE
big_endian_data: Rbv’← Rbv XOR 1112
little_endian_data: Rbv’← Rbv
ENDCASE
CASE
MSKBL: byte_mask ← 0000 0000 0000 00012
MSKWx: byte_mask ← 0000 0000 0000 00112
MSKLx: byte_mask ← 0000 0000 0000 11112
MSKQx: byte_mask ← 0000 0000 1111 11112
ENDCASE
byte_mask ← LEFT_SHIFT(byte_mask, Rbv’<2:0>)
CASE
MSKxL:
Rc ← BYTE_ZAP(Rav, byte_mask<7:0>)
MSKxH:
Rc ← BYTE_ZAP(Rav, byte_mask<15:8>)
ENDCASE

Exceptions:
None

Instruction mnemonics:
MSKBL
MSKWL
MSKLL
MSKQL
MSKWH
MSKLH
MSKQH

Mask Byte Low
Mask Word Low
Mask Longword Low
Mask Quadword Low
Mask Word High
Mask Longword High
Mask Quadword High

Qualifiers:
None

Instruction Descriptions 4–57

Description:
MSKxL and MSKxH set selected bytes of register Ra to zero, storing the result in register Rc.
Register Rbv’<2:0> selects the starting position of the field of zero bytes, and the function
code selects the maximum width: 1, 2, 4, or 8 bytes. The instructions generate a byte, word,
longword, or quadword field of zeros that can spread across two registers at an arbitrary byte
alignment.

Notes:
The comments in the examples below assume that the effective address (ea) of X(R11) is such
that (ea mod 8) = 5, the value of the aligned quadword containing X(R11) is CBAx xxxx, the
value of the aligned quadword containing X+7(R11) is yyyH GFED, the value to be stored
from R5 is HGFE DCBA, and the datum is little-endian. Slight modifications similar to those
in Section 4.6.2 apply to big-endian data.
The examples below are the most general case; if more information is known about the value
or intended alignment of X, shorter sequences can be used.
The intended sequence for storing an unaligned quadword R5 at address X(R11) is:
LDA
LDQ_U
LDQ_U
INSQH
INSQL
MSKQH
MSKQL
OR
OR
STQ_U
STQ_U

R6, X(R11)
R2, X+7(R11)
R1, X(R11)
R5, R6, R4
R5, R6, R3
R2, R6, R2
R1, R6, R1
R2, R4, R2
R1, R3, R1
R2, X+7(R11)
R1, X(R11)

; R6<2:0> = (X mod 8) = 5
; Ignores va<2:0>, R2 = yyyH GFED
; Ignores va<2:0>, R1 = CBAx xxxx
; R4 = 000H GFED
; R3 = CBA0 0000
; R2 = yyy0 0000
; R1 = 000x xxxx
; R2 = yyyH GFED
; R1 = CBAx xxxx
; Must store high then low for
; degenerate case of aligned QW

The intended sequence for storing an unaligned longword R5 at X is:
LDA
LDQ_U
LDQ_U
INSLH
INSLL
MSKLH
MSKLL
OR
OR
STQ_U
STQ_U

R6, X(R11)
R2, X+3(R11)
R1, X(R11)
R5, R6, R4
R5, R6, R3
R2, R6, R2
R1, R6, R1
R2, R4, R2
R1, R3, R1
R2, X+3(R11)
R1, X(R11)

4–58 Alpha Architecture Handbook

; R6<2:0> = (X mod 8) = 5
; Ignores va<2:0>, R2 = yyyy yyyD
; Ignores va<2:0>, R1 = CBAx xxxx
; R4 = 0000 000D
; R3 = CBA0 0000
; R2 = yyyy yyy0
; R1 = 000x xxxx
; R2 = yyyy yyyD
; R1 = CBAx xxxx
; Must store high then low for
; degenerate case of aligned

For software that is not designed to use the BWX extension, the intended sequence for storing
an unaligned word R5 at X is:
LDA
LDQ_U
LDQ_U
INSWH
INSWL
MSKWH
MSKWL
OR
OR
STQ_U
STQ_U

R6, X(R11)
R2, X+1(R11)
R1, X(R11)
R5, R6, R4
R5, R6, R3
R2, R6, R2
R1, R6, R1
R2, R4, R2
R1, R3, R1
R2, X+1(R11)
R1, X(R11)

; R6<2:0> = (X mod 8) = 5
; Ignores va<2:0>, R2 = yBAx xxxx
; Ignores va<2:0>, R1 = yBAx xxxx
; R4 = 0000 0000
; R3 = 0BA0 0000
; R2 = yBAx xxxx
; R1 = y00x xxxx
; R2 = yBAx xxxx
; R1 = yBAx xxxx
; Must store high then low for
; degenerate case of aligned

For software that is not designed to use the BWX extension, the intended sequence for storing
a byte R5 at X is:
LDA
LDQ_U
INSBL
MSKBL
OR
STQ_U

R6, X(R11)
R1, X(R11)
R5, R6, R3
R1, R6, R1
R1, R3, R1
R1, X(R11)

; R6<2:0> = (X mod 8) = 5
; Ignores va<2:0>, R1 = yyAx xxxx
; R3 = 00A0 0000
; R1 = yy0x xxxx
; R1 = yyAx xxxx
;

Instruction Descriptions 4–59

4.6.5 Sign Extend
Format:
SEXTx

Rb.rq,Rc.wq

!Operate format

SEXTx

#b.ib,Rc.wq

!Operate format

Operation:
CASE
SEXTB:
SEXTW:
ENDCASE

Rc ← SEXT(Rbv<07:0>)
Rc ← SEXT(Rbv<15:0>)

Exceptions:
None

Instruction mnemonics:
SEXTB
SEXTW

Sign Extend Byte
Sign Extend Word

Qualifiers:
None

Description:
The byte or word in register Rb is sign-extended to 64 bits and written to register Rc. Ra must
be R31.

Implementation Note:
The SEXTB and SEXTW instructions are supported in hardware on Alpha
implementations for which the AMASK instruction returns bit 0 set. SEXTB and SEXTW
are supported with software emulation in Alpha implementations for which AMASK does
not return bit 0 set. Software emulation of SEXTB and SEXTW is significantly slower
than hardware support.

4–60 Alpha Architecture Handbook

4.6.6 Zero Bytes
Format:
ZAPx

Ra.rq,Rb.rq,Rc.wq

!Operate format

ZAPx

Ra.rq,#b.ib,Rc.wq

!Operate format

Operation:
CASE
ZAP:
Rc ← BYTE_ZAP(Rav, Rbv<7:0>)
ZAPNOT:
Rc ← BYTE_ZAP(Rav, NOT Rbv<7:0>)
ENDCASE

Exceptions:
None

Instruction mnemonics:
ZAP
ZAPNOT

Zero Bytes
Zero Bytes Not

Qualifiers:
None

Description:
ZAP and ZAPNOT set selected bytes of register Ra to zero and store the result in register Rc.
Register Rb<7:0> selects the bytes to be zeroed. Bit 0 of Rbv corresponds to byte 0, bit 1 of
Rbv corresponds to byte 1, and so on. A result byte is set to zero if the corresponding bit of
Rbv is a one for ZAP and a zero for ZAPNOT.

Instruction Descriptions 4–61

4.7 Floating-Point Instructions
Alpha provides instructions for operating on floating-point operands in each of four data
formats:

•

F_floating (VAX single)

•

G_floating (VAX double, 11-bit exponent)

•

S_floating (IEEE single)

•

T_floating (IEEE double, 11-bit exponent)

Data conversion instructions are also provided to convert operands between floating-point and
quadword integer formats, between double and single floating, and between quadword and
longword integers.

Note:
D_floating is a partially supported datatype; no D_floating arithmetic operations are
provided in the architecture. For backward compatibility, exact D_floating arithmetic may
be provided via software emulation. D_floating "format compatibility," in which binary
files of D_floating numbers may be processed but without the last 3 bits of fraction
precision, can be obtained via conversions to G_floating, G arithmetic operations, then
conversion back to D_floating.
The choice of data formats is encoded in each instruction. Each instruction also encodes the
choice of rounding mode and the choice of trapping mode.
All floating-point operate instructions (not including loads or stores) that yield an F_floating or
G_floating zero result must materialize a true zero.

4.7.1 Single-Precision Operations
Single-precision values (F_floating or S_floating) are stored in the floating-point registers in
canonical form, as subsets of double-precision values, with 11-bit exponents restricted to the
corresponding single-precision range, and with the 29 low-order fraction bits restricted to be all
zero.
Single-precision operations applied to canonical single-precision values give single-precision
results. Single-precision operations applied to non-canonical operands give UNPREDICTABLE results.
Longword integer values in floating-point registers are stored in bits <63:62,58:29>, with bits
<61:59> ignored and zeros in bits <28:0>.

4.7.2 Subsets and Faults
All floating-point operations may take floating disabled faults. Any subsetted floating-point
instruction may take an Illegal Instruction Trap. These faults are not explicitly listed in the
description of each instruction.

4–62 Alpha Architecture Handbook

All floating-point loads and stores may take memory management faults (access control violation, translation not valid, fault on read/write, data alignment).
The floating-point enable (FEN) internal processor register (IPR) allows system software to
restrict access to the floating-point registers.
If a floating-point instruction is implemented and FEN = 0, attempts to execute the instruction
cause a floating disabled fault.
If a floating-point instruction is not implemented, attempts to execute the instruction cause an
Illegal Instruction Trap. This rule holds regardless of the value of FEN.
An Alpha implementation may provide both VAX and IEEE floating-point operations, either,
or none.
Some floating-point instructions are common to the VAX and IEEE subsets, some are VAX
only, and some are IEEE only. These are designated in the descriptions that follow. If either
subset is implemented, all the common instructions must be implemented.
An implementation that includes IEEE floating-point may subset the ability to perform rounding to plus infinity and minus infinity. If not implemented, instructions requesting these
rounding modes take Illegal Instruction Trap.
An implementation that includes IEEE floating-point may implement any subset of the Trap
Disable flags (DNOD, DZED, INED, INVD, OVFD, and UNFD) and Denormal Control flags
(DNZ and UNDZ) in the FPCR:

•

If a Trap Disable flag is not implemented, then the corresponding trap occurs as usual.

•

If DNZ is not implemented, then any IEEE operation with a denormal input must take
an Invalid Operation Trap.

•

If UNDZ is not implemented, then any IEEE operation that includes a /S qualifier that
underflows must take an Underflow Trap.

•

If DZED is implemented, then IEEE division of 0/0 must be treated as an invalid operation instead of a division by zero.

Any unimplemented bits in the FPCR are read as zero and ignored when set.

4.7.3 Definitions
The following definitions apply to Alpha floating-point support.

Alpha finite number
A floating-point number with a definite, in-range value. Specifically, all numbers in the inclusive ranges –MAX through –MIN, zero, and +MIN through +MAX, where MAX is the largest
non-infinite representable floating-point number and MIN is the smallest non-zero representable normalized floating-point number.

Instruction Descriptions 4–63

For VAX floating-point, finites do not include reserved operands or dirty zeros (this differs
from the usual VAX interpretation of dirty zeros as finite). For IEEE floating-point, finites do
not include infinites, NaNs, or denormals, but do include minus zero.

denormal
An IEEE floating-point bit pattern that represents a number whose magnitude lies between
zero and the smallest finite number.

dirty zero
A VAX floating-point bit pattern that represents a zero value, but not in true-zero form.

infinity
An IEEE floating-point bit pattern that represents plus or minus infinity.

LSB
The least significant bit. For a positive finite representable number A, A + 1 LSB is the next
larger representative number, and A + ½ LSB is exactly halfway between A and the next larger
representable number. For a positive representable number A whose fraction field is not all
zeros, A – 1 LSB is the next smaller representable number, and A – ½ LSB is exactly halfway
between A and the next smaller representable number.

non-finite number
An IEEE infinity, NaN, denormal number, or a VAX dirty zero or reserved operand.

Not-a-Number
An IEEE floating-point bit pattern that represents something other than a number. This comes
in two forms: signaling NaNs (for Alpha, those with an initial fraction bit of 0) and quiet NaNs
(for Alpha , those with initial fraction bit of 1).

representable result
A real number that can be represented exactly as a VAX or IEEE floating-point number, with
finite precision and bounded exponent range.

reserved operand
A VAX floating-point bit pattern that represents an illegal value.

trap shadow
The set of instructions potentially executed after an instruction that signals an arithmetic trap
but before the trap is actually taken.

true result
The mathematically correct result of an operation, assuming that the input operand values are
exact. The true result is typically rounded to the nearest representable result.

4–64 Alpha Architecture Handbook

true zero
The value +0, represented as exactly 64 zeros in a floating-point register.

4.7.4 Encodings
Floating-point numbers are represented with three fields: sign, exponent, and fraction. The sign
is 1 bit; the exponent is 8, 11, or 15 bits; and the fraction is 23, 52, 55, or 112 bits. Some
encodings represent special values:

Sign

Exponent

Fraction

Vax
Meaning

VAX
Finite

IEEE
Meaning

IEEE
Finite

All-1’s

Non-zero

Finite

Yes

+/–NaN

All-1’s

Finite

Yes

+/–Infinity

Non-zero

Dirty zero

+Denormal

Non-zero

Resv. operand

–Denormal

True zero

Yes

Resv. operand

–0

Yes

Other

Finite

Yes

finite

Yes

The values of MIN and MAX for each of the five floating-point data formats are:

Data
Format

MIN

MAX

F_floating

2**–127 * 0.5
(0.293873588e–38)

2**127 *(1.0 – 2**–24)
(1.7014117e38)

G_floating

2**–1023 * 0.5
(0.5562684646268004e–308)

2**1023 * (1.0 – 2**–53)
(0.89884656743115785407e308)

S_floating

2**–126 * 1.0
(1.17549435e–38)

2**127 * (2.0 – 2**–23)
(3.40282347e38)

T_floating

2**–1022 * 1.0
(2.2250738585072013e–308)

2**1023 * (2.0 – 2**–52)
(1.7976931348623158e308)

X_floating

2**–16382*1.0

2**16383*(2.0–2**–112)

(See below †)

(See below‡)

†
‡

(1.18973149535723176508575932662800702e4932)
(3.36210314311209350626267781732175260e–4932)

Instruction Descriptions 4–65

4.7.5 Rounding Modes
All rounding modes map a true result that is exactly representable to that representable value.

VAX Rounding Modes
For VAX floating-point operations, two rounding modes are provided and are specified in each
instruction: normal (biased) rounding and chopped rounding.
Normal VAX rounding maps the true result to the nearest of two representable results, with
true results exactly halfway between mapped to the larger in absolute value (sometimes called
biased rounding away from zero); maps true results ≥ MAX + 1/2 LSB in magnitude to an
overflow; maps true results < MIN – 1/4 LSB in magnitude to an underflow.
Chopped VAX rounding maps the true result to the smaller in magnitude of two surrounding
representable results; maps true results ≥ MAX + 1 LSB in magnitude to an overflow; maps
true results < MIN in magnitude to an underflow.

IEEE Rounding Modes
For IEEE floating-point operations, four rounding modes are provided: normal rounding (unbiased round to nearest), rounding toward minus infinity, round toward zero, and rounding
toward plus infinity. The first three can be specified in the instruction. Rounding toward plus
infinity can be obtained by setting the Floating-point Control Register (FPCR) to select it and
then specifying dynamic rounding mode in the instruction (See Section 4.7.8). Alpha IEEE
arithmetic does rounding before detecting overflow/underflow.
Normal IEEE rounding maps the true result to the nearest of two representable results, with
true results exactly halfway between mapped to the one whose fraction ends in 0 (sometimes
called unbiased rounding to even); maps true results ≥ MAX + 1/2 LSB in magnitude to an
overflow; maps true results < MIN – 1/2 LSB in magnitude to an underflow.
Plus infinity IEEE rounding maps the true result to the larger of two surrounding representable
results; maps true results > MAX in magnitude to an overflow; maps positive true results
≤ +MIN – 1 LSB to an underflow; and maps negative true results > –MIN to an underflow.
Minus infinity IEEE rounding maps the true result to the smaller of two surrounding representable results; maps true results > MAX in magnitude to an overflow; maps positive true results
< +MIN to an underflow; and maps negative true results ≥ –MIN + 1 LSB to an underflow.
Chopped IEEE rounding maps the true result to the smaller in magnitude of two surrounding
representable results; maps true results ≥ MAX + 1 LSB in magnitude to an overflow; and
maps non-zero true results < MIN in magnitude to an underflow.
Dynamic rounding mode uses the IEEE rounding mode selected by the FPCR register and is
described in more detail in Section 4.7.8.

4–66 Alpha Architecture Handbook

The following tables summarize the floating-point rounding modes:

VAX Rounding Mode

Instruction Notation

Normal rounding

(No qualifier)

Chopped

IEEE Rounding Mode

Instruction Notation

Normal rounding

(No qualifier)

Dynamic rounding

Plus infinity

/D and ensure that FPCR<DYN> = ‘11’

Minus infinity

Chopped

4.7.6 Computational Models
The Alpha architecture provides a choice of floating-point computational models.
There are two computational models available on systems that implement the VAX floating-point subset:

•

VAX-format arithmetic with precise exceptions

•

High-performance VAX-format arithmetic

There are three computational models available on systems that implement the IEEE floating-point subset:

•

IEEE compliant arithmetic

•

IEEE compliant arithmetic without inexact exception

•

High-performance IEEE-format arithmetic

4.7.6.1 VAX-Format Arithmetic with Precise Exceptions
This model provides floating-point arithmetic that is fully compatible with the floating-point
arithmetic provided by the VAX architecture. It provides support for VAX non-finites and
gives precise exceptions.
This model is implemented by using VAX floating-point instructions with the /S, /SU, and /SV
trap qualifiers. Each instruction can determine whether it also takes an exception on underflow
or integer overflow. The performance of this model depends on how often computations
involve non-finite operands. Performance also depends on how an Alpha system chooses to
trade off implementation complexity between hardware and operating system completion handlers (see Section 4.7.7.3).

Instruction Descriptions 4–67

4.7.6.2 High-Performance VAX-Format Arithmetic
This model provides arithmetic operations on VAX finite numbers. An imprecise arithmetic
trap is generated by any operation that involves non-finite numbers, floating overflow, and
divide-by-zero exceptions.
This model is implemented by using VAX floating-point instructions with a trap qualifier other
than /S, /SU, or /SV. Each instruction can determine whether it also traps on underflow or integer overflow. This model does not require the overhead of an operating system completion
handler and can be the faster of the two VAX models.

4.7.6.3 IEEE-Compliant Arithmetic
This model provides floating-point arithmetic that fully complies with the IEEE Standard for
Binary Floating-Point Arithmetic. It provides all of the exception status flags that are in the
standard. It provides a default where all traps and faults are disabled and where IEEE
non-finite values are used in lieu of exceptions.
Alpha operating systems provide additional mechanisms that allow the user to specify dynamically which exception conditions should trap and which should proceed without trapping. The
operating systems also include mechanisms that allow alternative handling of denormal values. See Appendix B and the appropriate operating system documentation for a description of
these mechanisms.
This model is implemented by using IEEE floating-point instructions with the /SUI
or /SVI trap qualifiers. The performance of this model depends on how often computations
involve inexact results and non-finite operands and results. Performance also depends on how
the Alpha system chooses to trade off implementation complexity between hardware and operating system completion handlers (see Section 4.7.7.3). This model provides acceptable
performance on Alpha systems that implement the inexact disable (INED) bit in the FPCR.
Performance may be slow if the INED bit is not implemented.

4.7.6.4 IEEE-Compliant Arithmetic Without Inexact Exception
This model is similar to the model in Section 4.7.6.3, except this model does not signal inexact
results either by the inexact status flag or by trapping. Combining routines that are compiled
with this model and routines that are compiled with the model in Section 4.7.6.3 can give an
application better control over testing when an inexact operation will affect computational
accuracy.
This model is implemented by using IEEE floating-point instructions with the /SU or /SV trap
qualifiers. The performance of this model depends on how often computations involve
non-finite operands and results. Performance also depends on how an Alpha system chooses to
trade off implementation complexity between hardware and operating system completion handlers (see Section 4.7.7.3).

4–68 Alpha Architecture Handbook

4.7.6.5 High-Performance IEEE-Format Arithmetic
This model provides arithmetic operations on IEEE finite numbers and notifies applications of
all exceptional floating-point operations. An imprecise arithmetic trap is generated by any
operation that involves non-finite numbers, floating overflow, divide-by-zero, and invalid
operations. Underflow results are set to zero. Conversion to integer results that overflow are set
to the low-order bits of the integer value.
This model is implemented by using IEEE floating-point instructions with a trap qualifier other
than /SU, /SV, /SUI, or /SVI. Each instruction can determine whether it also traps on underflow or integer overflow. This model does not require the overhead of an operating system
completion handler and can be the fastest of the three IEEE models.

4.7.7 Trapping Modes
There are six exceptions that can be generated by floating-point operate instructions, all signaled by an arithmetic exception trap. These exceptions are:

•

Invalid operation

•

Division by zero

•

Overflow

•

Underflow

•

Inexact result

•

Integer overflow (conversion to integer only)

4.7.7.1 VAX Trapping Modes
This section describes the characteristics of the four VAX trapping modes, which are summarized in Table 4–8.
When no trap mode is specified (the default):

•
•

•
•
•
•

Arithmetic is performed on VAX finite numbers.
Operations give imprecise traps whenever the following occur:
– an operand is a non-finite number
– a floating overflow
– a divide-by-zero
Traps are imprecise and it is not always possible to determine which instruction triggered a trap or the operands of that instruction.
An underflow produces a zero result without trapping.
A conversion to integer that overflows uses the low-order bits of the integer as the
result without trapping.
The result of any operation that traps is UNPREDICTABLE.

Instruction Descriptions 4–69

When /U or /V mode is specified:

•
•

•
•
•
•

Arithmetic is performed on VAX finite numbers.
Operations give imprecise traps whenever the following occur:
– an operand is a non-finite number
– an underflow
– an integer overflow
– a floating overflow
– a divide-by-zero
Traps are imprecise and it is not always possible to determine which instruction triggered a trap or the operands of that instruction.
An underflow trap produces a zero result.
A conversion to integer trapping with an integer overflow produces the low-order bits
of the integer value.
The result of any other operation that traps is UNPREDICTABLE.

When /S mode is specified:

•
•
•

•
•
•
•

Arithmetic is performed on all VAX values, both finite and non-finite.
A VAX dirty zero is treated as zero.
Exceptions are signaled for:
– a VAX reserved operand, which generates an invalid operation exception
– a floating overflow
– a divide-by-zero
Exceptions are precise and an application can locate the instruction that caused the
exception, along with its operand values. See Section 4.7.7.3.
An operation that underflows produces a zero result without taking an exception.
A conversion to integer that overflows uses the low-order bits of the integer as the
result, without taking an exception.
When an operation takes an exception, the result of the operation is UNPREDICTABLE.

When /SU or /SV mode is specified:

•
•
•

•
•
•
•

Arithmetic is performed on all VAX values, both finite and non-finite.
A VAX dirty zero is treated as zero.
Exceptions are signaled for:
– a VAX reserved operand, which generates an invalid operation exception
– an underflow
– an integer overflow
– a floating overflow
– a divide-by-zero
Exceptions are precise and an application can locate the instruction that caused the
exception, along with its operand values. See Section 4.7.7.3.
An underflow exception produces a zero.
A conversion to integer exception with integer overflow produces the low-order bits of
the integer value.
The result of any other operation that takes an exception is UNPREDICTABLE.

4–70 Alpha Architecture Handbook

A summary of the VAX trapping modes, instruction notation, and their meaning follows in
Table 4–8:

Table 4–8: VAX Trapping Modes Summary
Trap Mode

Notation

Meaning

Underflow disabled

No qualifier
/S

Imprecise
Precise exception completion

Underflow enabled

/U
/SU

Imprecise
Precise exception completion

Integer overflow disabled

No qualifier
/S

Imprecise
Precise exception completion

Integer overflow enabled

/V
/SV

Imprecise
Precise exception completion

4.7.7.2 IEEE Trapping Modes
This section describes the characteristics of the four IEEE trapping modes, which are summarized in Table 4–9.
When no trap mode is specified (the default):

•
•

•
•
•
•

Arithmetic is performed on IEEE finite numbers.
Operations give imprecise traps whenever the following occur:
– an operand is a non-finite number
– a floating overflow
– a divide-by-zero
– an invalid operation
Traps are imprecise, and it is not always possible to determine which instruction triggered a trap or the operands of that instruction.
An underflow produces a zero result without trapping.
A conversion to integer that overflows uses the low-order bits of the integer as the
result without trapping.
When an operation traps, the result of the operation is UNPREDICTABLE.

When /U or /V mode is specified :

•
•

Arithmetic is performed on IEEE finite numbers.
Operations give imprecise traps whenever the following occur:
– an operand is a non-finite number
– an underflow
– an integer overflow
– a floating overflow
– a divide-by-zero
– an invalid operation
Instruction Descriptions 4–71

•
•
•
•

Traps are imprecise, and it is not always possible to determine which instruction triggered a trap or the operands of that instruction.
An underflow trap produces a zero.
A conversion to integer trap with an integer overflow produces the low-order bits of the
integer.
The result of any other operation that traps is UNPREDICTABLE.

When /SU or /SV mode is specified:

•
•

•

Arithmetic is performed on all IEEE values, both finite and non-finite.
Alpha systems support all IEEE features except inexact exception (which requires /SUI
or /SVI):
– The IEEE standard specifies a default where exceptions do not fault or trap.In combination with the FPCR, this mode allows disabling exceptions and producing
IEEE compliant nontrapping results. See Sections 4.7.7.10 and 4.7.7.11.
– Each Alpha operating system provides a way to optionally signal IEEE floatingpoint exceptions. This mode enables the IEEE status flags that keep a record of
each exception that is encountered. An Alpha operating system uses the IEEE floating-point control (FP_C) quadword, described in Section B.2.1, to maintain the
IEEE status flags and to enable calls to IEEE user signal handlers.
Exceptions signaled in this mode are precise and an application can locate the instruction that caused the exception, along with its operand values. See Section 4.7.7.3.

When /SUI or /SVI mode is specified:

•
•

Arithmetic is performed on all IEEE values, both finite and non-finite.
Inexact exceptions are supported, along with all the other IEEE features supported by
the /SU or /SV mode.

A summary of the IEEE trapping modes, instruction notation, and their meaning follows in
Table 4–9:

Table 4–9: Summary of IEEE Trapping Modes
Trap Mode

Notation

Meaning

Underflow disabled and
inexact disabled

No qualifier

Imprecise

Underflow enabled and
inexact disabled

/U
/SU

Imprecise
Precise exception completion

Underflow enabled and
inexact enabled

/SUI

Precise exception completion

Integer overflow disabled and
inexact disabled

No qualifier

Imprecise

4–72 Alpha Architecture Handbook

Table 4–9: Summary of IEEE Trapping Modes (Continued)
Trap Mode

Notation

Meaning

Integer overflow enabled and
inexact disabled

/V
/SV

Imprecise
Precise exception completion

Integer overflow enabled and
inexact enabled

/SVI

Precise exception completion

4.7.7.3 Arithmetic Trap Completion
Because floating-point instructions may be pipelined, the trap PC can be an arbitrary number
of instructions past the one triggering the trap. Those instructions that are executed after the
trigger instruction of an arithmetic trap are collectively referred to as the trap shadow of the
trigger instruction.
Marking floating-point instructions for exception completion with any valid qualifier combination that includes the /S qualifier enables the completion of the triggering instruction. For any
instruction so marked, the output register for the triggering instruction cannot also be one of
the input registers, so that an input register cannot be overwritten and the input value is available after a trap occurs.
See Section B.2 for more information.
The AMASK instruction reports how the arithmetic trap should be completed:

•

If AMASK returns with bit 9 clear, floating-point traps are imprecise. Exception completion requires that generated code must obey the trap shadow rules in Section
4.7.7.3.1, with a trap shadow length as described in Section 4.7.7.3.2.

•

If AMASK returns with bit 9 set, the hardware implements precise floating-point traps.
If the instruction has any valid qualifier combination that includes /S, the trap PC points
to the instruction that immediately follows the instruction that triggered the trap. The
trap shadow contains zero instructions; exception completion does not require that the
generated code follow the conditions in Section 4.7.7.3.1 and the length rules in Section
4.7.7.3.2.

4.7.7.3.1 Trap Shadow Rules
For an operating system (OS) completion handler to complete non-finite operands and exceptions, the following conditions must hold.
Conditions 1 and 2, below, allow an OS completion handler to locate the trigger instruction by
doing a linear scan backwards from the trap PC while comparing destination registers in the
trap shadow with the registers that are specified in the register write mask parameter to the
arithmetic trap.

Instruction Descriptions 4–73

Condition 3 allows an OS completion handler to emulate the trigger instruction with its original input operand values.
Condition 4 allows the handler to re-execute instructions in the trap shadow with their original
operand values.
Condition 5 prevents any unusual side effects that would cause problems on repeated execution of the instructions in the trap shadow.
Conditions:

1. The destination register of the trigger instruction may not be used as the destination register of any instruction in the trap shadow.
2. The trap shadow may not include any branch or jump instructions.
3. An instruction in the trap shadow may not modify an input to the trigger instruction.
4. The value in a register or memory location that is used as input to some instruction in
the trap shadow may not be modified by a subsequent instruction in the trap shadow
unless that value is produced by an earlier instruction in the trap shadow.
5. The trap shadow may not contain any instructions with side effects that interact with
earlier instructions in the trap shadow or with other parts of the system. Examples of
operations with prohibited side effects are:
–

Modifications of the stack pointer or frame pointer that can change the accessibility
of stack variables and the exception context that is used by earlier instructions in
the trap shadow.

–

Modifications of volatile values and access to I/O device registers.

–

If order of exception reporting is important, taking an arithmetic trap by an integer
instruction or by a floating-point instruction that does not include a /S qualifier,
either of which can report exceptions out of order.

An instruction may be in the trap shadows of multiple instructions that include a /S qualifier.
That instruction must obey all conditions for all those trap shadows. For example, the destination register of an instruction in multiple trap shadows must be different than the destination
registers of each possible trigger instruction.
4.7.7.3.2 Trap Shadow Length Rules
The trap shadow length rules in Table 4–10 apply only to those floating-point instructions with
any valid qualifier combination that includes a /S trap qualifier. Further, the instruction to
which the trap shadow extends is not part of the trap shadow and that instruction is not executed prior to the arithmetic trap that is signaled by the trigger instruction.
Implementation notes:

•

On Alpha implementations for which the IMPLVER instruction returns the value 0, the
trap shadow of an instruction may extend after the result is consumed by a floating-point STx instruction. On all other implementations, the trap shadow ends when a
result is consumed.

•

Because Alpha implementations need not execute instructions that have R31 or F31 as
the destination operand, instructions with such an destination should not be thought to
end a trap shadow.

4–74 Alpha Architecture Handbook

Table 4–10: Trap Shadow Length Rules
Floating-Point
Instruction Group

Trap Shadow Extends Until Any of the Following
Occurs:

Floating-point operate
(except DIVx and SQRTx)

•

Encountering a CALL_PAL, EXCB, or TRAPB
instruction.

•

The result is consumed by any instruction except
floating-point STx.

•

The fourth instruction† after the result is consumed by
a floating-point STx instruction.
Or, following the floating-point STx of the result, the
result of a LDx that loads the stored value is
consumed by any instruction.

•

The result of a subsequent floating-point operate
instruction is consumed by any instruction except
floating-point STx.

•

The second instruction† after the result of a subsequent floating-point operate instruction is consumed
by a floating-point STx instruction.

•

The result of a subsequent floating-point DIVx or
SQRTx instruction is consumed by any instruction.

•

Encountering a CALL_PAL, EXCB, or TRAPB
instruction.

•

The result is consumed by any instruction except
floating-point STx.

•

The fourth instruction† after the result is consumed by
a floating-point STx instruction.

Floating-point DIVx

Or, following the floating-point STx of the result, the
result of a LDx that loads the stored value is
consumed by any instruction.

•

The result of a subsequent floating-point DIVx is consumed by any instruction.

Instruction Descriptions 4–75

Table 4–10: Trap Shadow Length Rules (Continued)
Floating-Point
Instruction Group

Trap Shadow Extends Until Any of the Following
Occurs:

Floating-point SQRTx

†

•

Encountering a CALL_PAL, EXCB, or TRAPB
instruction.

•

The result is consumed by any instruction.

•

The result of a subsequent SQRTx instruction is consumed by any instruction.

The length of four instructions is a conservative estimate of how far the trap shadow may
extend past a consuming floating-point STx instruction. The length of two instructions is a
conservative estimate of how far the trap shadow may extend after a subsequent floating-point operate instruction is consumed by a floating-point STx instruction. Compilers can
make a more precise estimate by consulting the DECchip 21064 and DECchip 21064A
Alpha AXP Microprocessors Hardware Reference Manual, EC-QD2RA-TE.

4.7.7.4 Invalid Operation (INV) Arithmetic Trap
An invalid operation arithmetic trap is signaled if an operand is a non-finite number or if an
operand is invalid for the operation to be performed. (Note that CMPTxy does not trap on plus
or minus infinity.) Invalid operations are:

•

Any operation on a signaling NaN.

•

Addition of unlike-signed infinities or subtraction of like-signed infinities, such as
(+infinity + –infinity) or (+infinity – +infinity).

•

Multiplication of 0∗infinity.

•

IEEE division of 0/0 or infinity/infinity.

•

Conversion of an infinity or NaN to an integer.

•

CMPTLE or CMPTLT when either operand is a NaN.

•

SQRTx of a negative non-zero number.

The instruction cannot disable the trap and, if the trap occurs, an UNPREDICTABLE value is
stored in the result register. However, under some conditions, the FPCR can dynamically disable the trap, as described in Section 4.7.7.10, producing a correct IEEE result, as described in
Section 4.7.10.
IEEE-compliant system software must also supply an invalid operation indication to the user
for x REM 0 and for conversions to integer that take an integer overflow trap.
If an implementation does not support the DZED (division by zero disable) bit, it may respond
to the IEEE division of 0/0 by delivering a division by zero trap to the operating system, which
IEEE compliant software must change to an invalid operation trap for the user.

4–76 Alpha Architecture Handbook

An implementation may choose not to take an INV trap for a valid IEEE operation that
involves denormal operands if:

•

The instruction is modified by any valid qualifier combination that includes the /S
(exception completion) qualifier.

•

The implementation supports the DNZ (denormal operands to zero) bit and DNZ is set.

•

The instruction produces the result and exceptions required by Section 4.7.10, as modified by the DNZ bit described in Section 4.7.7.11.

An implementation may choose not to take an INV trap for a valid IEEE operation that
involves denormal operands, and direct hardware implementation of denormal arithmetic is
permitted if:

•

The instruction is modified by any valid qualifier combination that includes the /S
(exception completion) qualifier.

•

The implementation supports both the DNOD (denormal operand exception disable) bit
and the DNZ (denormal operands to zero) bit and DNOD is set while DNZ is clear.

•

The instruction produces the result and exceptions required by Section 4.7.10, possibly
modified by the UDNZ bit described in Section 4.7.7.11.

Regardless of the setting of the INVD (invalid operation disable) bit, the implementation may
choose not to trap on valid operations that involve quiet NaNs and infinities as operands for
IEEE instructions that are modified by any valid qualifier combination that includes the /S
(exception completion) qualifier.

4.7.7.5 Division by Zero (DZE) Arithmetic Trap
A division by zero arithmetic trap is taken if the numerator does not cause an invalid operation
trap and the denominator is zero.
The instruction cannot disable the trap and, if the trap occurs, an UNPREDICTABLE value is
stored in the result register. However, under some conditions, the FPCR can dynamically disable the trap, as described in Section 4.7.7.10, producing a correct IEEE result, as described in
Section 4.7.10.
If an implementation does not support the DZED (division by zero disable) bit, it may respond
to the IEEE division of 0/0 by delivering a division by zero trap to the operating system, which
IEEE compliant software must change to an invalid operation trap for the user.

4.7.7.6 Overflow (OVF) Arithmetic Trap
An overflow arithmetic trap is signaled if the rounded result exceeds in magnitude the largest
finite number of the destination format.
The instruction cannot disable the trap and, if the trap occurs, an UNPREDICTABLE value is
stored in the result register. However, under some conditions, the FPCR can dynamically disable the trap, as described in Section 4.7.7.10, producing a correct IEEE result, as described in
Section 4.7.10.

Instruction Descriptions 4–77

4.7.7.7 Underflow (UNF) Arithmetic Trap
An underflow occurs if the rounded result is smaller in magnitude than the smallest finite number of the destination format.
If an underflow occurs, a true zero (64 bits of zero) is always stored in the result register. In the
case of an IEEE operation that takes an underflow arithmetic trap, a true zero is stored even if
the result after rounding would have been –0 (underflow below the negative denormal range).
If an underflow occurs and underflow traps are enabled by the instruction, an underflow arithmetic trap is signaled. However, under some conditions, the FPCR can dynamically disable the
trap, as described in Section 4.7.7.10, producing the result described in Section 4.7.10, as modified by the UNDZ bit described in Section 4.7.7.11.

4.7.7.8 Inexact Result (INE) Arithmetic Trap
An inexact result occurs if the infinitely precise result differs from the rounded result.
If an inexact result occurs, the normal rounded result is still stored in the result register. If an
inexact result occurs and inexact result traps are enabled by the instruction, an inexact result
arithmetic trap is signaled. However, under some conditions, the FPCR can dynamically disable the trap; see Section 4.7.7.10 for information.

4.7.7.9 Integer Overflow (IOV) Arithmetic Trap
In conversions from floating to quadword integer, an integer overflow occurs if the rounded
result is outside the range –2**63..2**63–1. In conversions from quadword integer to longword integer, an integer overflow occurs if the result is outside the range –2**31..2**31–1.
If an integer overflow occurs in CVTxQ or CVTQL, the true result truncated to the low-order
64 or 32 bits respectively is stored in the result register.
If an integer overflow occurs and integer overflow traps are enabled by the instruction, an integer overflow arithmetic trap is signaled.

4.7.7.10 IEEE Floating-Point Trap Disable Bits
In the case of IEEE exception completion modes, any of the traps described in Sections 4.7.7.4
through 4.7.7.9 may be disabled by setting the appropriate trap disable bit in the FPCR. The
trap disable bits only affect the IEEE trap modes when the instruction is modified by any valid
qualifier combination that includes the /S (exception completion) qualifier. The trap disable
bits (DNOD, DZED, INED, INVD, OVFD, and UNFD) do not affect any of the VAX trap
modes.
If a trap disable bit is set and the corresponding trap condition occurs, the hardware implementation sets the result of the operation to the nontrapping result value as specified in the IEEE
standard and Section 4.7.10 and modified by the denormal control bits. If the implementation
is unable to calculate the required result, it ignores the trap disable bit and signals a trap as
usual.
Note that a hardware implementation may choose to support any subset of the trap disable bits,
including the empty subset.
4–78 Alpha Architecture Handbook

4.7.7.11 IEEE Denormal Control Bits
In the case of IEEE exception completion modes, the handling of denormal operands and
results is controlled by the DNZ and UNDZ bits in the FPCR. These denormal control bits only
affect denormal handling by IEEE instructions that are modified by any valid qualifier combination that includes the /S (exception completion) qualifier.
The denormal control bits apply only to the IEEE operate instructions – ADD, SUB, MUL,
DIV, SQRT, CMPxx, and CVT with floating-point source operand.
If both the UNFD (underflow disable) bit and the UNDZ (underflow to zero) bit are set in the
FPCR, the implementation sets the result of an underflow operation to a true zero result. The
zeroing of a denormal result by UNDZ must also be treated as an inexact result.
If the DNZ (denormal operands to zero) bit is set in the FPCR, the implementation treats each
denormal operand as if it were a signed zero value. The source operands in the register are not
changed. If DNZ is set, IEEE operations with any valid qualifier combination that includes a /S
qualifier signal arithmetic traps as if any denormal operand were zero; that is, with DNZ set:

•

An IEEE operation with a denormal operand never generates an overflow, underflow, or
inexact result arithmetic trap.

•

Dividing by a denormal operand is a division by zero or invalid operation as appropriate.

•

Multiplying a denormal by infinity is an invalid operation.

•

A SQRT of a negative denormal produces a –0 instead of an invalid operation.

•

A denormal operand, treated as zero, does not take the denormal operand exception trap
controlled by the DNOD bit in the FPCR.

Note that a hardware implementation may choose to support any subset of the denormal control bits, including the empty subset.

4.7.8 Floating-Point Control Register (FPCR)
When an IEEE floating-point operate instruction specifies dynamic mode (/D) in its function
field (function field bits <12:11> = 11), the rounding mode to be used for the instruction is
derived from the FPCR register. The layout of the rounding mode bits and their assignments
matches exactly the format used in the 11-bit function field of the floating-point operate
instructions. The function field is described in Section 4.7.9.
In addition, the FPCR gives a summary of each exception type for the exception conditions
detected by all IEEE floating-point operates thus far, as well as an overall summary bit that
indicates whether any of these exception conditions has been detected. The individual exception bits match exactly in purpose and order the exception bits found in the exception summary
quadword that is pushed for arithmetic traps. However, for each instruction, these exception
bits are set independent of the trapping mode specified for the instruction. Therefore, even
though trapping may be disabled for a certain exceptional condition, the fact that the exceptional condition was encountered by an instruction is still recorded in the FPCR.
Floating-point operates that belong to the IEEE subset and CVTQL, which belongs to both

Instruction Descriptions 4–79

VAX and IEEE subsets, appropriately set the FPCR exception bits. It is UNPREDICTABLE
whether floating-point operates that belong only to the VAX floating-point subset set the
FPCR exception bits.
Alpha floating-point hardware only transitions these exception bits from zero to one. Once set
to one, these exception bits are only cleared when software writes zero into these bits by writing a new value into the FPCR.
Section 4.7.2 allows certain of the FPCR bits to be subsetted.
The format of the FPCR is shown in Figure 4–1 and described in Table 4–11.

Figure 4–1: Floating-Point Control Register (FPCR) Format
0

63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 48 47 46

S I U U DYN I I U O D I O D I D D
U N N N _RM O N N V Z N V Z N N N
ME F D
VE F F E V F E VZO
DDD D
DD Z

RAZ/IGN

Table 4–11: Floating-Point Control Register (FPCR) Bit Descriptions
Bit

Description (Meaning When Set)

Summary Bit (SUM). Records bitwise OR of FPCR exception bits. Equal to
FPCR<57 |56 | 55 | 54 | 53 | 52>.

Inexact Disable (INED)†. Suppress INE trap and place correct IEEE nontrapping
result in the destination register.

Underflow Disable (UNFD)†. Suppress UNF trap and place correct IEEE nontrapping result in the destination register if the implementation is capable of producing correct IEEE nontrapping result. The correct result value is determined
according to the value of the UNDZ bit.

Underflow to Zero (UNDZ)†. When set together with UNFD, on underflow, the
hardware places a true zero (64 bits of zero) in the destination register rather than
the result specified by the IEEE standard.

59–58

Dynamic Rounding Mode (DYN). Indicates the rounding mode to be used by an
IEEE floating-point operate instruction when the instruction’s function field specifies dynamic mode (/D). Assignments are:

DYN

IEEE Rounding Mode Selected

00
01
10
11

Chopped rounding mode
Minus infinity
Normal rounding
Plus infinity

4–80 Alpha Architecture Handbook

Table 4–11: Floating-Point Control Register (FPCR) Bit Descriptions (Continued)
Bit

Description (Meaning When Set)

Integer Overflow (IOV). An integer arithmetic operation or a conversion from
floating to integer overflowed the destination precision.

Inexact Result (INE). A floating arithmetic or conversion operation gave a result
that differed from the mathematically exact result.

Underflow (UNF). A floating arithmetic or conversion operation underflowed the
destination exponent.

Overflow (OVF). A floating arithmetic or conversion operation overflowed the
destination exponent.

Division by Zero (DZE). An attempt was made to perform a floating divide operation with a divisor of zero.

Invalid Operation (INV). An attempt was made to perform a floating arithmetic,
conversion, or comparison operation, and one or more of the operand values were
illegal.

Overflow Disable (OVFD)†. Suppress OVF trap and place correct IEEE nontrapping result in the destination register if the implementation is capable of producing correct IEEE nontrapping results.

Division by Zero Disable (DZED)†. Suppress DZE trap and place correct IEEE
nontrapping result in the destination register if the implementation is capable of
producing correct IEEE nontrapping results.

Invalid Operation Disable (INVD)†. Suppress INV trap and place correct IEEE
nontrapping result in the destination register if the implementation is capable of
producing correct IEEE nontrapping results.

Denormal Operands to Zero (DNZ)†. Treat all denormal operands as a signed zero
value with the same sign as the denormal.

Denormal Operand Exception Disable (DNOD)†. Suppress INV trap for valid
operations that involve denormal operand values and place the correct IEEE nontrapping result in the destination register if the implementation is capable of processing the denormal operand. If the result of the operation underflows, the
correct result is determined according to the value of the UNDZ bit. If DNZ is set,
DNOD has no effect because a denormal operand is treated as having a zero value
instead of a denormal value.

46–0

Reserved. Read as Zero. Ignored when written.

†

Bit only has meaning for IEEE instructions when any valid qualifier combination that
includes exception completion (/S) is specified.

FPCR is read from and written to the floating-point registers by the MT_FPCR and MF_FPCR
instructions respectively, which are described in Section 4.7.8.1.

Instruction Descriptions 4–81

FPCR and the instructions to access it are required for an implementation that supports floating-point (see Section 4.7.8). On implementations that do not support floating-point, the
instructions that access FPCR (MF_FPCR and MT_FPCR) take an Illegal Instruction Trap.

Software Note:
Support for FPCR is required on a system that supports the OpenVMS Alpha operating
system even if that system does not support floating-point.

4.7.8.1 Accessing the FPCR
Because Alpha floating-point hardware can overlap the execution of a number of floating-point instructions, accessing the FPCR must be synchronized with other floating-point
instructions. An EXCB instruction must be issued both prior to and after accessing the FPCR
to ensure that the FPCR access is synchronized with the execution of previous and subsequent
floating-point instructions; otherwise synchronization is not ensured.
Issuing an EXCB followed by an MT_FPCR followed by another EXCB ensures that only
floating-point instructions issued after the second EXCB are affected by and affect the new
value of the FPCR. Issuing an EXCB followed by an MF_FPCR followed by another EXCB
ensures that the value read from the FPCR only records the exception information for floating-point instructions issued prior to the first EXCB.
Consider the following example:
ADDT/D
EXCB
MT_FPCR F1,F1,F1
EXCB
SUBT/D

;1
;2

Without the first EXCB, it is possible in an implementation for the ADDT/D to execute in parallel with the MT_FPCR. Thus, it would be UNPREDICTABLE whether the ADDT/D was
affected by the new rounding mode set by the MT_FPCR and whether fields cleared by the
MT_FPCR in the exception summary were subsequently set by the ADDT/D.
Without the second EXCB, it is possible in an implementation for the MT_FPCR to execute in
parallel with the SUBT/D. Thus, it would be UNPREDICTABLE whether the SUBT/D was
affected by the new rounding mode set by the MT_FPCR and whether fields cleared by the
MT_FPCR in the exception summary field of FPCR were previously set by the SUBT/D.
Specifically, code should issue an EXCB before and after it accesses the FPCR if that code
needs to see valid values in FPCR bits <63> and <57:52>. An EXCB should be issued before
attempting to write the FPCR if the code expects changes to bits <59:52> not to have dependencies with prior instructions. An EXCB should be issued after attempting to write the FPCR
if the code expects subsequent instructions to have dependencies with changes to bits <59:52>.

4–82 Alpha Architecture Handbook

4.7.8.2 Default Values of the FPCR
Processor initialization leaves the value of FPCR UNPREDICTABLE.

Software Note:
Compaq software should initialize FPCR<DYN> = 10 during program activation. Using
this default, a program can be coded to use only dynamic rounding without the need to
explicitly set the rounding mode to normal rounding in its start-up code.
Program activation normally clears all other fields in the FPCR. However, this behavior
may depend on the operating system.

4.7.8.3 Saving and Restoring the FPCR
The FPCR must be saved and restored across context switches so that the FPCR value of one
process does not affect the rounding behavior and exception summary of another process.
The dynamic rounding mode put into effect by the programmer (or initialized by image activation) is valid for the entirety of the program and remains in effect until subsequently changed
by the programmer or until image run-down occurs.

Software Notes:
The following software notes apply to saving and restoring the FPCR:
1. The IEEE standard precludes saving and restoring the FPCR across subroutine calls.
2. The IEEE standard requires that an implementation provide status flags that are set
whenever the corresponding conditions occur and are reset only at the user’s request.
The exception bits in the FPCR do not satisfy that requirement, because they can be
spuriously set by instructions in a trap shadow that should not have been executed had
the trap been taken synchronously.
The IEEE status flags can be provided by software (as software status bits) as follows:
Trap interface software (usually the operating system) keeps a set of software
status bits and a mask of the traps that the user wants to receive. Code is generated
with the /SUI qualifiers. For a particular exception, the software clears the
corresponding trap disable bit if either the corresponding software status bit is 0 or
if the user wants to receive such traps. If a trap occurs, the software locates the
offending instruction in the trap shadow, simulates it and sets any of the software
status bits that are appropriate. Then, the software either delivers the trap to the
user program or disables further delivery of such traps. The user program must
interface to this trap interface software to set or clear any of the software status bits
or to enable or disable floating-point traps. See Section B.2.
When such a scheme is being used, the trap disable bits and denormal control bits
should be modified only by the trap interface software. If the disable bits are
spuriously cleared, unnecessary traps may occur. If they are spuriously set, the
software may fail to set the correct values in the software status bits. Programs should
call routines in the trap interface software to set or clear bits in the FPCR.

Instruction Descriptions 4–83

Compaq software may choose to initialize the software status bits and the trap disable
bits to all 1’s to avoid any initial trapping when an exception condition first occurs. Or,
software may choose to initialize those bits to all 0’s in order to provide a summary of
the exception behavior when the program terminates.
In any event, the exception bits in the FPCR are still useful to programs. A program
can clear all of the exception bits in the FPCR, execute a single floating-point
instruction, and then examine the status bits to determine which hardware-defined
exceptions the instruction encountered. For this operation to work in the presence of
various implementation options, the single instruction should be followed by a TRAPB
or EXCB instruction, and exception completion by the system software should save
and restore the FPCR registers without other modifications.
3. Because of the way the LDS and STS instructions manipulate bits <61:59> of floating-point registers, they should not be used to manipulate FPCR values.

4.7.9 Floating-Point Instruction Function Field Format
The function code for IEEE and VAX floating-point instructions, bits <15..5>, contain the
function field. That field is shown in Figure 4–2 and described for IEEE floating-point in Table
4–12 and for VAX floating-point in Table 4–13. Function codes for the independent floating-point instructions, those with opcode 1716, do not correspond to the function fields below.
The function field contains subfields that specify the trapping and rounding modes that are
enabled for the instruction, the source datatype, and the instruction class.

Figure 4–2: Floating-Point Instruction Function Field
31

26 25

Opcode

21 20

16 15 13 12 11 10 9 8

T
R
P

4–84 Alpha Architecture Handbook

R
N
D

S
R
C

5 4

F
N
C

Table 4–12: IEEE Floating-Point Function Field Bit Summary
Bits

Field

Meaning†

15–13

TRP

Trapping modes:
Contents

Meaning for Opcodes 1416 and 1616

000
001

Imprecise (default)
Underflow enable (/U) — floating-point output
Integer overflow enable (/V) — integer output
UNPREDICTABLE for opcode 1616 instructions
Reserved for opcode 1416 instructions

010
011

UNPREDICTABLE for opcode 1616 instructions
Reserved for opcode 1416 instructions

100

UNPREDICTABLE for opcode 1616 instructions
Reserved for opcode 1416 instructions

101

/SU — floating-point output
/SV — integer output
UNPREDICTABLE for opcode 1616 instructions
Reserved for opcode 1416 instructions

110
111

12–11

10–9

RND

SRC

/SUI — floating-point output
/SVI — integer output

Rounding modes:
Contents

Meaning for Opcodes 1616 and 1416

00
01
10
11

Chopped (/C)
Minus infinity (/M)
Normal (default)
Dynamic (/D)

Source datatype:
Contents

Meaning for
Opcode 1616

Meaning for
Opcode 1416

00
01
10
11

S_floating
Reserved
T_floating
Q_fixed

S_floating
Reserved
T_floating
Reserved

Instruction Descriptions 4–85

Table 4–12: IEEE Floating-Point Function Field Bit Summary (Continued)
Bits

Field

Meaning†

8–5

FNC

Instruction class:

†

Contents

Meaning for
Opcode 1616

Meaning for
Opcode 1416

0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110

ADDx
SUBx
MULx
DIVx
CMPxUN
CMPxEQ
CMPxLT
CMPxLE
Reserved
Reserved
Reserved
Reserved
CVTxS
Reserved
CVTxT

Reserved
Reserved
Reserved
Reserved
ITOFS/ITOFT
Reserved
Reserved
Reserved
Reserved
Reserved
Reserved
SQRTS/SQRTT
Reserved
Reserved
Reserved

1111

CVTxQ

Reserved

Encodings for the instructions CVTST and CVTST/S are exceptions to this table; use the
encodings in Section C.1.

4–86 Alpha Architecture Handbook

Table 4–13: VAX Floating-Point Function Field Bit Summary
Bits

Field

Meaning

15–13

TRP

Trapping modes:
Contents

Meaning for Opcodes 1416 and 1516

000
001

Imprecise (default)
Underflow enable (/U) – floating-point output
Integer overflow enable (/V) – integer output
UNPREDICTABLE for opcode 1516 instructions
Reserved for opcode 1416 instructions

010
011

UNPREDICTABLE for opcode 1516 instructions
Reserved for opcode 1416 instructions

100
101

/S – Exception completion enable
/SU – floating-point output
/SV – integer output
UNPREDICTABLE for opcode 1516 instructions
Reserved for opcode 1416 instructions

110
111

12–11

10–9

RND

SRC

UNPREDICTABLE for opcode 1516 instructions
Reserved for opcode 1416 instructions

Rounding modes:
Contents

Meaning for Opcodes 1516 and 1416

00
01
10
11

Chopped (/C)
UNPREDICTABLE
Normal (default)
UNPREDICTABLE

Source datatype:†
Contents

Meaning for Opcode 15 16

Meaning for Opcode 1416

00
01
10
11

F_floating
D_floating
G_floating
Q_fixed

F_floating
F_floating
G_floating
Reserved

Instruction Descriptions 4–87

Table 4–13: VAX Floating-Point Function Field Bit Summary (Continued)
Bits

Field

Meaning

8–5

FNC

Instruction class:

†

Contents

Meaning for
Opcode 1516

Meaning for
Opcode 1416

0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111

ADDx
SUBx
MULx
DIVx
CMPxUN
CMPxEQ
CMPxLT
CMPxLE
Reserved
Reserved
Reserved
Reserved
CVTxF
CVTxD
CVTxG
CVTxQ

Reserved
Reserved
Reserved
Reserved
ITOFF
Reserved
Reserved
Reserved
Reserved
Reserved
SQRTF/SQRTG
Reserved
Reserved
Reserved
Reserved
Reserved

In the SRC field, both 00 and 01 specify the F_floating source datatype for opcode 14 16.

4.7.10 IEEE Standard
The IEEE Standard for Binary Floating-Point Arithmetic (ANSI/IEEE Standard 754-1985) is
included by reference.
This standard leaves certain operations as implementation dependent. The remainder of this
section specifies the behavior of the Alpha architecture in these situations. Note that this
behavior may be supplied by either hardware (if the invalid operation disable, or INVD, bit is
implemented) or by software. See Sections 4.7.7.10, 4.7.7.11, 4.7.8, 4.7.8.3, and Section B.1.

4.7.10.1 Conversion of NaN and Infinity Values
Conversion of a NaN or an Infinity value to an integer gives a result of zero.
Conversion of a NaN value from S_floating to T_floating gives a result identical to the input,
except that the most significant fraction bit (bit 51) is set to indicate a quiet NaN.
Conversion of a NaN value from T_floating to S_floating gives a result identical to the input,
except that the most significant fraction bit (bit 51) is set to indicate a quiet NaN, and bits
<28:0> are cleared to zero.

4–88 Alpha Architecture Handbook

4.7.10.2 Copying NaN Values
Copying a NaN value without changing its precision does not cause an invalid operation
exception.

4.7.10.3 Generating NaN Values
When an operation is required to produce a NaN and none of its inputs are NaN values, the
result of the operation is the quiet NaN value that has the sign bit set to one, all exponent bits
set to one (to indicate a NaN), the most significant fraction bit set to one (to indicate that the
NaN is quiet), and all other fraction bits cleared to zero. This value is referred to as "the canonical quiet NaN."

4.7.10.4 Propagating NaN Values
When an operation is required to produce a NaN and one or both of its inputs are NaN values,
the IEEE standard requires that quiet NaN values be propagated when possible. With the Alpha
architecture, the result of such an operation is a NaN generated according to the first of the following rules that is applicable:
1. If the operand in the Fb register of the operation is a quiet NaN, that value is used as the
result.
2. If the operand in the Fb register of the operation is a signaling NaN, the result is the
quiet NaN formed from the Fb value by setting the most significant fraction bit (bit 51)
to a one bit.
3. If the operation uses its Fa operand and the value in the Fa register is a quiet NaN, that
value is used as the result.
4. If the operation uses its Fa operand and the value in the Fa register is a signaling NaN,
the result is the quiet NaN formed from the Fa value by setting the most significant
fraction bit (bit 51) to a one bit.
5. The result is the canonical quiet NaN.

Instruction Descriptions 4–89

4.8 Memory Format Floating-Point Instructions
The instructions in this section move data between the floating-point registers and memory.
They use the Memory instruction format. They do not interpret the bits moved in any way; specifically, they do not trap on non-finite values.
The instructions are summarized in Table 4–14.

Table 4–14: Memory Format Floating-Point Instructions Summary
Mnemonic

Operation

Subset

LDF

Load F_floating

VAX

LDG

Load G_floating (Load D_floating)

VAX

LDS

Load S_floating (Load Longword Integer)

Both

LDT

Load T_floating (Load Quadword Integer)

Both

STF

Store F_floating

VAX

STG

Store G_floating (Store D_floating)

VAX

STS

Store S_floating (Store Longword Integer)

Both

STT

Store T_floating (Store Quadword Integer)

Both

4–90 Alpha Architecture Handbook

4.8.1 Load F_floating
Format:
LDF

!Memory format

Fa.wf,disp.ab(Rb.ab)

Operation:
va ←

{Rbv + SEXT(disp)}

CASE
big_endian_data: va’ ← va XOR 1002
little_endian_data: va’ ← va
ENDCASE
Fa ← (va’)<15> || MAP_F((va’)<14:7>) || (va’)<6:0> ||
(va’)<31:16> || 0<28:0>

Exceptions:
Access Violation
Fault on Read
Alignment
Translation Not Valid

Instruction mnemonics:
LDF

Load F_floating

Qualifiers:
None

Description:
LDF fetches an F_floating datum from memory and writes it to register Fa. If the data is not
naturally aligned, an alignment exception is generated.
The MAP_F function causes the 8-bit memory-format exponent to be expanded to an 11-bit
register-format exponent according to Table 2–1.
The virtual address is computed by adding register Rb to the sign-extended 16-bit displacement. For a big-endian longword access, va<2> (bit 2 of the virtual address) is inverted, and
any memory management fault is reported for va (not va’). The source operand is fetched
from memory and the bytes are reordered to conform to the F_floating register format. The
result is then zero-extended in the low-order longword and written to register Fa.

Instruction Descriptions 4–91

4.8.2 Load G_floating
Format:
LDG

Fa.wg,disp.ab(Rb.ab)

!Memory format

Operation:
va ← {Rbv + SEXT(disp)}
Fa ← (va)<15:0> || (va)<31:16> || (va)<47:32> || (va)<63:48>

Exceptions:
Access Violation
Fault on Read
Alignment
Translation Not Valid

Instruction mnemonics:
LDG

Load G_floating (Load D_floating)

Qualifiers:
None

Description:
LDG fetches a G_floating (or D_floating) datum from memory and writes it to register Fa. If
the data is not naturally aligned, an alignment exception is generated.
The virtual address is computed by adding register Rb to the sign-extended 16-bit displacement. The source operand is fetched from memory, the bytes are reordered to conform to the
G_floating register format (also conforming to the D_floating register format), and the result is
then written to register Fa.

4–92 Alpha Architecture Handbook

4.8.3 Load S_floating
Format:
LDS

!Memory format

Fa.ws,disp.ab(Rb.ab)

Operation:
va ← {Rbv + SEXT(disp)}
CASE
big_endian_data: va’ ← va XOR 1002
little_endian_data: va’ ← va
ENDCASE
Fa ← (va’)<31> || MAP_S((va’)<30:23>) || (va’)<22:0> || 0<28:0>

Exceptions:
Access Violation
Fault on Read
Alignment
Translation Not Valid

Instruction mnemonics:
LDS

Load S_floating (Load Longword Integer)

Qualifiers:
None

Description:
LDS fetches a longword (integer or S_floating) from memory and writes it to register Fa. If the
data is not naturally aligned, an alignment exception is generated. The MAP_S function causes
the 8-bit memory-format exponent to be expanded to an 11-bit register-format exponent
according to Table 2–2.
The virtual address is computed by adding register Rb to the sign-extended 16-bit displacement. For a big-endian longword access, va<2> (bit 2 of the virtual address) is inverted, and
any memory management fault is reported for va (not va’). The source operand is fetched
from memory, is zero-extended in the low-order longword, and then written to register Fa.
Longword integers in floating registers are stored in bits <63:62,58:29>, with bits <61:59>
ignored and zeros in bits <28:0>.

Instruction Descriptions 4–93

4.8.4 Load T_floating
Format:
LDT

Fa.wt,disp.ab(Rb.ab)

!Memory format

Operation:
va ← {Rbv + SEXT(disp)}
Fa ← (va)<63:0>

Exceptions:
Access Violation
Fault on Read
Alignment
Translation Not Valid

Instruction mnemonics:
LDT

Load T_floating (Load Quadword Integer)

Qualifiers:
None

Description:
LDT fetches a quadword (integer or T_floating) from memory and writes it to register Fa. If
the data is not naturally aligned, an alignment exception is generated.
The virtual address is computed by adding register Rb to the sign-extended 16-bit displacement. The source operand is fetched from memory and written to register Fa.

4–94 Alpha Architecture Handbook

4.8.5 Store F_floating
Format:
STF

!Memory format

Fa.rf,disp.ab(Rb.ab)

Operation:
va ← {Rbv + SEXT(disp)}
CASE
big_endian_data: va’ ← va XOR 1002
little_endian_data: va’ ← va
ENDCASE
(va’)<31:0> ← Fav<44:29> || Fav<63:62> || Fav<58:45>

Exceptions:
Access Violation
Fault on Write
Alignment
Translation Not Valid

Instruction mnemonics:
STF

Store F_floating

Qualifiers:
None

Description:
STF stores an F_floating datum from Fa to memory. If the data is not naturally aligned, an
alignment exception is generated.
The virtual address is computed by adding register Rb to the sign-extended 16-bit displacement. For a big-endian longword access, va<2> (bit 2 of the virtual address) is inverted, and
any memory management fault is reported for va (not va’). The bits of the source operand are
fetched from register Fa, the bits are reordered to conform to F_floating memory format, and
the result is then written to memory. Bits <61:59> and <28:0> of Fa are ignored. No checking
is done.

Instruction Descriptions 4–95

4.8.6 Store G_floating
Format:
STG

Fa.rg,disp.ab(Rb.ab)

!Memory format

Operation:
va ← {Rbv + SEXT(disp)}
(va)<63:0> ← Fav<15:0> || Fav<31:16> || Fav<47:32> || Fav<63:48>

Exceptions:
Access Violation
Fault on Write
Alignment
Translation Not Valid

Instruction mnemonics:
STG

Store G_floating (Store D_floating)

Qualifiers:
None

Description:
STG stores a G_floating (or D_floating) datum from Fa to memory. If the data is not naturally
aligned, an alignment exception is generated.
The virtual address is computed by adding register Rb to the sign-extended 16-bit displacement. The source operand is fetched from register Fa, the bytes are reordered to conform to the
G_floating memory format (also conforming to the D_floating memory format), and the result
is then written to memory.

4–96 Alpha Architecture Handbook

4.8.7 Store S_floating
Format:
STS

!Memory format

Fa.rs,disp.ab(Rb.ab)

Operation:
va ← {Rbv + SEXT(disp)}
CASE
big_endian_data: va’ ← va XOR 1002
little_endian_data: va’ ← va
ENDCASE
(va’)<31:0> ← Fav<63:62> || Fav<58:29>

Exceptions:
Access Violation
Fault on Write
Alignment
Translation Not Valid

Instruction mnemonics:
STS

Store S_floating (Store Longword Integer)

Qualifiers:
None

Description:
STS stores a longword (integer or S_floating) datum from Fa to memory. If the data is not naturally aligned, an alignment exception is generated.
The virtual address is computed by adding register Rb to the sign-extended 16-bit displacement. For a big-endian longword access, va<2> (bit 2 of the virtual address) is inverted, and
any memory management fault is reported for va (not va’). The bits of the source operand are
fetched from register Fa, the bits are reordered to conform to S_floating memory format, and
the result is then written to memory. Bits <61:59> and <28:0> of Fa are ignored. No checking
is done.

Instruction Descriptions 4–97

4.8.8 Store T_floating
Format:
STT

Fa.rt,disp.ab(Rb.ab)

!Memory format

Operation:
va ← {Rbv + SEXT(disp)}
(va)<63:0> ← Fav<63:0>

Exceptions:
Access Violation
Fault on Write
Alignment
Translation Not Valid

Instruction mnemonics:
STT

Store T_floating (Store Quadword Integer)

Qualifiers:
None

Description:
STT stores a quadword (integer or T_floating) datum from Fa to memory. If the data is not naturally aligned, an alignment exception is generated.
The virtual address is computed by adding register Rb to the sign-extended 16-bit displacement. The source operand is fetched from register Fa and written to memory.

4–98 Alpha Architecture Handbook

4.9 Branch Format Floating-Point Instructions
Alpha provides six floating conditional branch instructions. These branch-format instructions
test the value of a floating-point register and conditionally change the PC.
They do not interpret the bits tested in any way; specifically, they do not trap on non-finite
values.
The test is based on the sign bit and whether the rest of the register is all zero bits. All 64 bits
of the register are tested. The test is independent of the format of the operand in the register.
Both plus and minus zero are equal to zero. A non-zero value with a sign of zero is greater than
zero. A non-zero value with a sign of one is less than zero. No reserved operand or non-finite
checking is done.
The floating-point branch operations are summarized in Table 4–15:

Table 4–15: Floating-Point Branch Instructions Summary
Mnemonic

Operation

Subset

FBEQ

Floating Branch Equal

Both

FBGE

Floating Branch Greater Than or Equal

Both

FBGT

Floating Branch Greater Than

Both

FBLE

Floating Branch Less Than or Equal

Both

FBLT

Floating Branch Less Than

Both

FBNE

Floating Branch Not Equal

Both

Instruction Descriptions 4–99

4.9.1 Conditional Branch
Format:
FBxx

Fa.rq,disp.al

!Branch format

Operation:
{update PC}
va ← PC + {4*SEXT(disp)}
IF TEST(Fav, Condition_based_on_Opcode) THEN
PC ← va

Exceptions:
None

Instruction mnemonics:
FBEQ
FBGE
FBGT
FBLE
FBLT
FBNE

Floating Branch Equal
Floating Branch Greater Than or Equal
Floating Branch Greater Than
Floating Branch Less Than or Equal
Floating Branch Less Than
Floating Branch Not Equal

Qualifiers:
None

Description:
Register Fa is tested. If the specified relationship is true, the PC is loaded with the target virtual address; otherwise, execution continues with the next sequential instruction.
The displacement is treated as a signed longword offset. This means it is shifted left two bits
(to address a longword boundary), sign-extended to 64 bits, and added to the updated PC to
form the target virtual address.
The conditional branch instructions are PC-relative only. The 21-bit signed displacement gives
a forward/backward branch distance of +/–1M instructions.

4–100 Alpha Architecture Handbook

Notes:
•

To branch properly on non-finite operands, compare to F31, then branch on the result of
the compare.

•

The largest negative integer (8000 0000 0000 000016) is the same bit pattern as floating
minus zero, so it is treated as equal to zero by the branch instructions. To branch properly on the largest negative integer, convert it to floating or move it to an integer register and do an integer branch.

Instruction Descriptions 4–101

4.10 Floating-Point Operate Format Instructions
The floating-point bit-operate instructions perform copy and integer convert operations on
64-bit register values. The bit-operate instructions do not interpret the bits moved in any way;
specifically, they do not trap on non-finite values.
The floating-point arithmetic-operate instructions perform add, subtract, multiply, divide, compare, register move, squre root, and floating convert operations on 64-bit register values in one
of the four specified floating formats.
Each instruction specifies the source and destination formats of the values, as well as the
rounding mode and trapping mode to be used. These instructions use the Floating-point Operate format.

Floating-point convert and square-root (FIX) extension implementation note:
The FIX extension to the architecture provides the FTOIx, ITOFx, and SQRTx
instructions. Alpha processors for which the AMASK instruction returns bit 1 set
implement these instructions. Those processors for which AMASK does not return bit 1 set
can take an Illegal Instruction trap, and software can emulate their function, if required.
AMASK is described in Sections 4.11.1 and D.3.
The floating-point operate instructions are summarized in Table 4–16.

Table 4–16: Floating-Point Operate Instructions Summary
Mnemonic

Operation

Subset

Bit and FPCR Operations:
CPYS

Copy Sign

Both

CPYSE

Copy Sign and Exponent

Both

CPYSN

Copy Sign Negate

Both

CVTLQ

Convert Longword to Quadword

Both

CVTQL

Convert Quadword to Longword

Both

FCMOVxx

Floating Conditional Move

Both

MF_FPCR

Move from Floating-point Control Register

Both

MT_FPCR

Move to Floating-point Control Register

Both

4–102 Alpha Architecture Handbook

Table 4–16: Floating-Point Operate Instructions Summary (Continued)
Mnemonic

Operation

Subset

Arithmetic Operations
ADDF

Add F_floating

VAX

ADDG

Add G_floating

VAX

ADDS

Add S_floating

IEEE

ADDT

Add T_floating

IEEE

CMPGxx

Compare G_floating

VAX

CMPTxx

Compare T_floating

IEEE

CVTDG

Convert D_floating to G_floating

VAX

CVTGD

Convert G_floating to D_floating

VAX

CVTGF

Convert G_floating to F_floating

VAX

CVTGQ

Convert G_floating to Quadword

VAX

CVTQF

Convert Quadword to F_floating

VAX

CVTQG

Convert Quadword to G_floating

VAX

CVTQS

Convert Quadword to S_floating

IEEE

CVTQT

Convert Quadword to T_floating

IEEE

CVTST

Convert S_floating to T_floating

IEEE

CVTTQ

Convert T_floating to Quadword

IEEE

CVTTS

Convert T_floating to S_floating

IEEE

DIVF

Divide F_floating

VAX

DIVG

Divide G_floating

VAX

DIVS

Divide S_floating

IEEE

DIVT

Divide T_floating

IEEE

FTOIS

Floating-point to integer register move, S_floating

IEEE

FTOIT

Floating-point to integer register move, T_floating

IEEE

ITOFF

Integer to floating-point register move, F_floating

VAX

ITOFS

Integer to floating-point register move, S_floating

IEEE

ITOFT

Integer to floating-point register move, T_floating

IEEE

Instruction Descriptions 4–103

Table 4–16: Floating-Point Operate Instructions Summary (Continued)
Mnemonic

Operation

Subset

Arithmetic Operations
MULF

Multiply F_floating

VAX

MULG

Multiply G_floating

VAX

MULS

Multiply S_floating

IEEE

MULT

Multiply T_floating

IEEE

SQRTF

Square root F_floating

VAX

SQRTG

Square root G_floating

VAX

SQRTS

Square root S_floating

IEEE

SQRTT

Square root T_floating

IEEE

SUBF

Subtract F_floating

VAX

SUBG

Subtract G_floating

VAX

SUBS

Subtract S_floating

IEEE

SUBT

Subtract T_floating

IEEE

4–104 Alpha Architecture Handbook

4.10.1 Copy Sign
Format:
CPYSy

Fa.rq,Fb.rq,Fc.wq

!Floating-point Operate format

Operation:
CASE
CPYS: Fc ← Fav<63> || Fbv<62:0>
CPYSN: Fc ← NOT(Fav<63>) || Fbv<62:0>
CPYSE: Fc ← Fav<63:52> || Fbv<51:0>
ENDCASE

Exceptions:
None

Instruction mnemonics:
CPYS
CPYSE
CPYSN

Copy Sign
Copy Sign and Exponent
Copy Sign Negate

Qualifiers:
None

Description:
For CPYS and CPYSN, the sign bit of Fa is fetched (and complemented in the case of CPYSN)
and concatenated with the exponent and fraction bits from Fb; the result is stored in Fc.
For CPYSE, the sign and exponent bits from Fa are fetched and concatenated with the fraction
bits from Fb; the result is stored in Fc.
No checking of the operands is performed.

Notes:
•

Register moves can be performed using CPYS Fx,Fx,Fy. Floating-point absolute value
can be done using CPYS F31,Fx,Fy. Floating-point negation can be done using
CPYSN Fx,Fx,Fy. Floating values can be scaled to a known range by using CPYSE.

Instruction Descriptions 4–105

4.10.2 Convert Integer to Integer
Format:
CVTxy

Fb.rq,Fc.wx

!Floating-point Operate format

Operation:
CASE
CVTQL: Fc ← Fbv<31:30> || 0<2:0> || Fbv<29:0> ||0<28:0>
CVTLQ: Fc ← SEXT(Fbv<63:62> || Fbv<58:29>)
ENDCASE

Exceptions:
Integer Overflow, CVTQL only

Instruction mnemonics:
CVTLQ
CVTQL

Convert Longword to Quadword
Convert Quadword to Longword

Qualifiers:
Trapping:

Exception Completion (/S) (CVTQL only)
Integer Overflow Enable (/V) (CVTQL only)

Description:
The two’s-complement operand in register Fb is converted to a two’s-complement result and
written to register Fc. Register Fa must be F31.
The conversion from quadword to longword is a repositioning of the low 32 bits of the operand, with zero fill and optional integer overflow checking. Integer overflow occurs if Fb is
outside the range –2**31..2**31–1. If integer overflow occurs, the truncated result is stored in
Fc, and an arithmetic trap is taken if enabled.
The conversion from longword to quadword is a repositioning of 32 bits of the operand, with
sign extension.

4–106 Alpha Architecture Handbook

4.10.3 Floating-Point Conditional Move
Format:
FCMOVxx

Fa.rq,Fb.rq,Fc.wq

!Floating-point Operate format

Operation:
IF TEST(Fav, Condition_based_on_Opcode) THEN
Fc ← Fbv

Exceptions:
None

Instruction mnemonics:
FCMOVEQ
FCMOVGE
FCMOVGT
FCMOVLE
FCMOVLT
FCMOVNE

FCMOVE if Register Equal to Zero
FCMOVE if Register Greater Than or Equal to Zero
FCMOVE if Register Greater Than Zero
FCMOVE if Register Less Than or Equal to Zero
FCMOVE if Register Less Than Zero
FCMOVE if Register Not Equal to Zero

Qualifiers:
None

Description:
Register Fa is tested. If the specified relationship is true, register Fb is written to register Fc;
otherwise, the move is suppressed and register Fc is unchanged. The test is based on the sign
bit and whether the rest of the register is all zero bits, as described for floating branches in Section 4.9.

Instruction Descriptions 4–107

Notes:
Except that it is likely in many implementations to be substantially faster, the instruction:
FCMOVxx Fa,Fb,Fc
is exactly equivalent to:
FByy Fa,label
CPYS Fb,Fb,Fc
label: ...

! yy = NOT xx

For example, a branchless sequence for:
F1=MAX(F1,F2)
is:
CMPxLT F1,F2,F3
FCMOVNE F3,F2,F1

4–108 Alpha Architecture Handbook

! F3=one if F1<F2; x=F/G/S/T
! Move F2 to F1 if F1<F2

4.10.4 Move from/to Floating-Point Control Register
Format:
Mx_FPCR

Fa.rq,Fa.rq,Fa.wq

!Floating-point Operate format

Operation:
CASE
MF_FPCR: Fa
← FPCR
MT_FPCR: FPCR ← Fav
ENDCASE

Exceptions:
None

Instruction mnemonics:
MF_FPCR
MT_FPCR

Move from Floating-point Control Register
Move to Floating-point Control Register

Qualifiers:
None

Description:
The Floating-point Control Register (FPCR) is read from (MF_FPCR) or written to
(MT_FPCR), a floating-point register. The floating-point register to be used is specified by the
Fa, Fb, and Fc fields all pointing to the same floating-point register. If the Fa, Fb, and Fc fields
do not all point to the same floating-point register, then it is UNPREDICTABLE which register is used. If the Fa, Fb, and Fc fields do not all point to the same floating-point register, the
resulting values in the Fc register and in FPCR are UNPREDICTABLE.
If t h e F c f i el d i s F 3 1 i n th e c as e o f M T _ F P C R , t h e r e s u ltin g v a lu e i n FP C R i s
UNPREDICTABLE.
The use of these instructions and the FPCR are described in Section 4.7.8.

Instruction Descriptions 4–109

4.10.5 VAX Floating Add
Format:
ADDx

Fa.rx,Fb.rx,Fc.wx

!Floating-point Operate format

Operation:
Fc ← Fav + Fbv

Exceptions:
Invalid Operation
Overflow
Underflow

Instruction mnemonics:
ADDF
ADDG

Add F_floating
Add G_floating

Qualifiers:
Rounding:
Trapping:

Chopped (/C)
Exception Completion (/S)
Underflow Enable (/U)

Description:
Register Fa is added to register Fb, and the sum is written to register Fc.
The sum is rounded or chopped to the specified precision, and then the corresponding range is
checked for overflow/underflow. The single-precision operation on canonical single-precision
values produces a canonical single-precision result.
An invalid operation trap is signaled if either operand has exp=0 and is not a true zero (that is,
VAX reserved operands and dirty zeros trap). The contents of Fc are UNPREDICTABLE if
this occurs. See Section 4.7.7 for details of the stored result on overflow or underflow.

4–110 Alpha Architecture Handbook

4.10.6 IEEE Floating Add
Format:
ADDx

Fa.rx,Fb.rx,Fc.wx

!Floating-point Operate format

Operation:
Fc ← Fav + Fbv

Exceptions:
Invalid Operation
Overflow
Underflow
Inexact Result

Instruction mnemonics:
ADDS
ADDT

Add S_floating
Add T_floating

Qualifiers:
Rounding:

Trapping:

Dynamic (/D)
Minus infinity (/M)
Chopped (/C)
Exception Completion (/S)
Underflow Enable (/U)
Inexact Enable (/I)

Description:
Register Fa is added to register Fb, and the sum is written to register Fc.
The sum is rounded to the specified precision and then the corresponding range is checked for
overflow/underflow. The single-precision operation on canonical single-precision values produces a canonical single-precision result.
See Section 4.7.7 for details of the stored result on overflow, underflow, or inexact result.

Instruction Descriptions 4–111

4.10.7 VAX Floating Compare
Format:
CMPGyy

Fa.rg,Fb.rg,Fc.wq

!Floating-point Operate format

Operation:
IF Fav SIGNED_RELATION Fbv THEN
Fc ← 4000 0000 0000 000016
ELSE
Fc ← 0000 0000 0000 000016

Exceptions:
Invalid Operation

Instruction mnemonics:
CMPGEQ
CMPGLE
CMPGLT

Compare G_floating Equal
Compare G_floating Less Than or Equal
Compare G_floating Less Than

Qualifiers:
Trapping:

Exception Completion (/S)

Description:
The two operands in Fa and Fb are compared. If the relationship specified by the qualifier is
true, a non-zero floating value (0.5) is written to register Fc; otherwise, a true zero is written to
Fc.
Comparisons are exact and never overflow or underflow. Three mutually exclusive relations
are possible: less than, equal, and greater than.
An invalid operation trap is signaled if either operand has exp=0 and is not a true zero (that is,
VAX reserved operands and dirty zeros trap). The contents of Fc are UNPREDICTABLE if
this occurs.

Notes:
•

Compare Less Than A,B is the same as Compare Greater Than B,A; Compare Less
Than or Equal A,B is the same as Compare Greater Than or Equal B,A. Therefore, only
the less-than operations are included.

4–112 Alpha Architecture Handbook

4.10.8 IEEE Floating Compare
Format:
CMPTyy

Fa.rx,Fb.rx,Fc.wq

!Floating-point Operate format

Operation:
IF Fav SIGNED_RELATION Fbv THEN
Fc ← 4000 0000 0000 000016
ELSE
Fc ← 0000 0000 0000 000016

Exceptions:
Invalid Operation

Instruction mnemonics:
CMPTEQ
CMPTLE
CMPTLT
CMPTUN

Compare T_floating Equal
Compare T_floating Less Than or Equal
Compare T_floating Less Than
Compare T_floating Unordered

Qualifiers:
Trapping:

Exception Completion (/SU)

Description:
The two operands in Fa and Fb are compared. If the relationship specified by the qualifier is
true, a non-zero floating value (2.0) is written to register Fc; otherwise, a true zero is written to
Fc.
Comparisons are exact and never overflow or underflow. Four mutually exclusive relations are
possible: less than, equal, greater than, and unordered. The unordered relation is true if one or
both operands are NaN. (This behavior must be provided by an operating system (OS) completion handler, since NaNs trap.) Comparisons ignore the sign of zero, so +0 = –0.
Comparisons with plus and minus infinity execute normally and do not take an invalid operation
trap.
Notes:

•

In order to use CMPTxx with exception completion handling, it is necessary to specify
the /SU IEEE trap mode, even though an underflow trap is not possible.

•

Compare Less Than A,B is the same as Compare Greater Than B,A; Compare Less
Than or Equal A,B is the same as Compare Greater Than or Equal B,A. Therefore, only
the less-than operations are included.

Instruction Descriptions 4–113

4.10.9 Convert VAX Floating to Integer
Format:
CVTGQ

Fb.rx,Fc.wq

!Floating-point Operate format

Operation:
Fc ← {conversion of Fbv}

Exceptions:
Invalid Operation
Integer Overflow

Instruction mnemonics:
CVTGQ

Convert G_floating to Quadword

Qualifiers:
Rounding:
Trapping:

Chopped (/C)
Exception Completion (/S)
Integer Overflow Enable (/V)

Description:
The floating operand in register Fb is converted to a two’s-complement quadword number and
written to register Fc. The conversion aligns the operand fraction with the binary point just to
the right of bit zero, rounds as specified, and complements the result if negative. Register Fa
must be F31.
An invalid operation trap is signaled if the operand has exp=0 and is not a true zero (that is,
VAX reserved operands and dirty zeros trap). The contents of Fc are UNPREDICTABLE if
this occurs.
See Section 4.7.7 for details of the stored result on integer overflow.

4–114 Alpha Architecture Handbook

4.10.10 Convert Integer to VAX Floating
Format:
CVTQy

Fb.rq,Fc.wx

!Floating-point Operate format

Operation:
Fc ← {conversion of Fbv<63:0>}

Exceptions:
None

Instruction mnemonics:
CVTQF
CVTQG

Convert Quadword to F_floating
Convert Quadword to G_floating

Qualifiers:
Rounding:

Chopped (/C)

Description:
The two’s-complement quadword operand in register Fb is converted to a single- or double-precision floating result and written to register Fc. The conversion complements a number
if negative, normalizes it, rounds to the target precision, and packs the result with an appropriate sign and exponent field. Register Fa must be F31.

Instruction Descriptions 4–115

4.10.11 Convert VAX Floating to VAX Floating
Format:
CVTxy

Fb.rx,Fc.wx

!Floating-point Operate format

Operation:
Fc ← {conversion of Fbv}

Exceptions:
Invalid Operation
Overflow
Underflow

Instruction mnemonics:
CVTDG
CVTGD
CVTGF

Convert D_floating to G_floating
Convert G_floating to D_floating
Convert G_floating to F_floating

Qualifiers:
Rounding:
Trapping:

Chopped (/C)
Exception Completion (/S)
Underflow Enable (/U)

Description:
The floating operand in register Fb is converted to the specified alternate floating format and
written to register Fc. Register Fa must be F31.
An invalid operation trap is signaled if the operand has exp=0 and is not a true zero (that is,
VAX reserved operands and dirty zeros trap). The contents of Fc are UNPREDICTABLE if
this occurs.
See Section 4.7.7 for details of the stored result on overflow or underflow.

Notes:
•

The only arithmetic operations on D_floating values are conversions to and from
G_floating. The conversion to G_floating rounds or chops as specified, removing three
fraction bits. The conversion from G_floating to D_floating adds three low-order zeros
as fraction bits, then the 8-bit exponent range is checked for overflow/underflow.

•

The conversion from G_floating to F_floating rounds or chops to single precision, then
the 8-bit exponent range is checked for overflow/underflow.

•

No conversion from F_floating to G_floating is required, since F_floating values are
always stored in registers as equivalent G_floating values.

4–116 Alpha Architecture Handbook

4.10.12 Convert IEEE Floating to Integer
Format:
CVTTQ

Fb.rx,Fc.wq

!Floating-point Operate format

Operation:
Fc ← {conversion of Fbv}

Exceptions:
Invalid Operation
Inexact Result
Integer Overflow

Instruction mnemonics:
CVTTQ

Convert T_floating to Quadword

Qualifiers:
Rounding:

Trapping:

Dynamic (/D)
Minus infinity (/M)
Chopped (/C)
Exception Completion (/S)
Integer Overflow Enable (/V)
Inexact Enable (/I)

Description:
The floating operand in register Fb is converted to a two’s-complement number and written to
register Fc. The conversion aligns the operand fraction with the binary point just to the right of
bit zero, rounds as specified, and complements the result if negative. Register Fa must be F31.
See Section 4.7.7 for details of the stored result on integer overflow and inexact result.

Instruction Descriptions 4–117

4.10.13 Convert Integer to IEEE Floating
Format:
CVTQy

Fb.rq,Fc.wx

!Floating-point Operate format

Operation:
Fc ← {conversion of Fbv<63:0>}

Exceptions:
Inexact Result

Instruction mnemonics:
CVTQS
CVTQT

Convert Quadword to S_floating
Convert Quadword to T_floating

Qualifiers:
Rounding:

Trapping:

Dynamic (/D)
Minus infinity (/M)
Chopped (/C)
Exception Completion (/S)
Inexact Enable (/I)

Description:
The two’s-complement operand in register Fb is converted to a single- or double-precision
floating result and written to register Fc. The conversion complements a number if negative,
normalizes it, rounds to the target precision, and packs the result with an appropriate sign and
exponent field. Register Fa must be F31.
See Section 4.7.7 for details of the stored result on inexact result.

Notes:
•

In order to use CVTQS or CVTQT with exception completion handling, it is necessary
to specify the /SUI IEEE trap mode, even though an underflow trap is not possible.

4–118 Alpha Architecture Handbook

4.10.14 Convert IEEE S_Floating to IEEE T_Floating
Format:
CVTST

Fb.rx,Fc.wx

! Floating-point Operate format

Operation:
Fc ← {conversion of Fbv}

Exceptions:
Invalid Operation

Instruction mnemonics:
CVTST

Convert S_floating to T_floating

Qualifiers:
Trapping:

Exception Completion (/S)

Description:
The S_floating operand in register Fb is converted to T_floating format and written to register
Fc. Register Fa must be F31.

Notes:
•

The conversion from S_floating to T_floating is exact. No rounding occurs. No underflow, overflow, or inexact result can occur. In fact, the conversion for finite values is the
identity transformation.

•

A trap handler can convert an S_floating denormal value into the corresponding
T_floating finite value by adding 896 to the exponent and normalizing.

Instruction Descriptions 4–119

4.10.15 Convert IEEE T_Floating to IEEE S_Floating
Format:
CVTTS

Fb.rx,Fc.wx

!Floating-point Operate format

Operation:
Fc ← {conversion of Fbv}

Exceptions:
Invalid Operation
Overflow
Underflow
Inexact Result

Instruction mnemonics:
CVTTS

Convert T_floating to S_floating

Qualifiers:
Rounding:

Trapping:

Dynamic (/D)
Minus infinity (/M)
Chopped (/C)
Exception Completion (/S)
Underflow Enable (/U)
Inexact Enable (/I)

Description:
The T_floating operand in register Fb is converted to S_floating format and written to register
Fc. Register Fa must be F31.
See Section 4.7.7 for details of the stored result on overflow, underflow, or inexact result.

4–120 Alpha Architecture Handbook

4.10.16 VAX Floating Divide
Format:
DIVx

Fa.rx,Fb.rx,Fc.wx

!Floating-point Operate format

Operation:
Fc ←

Fav / Fbv

Exceptions:
Invalid Operation
Division by Zero
Overflow
Underflow

Instruction mnemonics:
DIVF
DIVG

Divide F_floating
Divide G_floating

Qualifiers:
Rounding:
Trapping:

Chopped (/C)
Exception Completion (/S)
Underflow Enable (/U)

Description:
The dividend operand in register Fa is divided by the divisor operand in register Fb and the
quotient is written to register Fc.
The quotient is rounded or chopped to the specified precision and then the corresponding range
is checked for overflow/underflow. The single-precision operation on canonical single-precision values produces a canonical single-precision result.
An invalid operation trap is signaled if either operand has exp=0 and is not a true zero (that is,
VAX reserved operands and dirty zeros trap). The contents of Fc are UNPREDICTABLE if
this occurs.
A division by zero trap is signaled if Fbv is zero. The contents of Fc are UNPREDICTABLE if
this occurs.
See Section 4.7.7 for details of the stored result on overflow or underflow.

Instruction Descriptions 4–121

4.10.17 IEEE Floating Divide
Format:
DIVx

Fa.rx,Fb.rx,Fc.wx

!Floating-point Operate format

Operation:
Fc ← Fav / Fbv

Exceptions:
Invalid Operation
Division by Zero
Overflow
Underflow
Inexact Result

Instruction mnemonics:
DIVS
DIVT

Divide S_floating
Divide T_floating

Qualifiers:
Rounding:

Trapping:

Dynamic (/D)
Minus infinity (/M)
Chopped (/C)
Exception Completion (/S)
Underflow Enable (/U)
Inexact Enable (/I)

Description:
The dividend operand in register Fa is divided by the divisor operand in register Fb and the
quotient is written to register Fc.
The quotient is rounded to the specified precision and then the corresponding range is checked
for overflow/underflow. The single-precision operation on canonical single-precision values
produces a canonical single-precision result.
See Section 4.7.7 for details of the stored result on overflow, underflow, or inexact result.

4–122 Alpha Architecture Handbook

4.10.18 Floating-Point Register to Integer Register Move
Format:
FTOIx

Fa.rq,Rc.wq

!Floating-point Operate format

Operation:
CASE:
FTOIS:
Rc<63:32> ← SEXT(Fav<63>)
Rc<31:0> ← Fav<63:62> || Fav <58:29>
FTOIT:
Rc <- Fav
ENDCASE

Exceptions:
None

Instruction mnemonics:
FTOIS
FTOIT

Floating-point to Integer Register Move, S_floating
Floating-point to Integer Register Move, T_floating

Qualifiers:
None

Description:
Data in a floating-point register file is moved to an integer register file.
The Fb field must be F31.
The instructions do not interpret bits in the register files; specifically, the instructions do not
trap on non-finite values. Also, the instructions do not access memory.
FTOIS is exactly equivalent to the sequence:
STS
LDL

FTOIT is exactly equivalent to the sequence:
STT
LDQ

Software Note:
FTOIS and FTOIT are no slower than the corresponding store/load sequence and can be
significantly faster.

Instruction Descriptions 4–123

4.10.19 Integer Register to Floating-Point Register Move
Format:
ITOFx

Ra.rq,Fc.wq

!Floating-point Operate format

Operation:
CASE:
ITOFF:
Fc ← Rav<31> || MAP_F(Rav<30:23> || Rav<22:0> || 0<28:0>
ITOFS:
Fc ← Rav<31> || MAP_S(Rav<30:23> || Rav<22:0> || 0<28:0>
ITOFT:
Fc <- Rav
ENDCASE

Exceptions:
None

Instruction mnemonics:
ITOFF
ITOFS
ITOFT

Integer to Floating-point Register Move, F_floating
Integer to Floating-point Register Move, S_floating
Integer to Floating-point Register Move, T_floating

Qualifiers:
None

Description:
Data in an integer register file is moved to a floating-point register file.
The Rb field must be R31.
The instructions do not interpret bits in the register files; specifically, the instructions do not
trap on non-finite values. Also, the instructions do not access memory.
ITOFF is equivalent to the following sequence, except that the word swapping that LDF normally performs is not performed by ITOFF:
STL
LDF

4–124 Alpha Architecture Handbook

ITOFS is exactly equivalent to the sequence:
STL
LDS

ITOFT is exactly equivalent to the sequence:
STQ
LDT

Software Note:
ITOFF, ITOFS, and ITOFT are no slower than the corresponding store/load sequence and
can be significantly faster.

Instruction Descriptions 4–125

4.10.20 VAX Floating Multiply
Format:
MULx

Fa.rx,Fb.rx,Fc.wx

!Floating-point Operate format

Operation:
Fc ← Fav * Fbv

Exceptions:
Invalid Operation
Overflow
Underflow

Instruction mnemonics:
MULF
MULG

Multiply F_floating
Multiply G_floating

Qualifiers:
Rounding:
Trapping:

Chopped (/C)
Exception Completion (/S)
Underflow Enable (/U)

Description:
The multiplicand operand in register Fb is multiplied by the multiplier operand in register Fa
and the product is written to register Fc.
The product is rounded or chopped to the specified precision and then the corresponding range
is checked for overflow/underflow. The single-precision operation on canonical single-precision values produces a canonical single-precision result.
An invalid operation trap is signaled if either operand has exp=0 and is not a true zero (that is,
VAX reserved operands and dirty zeros trap). The contents of Fc are UNPREDICTABLE if
this occurs.
See Section 4.7.7 for details of the stored result on overflow or underflow.

4–126 Alpha Architecture Handbook

4.10.21 IEEE Floating Multiply
Format:
MULx

Fa.rx,Fb.rx,Fc.wx

!Floating-point Operate format

Operation:
Fc ← Fav * Fbv

Exceptions:
Invalid Operation
Overflow
Underflow
Inexact Result

Instruction mnemonics:
MULS
MULT

Multiply S_floating
Multiply T_floating

Qualifiers:
Rounding:

Trapping:

Dynamic (/D)
Minus infinity (/M)
Chopped (/C)
Exception Completion (/S)
Underflow Enable (/U)
Inexact Enable (/I)

Description:
The multiplicand operand in register Fb is multiplied by the multiplier operand in register Fa
and the product is written to register Fc.
The product is rounded to the specified precision and then the corresponding range is checked
for overflow/underflow. The single-precision operation on canonical single-precision values
produces a canonical single-precision result.
See Section 4.7.7 for details of the stored result on overflow, underflow, or inexact result.

Instruction Descriptions 4–127

4.10.22 VAX Floating Square Root
Format:
SQRTx

Fb.rx,Fc.wx

!Floating-point Operate format

Operation:
Fc ← Fb ** (1/2)

Exceptions:
Invalid operation

Instruction mnemonics:
SQRTF
SQRTG

Square root F_floating
Square root G_floating

Qualifiers:
Rounding:
Trapping:

Chopped (/C)
Exception Completion (/S)
Underflow Enable (/U) — See Notes below

Description:
The square root of the floating-point operand in register Fb is written to register Fc. (The Fa
field of this instruction must be set to a value of F31.)
The result is rounded or chopped to the specified precision. The single-precision operation on a
canonical single-precision value produces a canonical single-precision result.
An invalid operation is signaled if the operand has exp=0 and is not a true zero (that is, VAX
reserved operands and dirty zeros trap). An invalid operation is signaled if the sign of the operand is negative.
The contents of the Fc are UNPREDICTABLE if an invalid operation is signaled.

Notes:
•

Floating-point overflow and underflow are not possible for square root operation. The
underflow enable qualifier is ignored.

4–128 Alpha Architecture Handbook

4.10.23 IEEE Floating Square Root
Format:
SQRTx

Fb.rx,Fc.wx

!Floating-point Operate format

Operation:
Fc ← Fb ** (1/2)

Exceptions:
Inexact result
Invalid operation

Instruction mnemonics:
SQRTS
SQRTT

Square root S_floating
Square root T_floating

Qualifiers:
Rounding:

Trapping:

Chopped (/C)
Dynamic (/D)
Minus infinity (/M)
Inexact Enable (/I)
Exception Completion (/S)
Underflow Enable (/U) — See Notes below

Description:
The square root of the floating-point operand in register Fb is written to register Fc. (The Fa
field of this instruction must be set to a value of F31.)
The result is rounded to the specified precision. The single-precision operation on a canonical
single-precision value produces a canonical single-precision result.
An invalid operation is signaled if the sign of the operand is less than zero. However, SQRT
(–0) produces a result of –0.

Notes:
•

Floating-point overflow and underflow are not possible for square root operation. The
underflow enable qualifier is ignored.

Instruction Descriptions 4–129

4.10.24 VAX Floating Subtract
Format:
SUBx

Fa.rx,Fb.rx,Fc.wx

!Floating-point Operate format

Operation:
Fc ← Fav - Fbv

Exceptions:
Invalid Operation
Overflow
Underflow

Instruction mnemonics:
SUBF
SUBG

Subtract F_floating
Subtract G_floating

Qualifiers:
Rounding:
Trapping:

Chopped (/C)
Exception Completion (/S)
Underflow Enable (/U)

Description:
The subtrahend operand in register Fb is subtracted from the minuend operand in register Fa
and the difference is written to register Fc.
The difference is rounded or chopped to the specified precision and then the corresponding
range is checked for overflow/underflow. The single-precision operation on canonical single-precision values produces a canonical single-precision result.
An invalid operation trap is signaled if either operand has exp=0 and is not a true zero (that is,
VAX reserved operands and dirty zeros trap). The contents of Fc are UNPREDICTABLE if
this occurs.
See Section 4.7.7 for details of the stored result on overflow or underflow.

4–130 Alpha Architecture Handbook

4.10.25 IEEE Floating Subtract
Format:
SUBx

Fa.rx,Fb.rx,Fc.wx

!Floating-point Operate format

Operation:
Fc ← Fav - Fbv

Exceptions:
Invalid Operation
Overflow
Underflow
Inexact Result

Instruction mnemonics:
SUBS
SUBT

Subtract S_floating
Subtract T_floating

Qualifiers:
Rounding:

Trapping:

Dynamic (/D)
Minus infinity (/M)
Chopped (/C)
Exception Completion (/S)
Underflow Enable (/U)
Inexact Enable (/I)

Description:
The subtrahend operand in register Fb is subtracted from the minuend operand in register Fa
and the difference is written to register Fc.
The difference is rounded to the specified precision and then the corresponding range is
checked for overflow/underflow. The single-precision operation on canonical single-precision
values produces a canonical single-precision result.
See Section 4.7.7 for details of the stored result on overflow, underflow, or inexact result.

Instruction Descriptions 4–131

4.11 Miscellaneous Instructions
Alpha provides the miscellaneous instructions shown in Table 4–17.

Table 4–17: Miscellaneous Instructions Summary
Mnemonic

Operation

AMASK

Architecture Mask

CALL_PAL

Call Privileged Architecture Library Routine

ECB

Evict Cache Block

EXCB

Exception Barrier

FETCH

Prefetch Data

FETCH_M

Prefetch Data, Modify Intent

IMPLVER

Implementation Version

Memory Barrier

RPCC

Read Processor Cycle Counter

TRAPB

Trap Barrier

WH64

Write Hint — 64 Bytes

WMB

Write Memory Barrier

4–132 Alpha Architecture Handbook

4.11.1 Architecture Mask
Format:
AMASK

Rb.rq,Rc.wq

!Operate format

AMASK

#b.ib,Rc.wq

!Operate format

Operation:
Rc ← Rbv AND {NOT CPU_feature_mask}

Exceptions:
None

Instruction mnemonics:
AMASK

Architecture Mask

Qualifiers:
None

Description:
Rbv represents a mask of the requested architectural extensions. Bits are cleared that correspond to architectural extensions that are present. Reserved bits and bits that correspond to
absent extensions are copied unchanged. In either case, the result is placed in Rc. If the result
is zero, all requested features are present.
Software may specify an Rbv of all 1’s to determine the complete set of architectural extensions implemented by a processor. Assigned bit definitions are located in Section D.3.
Ra must be R31 or the result in Rc is UNPREDICTABLE and it is UNPREDICTABLE
whether an exception is signaled.

Software Note:
Use this instruction to make instruction-set decisions; use IMPLVER to make code-tuning
decisions.

Implementation Note:
Instruction encoding is implemented as follows:

•

On 21064/21064A/21066/21068/21066A (EV4/EV45/LCA/LCA45 chips), AMASK
copies Rbv to Rc.

•

On 21164 (EV5), AMASK copies Rbv to Rc.

Instruction Descriptions 4–133

•

On 21164A (EV56), 21164PC (PCA56), and 21264 (EV6), AMASK correctly indicates
support for architecture extensions by copying Rbv to Rc and clearing appropriate bits.

Bits are assigned and placed in Appendix D for architecture extensions as ECOs for those
extensions are passed. The low 8 bits are reserved for standard architecture extensions so
they can be tested with a literal; application-specific extensions are assigned from bit 8
upward.

4–134 Alpha Architecture Handbook

4.11.2 Call Privileged Architecture Library
Format:
CALL_PAL

!PAL format

fnc.ir

Operation:
{Stall instruction issuing until all
prior instructions are guaranteed to
complete without incurring exceptions.}
{Trap to PALcode.}

Exceptions:
None

Instruction mnemonics:
CALL_PAL

Call Privileged Architecture Library

Qualifiers:
None

Description:
The CALL_PAL instruction is not issued until all previous instructions are guaranteed to complete without exceptions. If an exception occurs, the continuation PC in the exception stack
frame points to the CALL_PAL instruction. The CALL_PAL instruction causes a trap to
PALcode.

Instruction Descriptions 4–135

4.11.3 Evict Data Cache Block
Format:
ECB

(Rb.ab)

! Memory format

Operation:
va ← Rbv
IF { va maps to memory space } THEN
Prepare to reuse cache resources that are occupied by the
the addressed byte.
END

Exceptions:
None

Instruction mnemonics:
ECB

Evict Cache Block

Qualifiers:
None

Description:
The ECB instruction provides a hint that the addressed location will not be referenced again in
the near future, so any cache space it occupies should be made available to cache other memory locations. If the cache copy of the location is dirty, the processor may start writing it back;
if the cache has multiple sets, the processor may arrange for the set containing the addressed
byte to be the next set allocated.
The ECB instruction does not generate exceptions; if it encounters data address translation
errors (access violation, translation not valid, and so forth) during execution, it is treated as a
NOP.
If the address maps to non-memory-like (I/O) space, ECB is treated as a NOP.

Software Note:
•

ECB makes a particular cache location available for reuse by evicting and invalidating
its contents. The intent is to give software more control over cache allocation policy in
set-associative caches so that "useful" blocks can be retained in the cache.

•

ECB is a performance hint — it does not serialize the eviction of the addressed cache
block with any preceding or following memory operation.

4–136 Alpha Architecture Handbook

•

ECB is not intended for flushing caches prior to power failure or low power operation
— CFLUSH is intended for that purpose.

Implementation Note:
Implementations with set-associative caches are encouraged to update their allocation
pointer so that the next D-stream reference that misses the cache and maps to this line is
allocated into the vacated set.

Instruction Descriptions 4–137

4.11.4 Exception Barrier
Format:
! Memory format

EXCB

Operation:
{EXCB does not appear to issue until completion of all
exceptions and dependencies on the Floating-point Control
Register (FPCR) from prior instructions.}

Exceptions:
None

Instruction mnemonics:
EXCB

Exception Barrier

Qualifiers:
None

Description:
The EXCB instruction allows software to guarantee that in a pipelined implementation, all previous instructions have completed any behavior related to exceptions or rounding modes before
any instructions after the EXCB are issued.
In particular, all changes to the Floating-point Control Register (FPCR) are guaranteed to have
been made, whether or not there is an associated exception. Also, all potential floating-point
exceptions and integer overflow exceptions are guaranteed to have been taken. EXCB is thus a
superset of TRAPB.
If a floating-point exception occurs for which trapping is enabled, the EXCB instruction acts
like a fault. In this case, the value of the Program Counter reported to the program may be the
address of the EXCB instruction (or earlier) but is never the address of an instruction following the EXCB.
The relationship between EXCB and the FPCR is described in Section 4.7.8.1.

4–138 Alpha Architecture Handbook

4.11.5 Prefetch Data
Format:
FETCHx

0(Rb.ab)

!Memory format

Operation:
va ← {Rbv}
{Optionally prefetch aligned 512-byte block surrounding va.}

Exceptions:
None

Instruction mnemonics:
FETCH
FETCH_M

Prefetch Data
Prefetch Data, Modify Intent

Qualifiers:
None

Description:
The virtual address is given by Rbv. This address is used to designate an aligned 512-byte
block of data. An implementation may optionally attempt to move all or part of this block (or a
larger surrounding block) of data to a part of the memory hierarchy that has faster-access, in
anticipation of subsequent Load or Store instructions that access that data.

Implementation Note:
FETCHx is intended to help software overlap memory latencies when such latencies are on
the order of at least 100 cycles. FETCHx is unlikely to help (or be implemented) for
significantly shorter memory latencies. Code scheduling and cache-line prefetching (See
Section A.3.5) should be used to overlap such shorter latencies.
Existing Alpha implementations (through the 21264) have memory latencies that are too
short to profitably implement FETCHx. Therefore, FETCHx does not improve memory
performance in existing Alpha implementations.
The FETCH instruction is a hint to the implementation that may allow faster execution. An
implementation is free to ignore the hint. If prefetching is done in an implementation, the order
of fetch within the designated block is UNPREDICTABLE.
The FETCH_M instruction gives the additional hint that modifications (stores) to some or all
of the data block are anticipated.

Instruction Descriptions 4–139

No exceptions are generated by FETCHx. If a Load (or Store in the case of FETCH_M) that
uses the same address would fault, the prefetch request is ignored. It is UNPREDICTABLE
whether a TB-miss fault is ever taken by FETCHx.

Implementation Note:
Implementations are encouraged to take the TB-miss fault, then continue the prefetch.

4–140 Alpha Architecture Handbook

4.11.6 Implementation Version
Format:
IMPLVER

!Operate format

Operation:
Rc ← value, which is defined in Appendix D

Exceptions:
None

Instruction mnemonics:
IMPLVER

Implementation Version

Description:
A small integer is placed in Rc that specifies the major implementation version of the processor on which it is executed. This information can be used to make code-scheduling or tuning
decisions, or the information can be used to branch to different pieces of code optimized for
different implementations.

Notes:
•

The value returned by IMPLVER does not identify the particular processor type.
Rather, it identifies a group of processors that can be treated similarly for performance
characteristics such as scheduling. Ra must be R31 and Rb must be the literal #1 or the
result in Rc is UNPREDICTABLE and it is UNPREDICTABLE whether an exception
is signaled.

Software Note:
Use this instruction to make code-tuning decisions; use AMASK to make instruction-set
decisions.

Instruction Descriptions 4–141

4.11.7 Memory Barrier
Format:
!Memory format

Operation:
{Guarantee that all subsequent loads or stores
will not access memory until after all previous
loads and stores have accessed memory, as
observed by other processors.}

Exceptions:
None

Instruction mnemonics:
MB

Memory Barrier

Qualifiers:
None

Description:
The use of the Memory Barrier (MB) instruction is required only in multiprocessor systems.
In the absence of an MB instruction, loads and stores to different physical locations are
allowed to complete out of order on the issuing processor as observed by other processors. The
MB instruction allows memory accesses to be serialized on the issuing processor as observed
by other processors. See Chapter 5 for details on using the MB instruction to serialize these
accesses. Chapter 5 also details coordinating memory accesses across processors.
Note that MB ensures serialization only; it does not necessarily accelerate the progress of
memory operations.

4–142 Alpha Architecture Handbook

4.11.8 Read Processor Cycle Counter
Format:
RPCC

!Memory format

Ra.wq

Operation:
Ra ← {cycle counter}

Exceptions:
None

Instruction mnemonics:
RPCC

Read Processor Cycle Counter

Qualifiers:
None

Description:
Register Ra is written with the processor cycle counter (PCC). The PCC register consists of
two 32-bit fields. The low-order 32 bits (PCC<31:0>) are an unsigned, wrapping counter,
PCC_CNT. The high-order 32 bits (PCC<63:32>), PCC_OFF, are operating-system dependent in their implementation.
See Section 3.1.5 for a description of the PCC.
If an operating system uses PCC_OFF to calculate the per-process or per-thread cycle count,
that count must be derived from the 32-bit sum of PCC_OFF and PCC_CNT. The following
example computes that cycle count, modulo 2**32, and returns the count value in R0. Notice
the care taken not to cause an unwanted sign extension.
RPCC
SLL
ADDQ
SRL

R0
R0, #32, R1
R0, R1, R0
R0, #32, R0

; Read the process cycle counter
; Line up the offset and count fields
; Do add
; Zero extend the count to 64 bits

The following example code returns the value of PCC_CNT in R0<31:0> and all zeros in
R0<63:32>.
RPCC R0
ZAPNOT R0,#15,R0

Instruction Descriptions 4–143

4.11.9 Trap Barrier
Format:
!Memory format

TRAPB

Operation:
{TRAPB does not appear to issue until all prior instructions
are guaranteed to complete without causing any arithmetic traps}.

Exceptions:
None

Instruction mnemonics:
TRAPB

Trap Barrier

Qualifiers:
None

Description:
The TRAPB instruction allows software to guarantee that in a pipelined implementation, all
previous arithmetic instructions will complete without incurring any arithmetic traps before the
TRAPB or any instructions after it are issued.
If an arithmetic exception occurs for which trapping is enabled, the TRAPB instruction acts
like a fault. In this case, the value of the Program Counter reported to the program may be the
address of the TRAPB instruction (or earlier) but is never the address of the instruction following the TRAPB.
This fault behavior by TRAPB allows software, using one TRAPB instruction for each exception domain, to isolate the address range in which an exception occurs. If the address of the
instruction following the TRAPB were allowed, there would be no way to distinguish an
exception in the address range preceding a label from an exception in the range that includes
the label along with the faulting instruction and a branch back to the label. This case arises
when the code is not following exception completion rules but is inserting TRAPB instructions to isolate exceptions to the proper scope.
Use of TRAPB should be compared with use of the EXCB instruction; see Section 4.11.4.

4–144 Alpha Architecture Handbook

4.11.10 Write Hint
Format:
WH64

! Memory format

(Rb.ab)

Operation:
va ← Rbv
IF { va maps to memory space } THEN
Write UNPREDICTABLE data to the aligned 64-byte region
containing the addressed byte.
END

Exceptions:
None

Instruction mnemonics:
WH64

Write Hint - 64 Bytes

Qualifiers:
None

Description:
The WH64 instruction provides a hint that the current contents of the aligned 64-byte block
containing the addressed byte will never be read again but will be overwritten in the near
future.
The processor may allocate cache resources to hold the block without reading its previous contents from memory; the contents of the block may be set to any value that does not introduce a
security hole, as described in Section 1.6.3.
The WH64 instruction does not generate exceptions; if it encounters data address translation
errors (access violation, translation not valid, and so forth), it is treated as a NOP.
If the address maps to non-memory-like (I/O) space, WH64 is treated as a NOP.

Software Note:
This instruction is a performance hint that should be used when writing a large continuous
region of memory. The intended code sequence consists of one WH64 instruction followed
by eight quadword stores for each aligned 64-byte region to be written.
Sometimes, the UNPREDICTABLE data will exactly match some or all of the previous
contents of the addressed block of memory.

Instruction Descriptions 4–145

Implementation Note:

If the 64-byte region containing the addressed byte is not in the data cache,
implementations are encouraged to allocate the region in the data cache without first
reading it from memory. However, if any of the addressed bytes exist in the caches of
other processors, they must be kept coherent with respect to those processors.
Processors with cache blocks smaller than 64 bytes are encouraged to implement WH64 as
defined. However, they may instead implement the instruction by allocating a smaller
aligned cache block for write access or by treating WH64 as a NOP.
Processors with cache blocks larger than 64 bytes are also encouraged to implement WH64
as defined. However, they may instead treat WH64 as a NOP.

4–146 Alpha Architecture Handbook

4.11.11 Write Memory Barrier
Format:
!Memory format

WMB

Operation:
{ Guarantee that
{ All preceding stores that access memory-like
{
regions are ordered before any subsequent stores
{
that access memory-like regions and
{ All preceding stores that access non-memory-like
{
regions are ordered before any subsequent stores
{
that access non-memory-like regions.

Exceptions:
None

Instruction mnemonics:
WMB

Write Memory Barrier

Qualifiers:
None

Description:
The WMB instruction provides a way for software to control write buffers. It guarantees that
writes preceding the WMB are not aggregated with writes that follow the WMB.
WMB guarantees that writes to memory-like regions that precede the WMB are ordered before
writes to memory-like regions that follow the WMB. Similarly, WMB guarantees that writes to
non-memory-like regions that precede the WMB are ordered before writes to non-memory-like regions that follow the WMB. It does not order writes to memory-like regions relative
to writes to non-memory-like regions.
WMB causes writes that are contained in buffers to be completed without unnecessary delay. It
is particularly suited for batching writes to high-performance I/O devices.
WMB prevents writes that precede the WMB from being merged with writes that follow the
WMB. In particular, two writes that access the same location and are separated by a WMB
cause two distinct and ordered write events.
In the absence of a WMB (or IMB or MB) instruction, stores to memory-like or non-memory-like regions can be aggregated and/or buffered and completed in any order.

Instruction Descriptions 4–147

The WMB instruction is the preferred method for providing high-bandwidth write streams
where order must be preserved between writes in that stream.
Notes:

WMB is useful for ordering streams of writes to a non-memory-like region, such as to memory-mapped control registers or to a graphics frame buffer. While both MB and WMB can
ensure that writes to a non-memory-like region occur in order, without being aggregated or
reordered, the WMB is usually faster and is never slower than MB.
WMB can correctly order streams of writes in programs that operate on shared sections of data
if the data in those sections are protected by a classic semaphore protocol. The following
example illustrates such a protocol:

Processor i

Processor j

⇒

<Acquire lock>
MB
<Read and write data in shared section>
WMB

The example above is similar to that in Section 5.5.4, except a WMB is substituted for the second MB in the lock-update-release sequence. It is correct to substitute WMB for the second
MB only if:
1. All data locations that are read or written in the critical section are accessed only after
acquiring a software lock by using lock_variable (and before releasing the software
lock).
2. For each read u of shared data in the critical section, there is a write v such that:
a. v is BEFORE the WMB
b. v follows u in processor issue sequence (see Section 5.6.1.1)
c. v either depends on u (see Section 5.6.1.7) or overlaps u (see Section 5.6.1), or
both.
3. Both lock_variable and all the shared data are in memory-like regions (or lock_variable
and all the shared data are in non-memory-like regions). If the lock_variable is in a
non-memory-like region, the atomic lock protocol must use some implementation-specific hardware support.
The substitution of a WMB for the second MB is usually faster and never slower.

4–148 Alpha Architecture Handbook

4.12 VAX Compatibility Instructions
Alpha provides the instructions shown in Table 4–18 for use in translated VAX code. These
instructions are not a permanent part of the architecture and will not be available in some
future implementations. They are intended to preserve customer assumptions about VAX
instruction atomicity in porting code from VAX to Alpha.
These instructions should be generated only by the VAX-to-Alpha software translator; they
should never be used in native Alpha code. Any native code that uses them may cease to work.

Table 4–18: VAX Compatibility Instructions Summary
Mnemonic

Operation

Read and Clear

Read and Set

Instruction Descriptions 4–149

4.12.1 VAX Compatibility Instructions
Format:
Rx

!Memory format

Ra.wq

Operation:
Ra ← intr_flag
intr_flag ← 0
intr_flag ← 1

!RC
!RS

Exceptions:
None

Instruction mnemonics:
RC
RS

Read and Clear
Read and Set

Qualifiers:
None

Description:
The intr_flag is returned in Ra and then cleared to zero (RC) or set to one (RS).
These instructions may be used to determine whether the sequence of Alpha instructions
between RS and RC (corresponding to a single VAX instruction) was executed without interruption or exception.
Intr_flag is a per-processor state bit. The intr_flag is cleared if that processor encounters a
CALL_PAL REI instruction.
It is UNPREDICTABLE whether a processor’s intr_flag is affected when that processor executes an LDx_L or STx_C instruction. A processor’s intr_flag is not affected when that
processor executes a normal load or store instruction.
A processor’s intr_flag is not affected when that processor executes a taken branch.

Notes:
•

These instructions are intended only for use by the VAX-to-Alpha software translator;
they should never be used by native code.

4–150 Alpha Architecture Handbook

4.13 Multimedia (Graphics and Video) Support
Alpha provides the following instructions that enhance support for graphics and video
algorithms:

Mnemonic

Operation

MINUB8

Vector Unsigned Byte Minimum

MINSB8

Vector Signed Byte Minimum

MINUW4

Vector Unsigned Word Minimum

MINSW4

Vector Signed Word Minimum

MAXUB8

Vector Unsigned Byte Maximum

MAXSB8

Vector Signed Byte Maximum

MAXUW4

Vector Unsigned Word Maximum

MAXSW4

Vector Signed Word Maximum

PERR

Pixel Error

PKLB

Pack Longwords to Bytes

PKWB

Pack Words to Bytes

UNPKBL

Unpack Bytes to Longwords

UNPKBW

Unpack Bytes to Words

The MIN and MAX instructions allow the clamping of pixel values to maximium values that
are allowed in different standards and stages of the CODECs.
The PERR instruction accelerates the macroblock search in motion estimation.
The pack and unpack (PKxB and UNPKBx) instructions accelerate the blocking of interleaved
YUV coordinates for processing by the CODEC.

Implementation Note:
Alpha processors for which the AMASK instruction returns bit 8 set implement these
instructions. Those processors for which AMASK does not return bit 8 set can take an
Illegal Instruction trap, and software can emulate their function, if required.

Instruction Descriptions 4–151

4.13.1 Byte and Word Minimum and Maximum
Format:
MINxxx

Ra.rq,Rb.rq,Rc.wq
Ra.rq,#b.ib,Rc.wq

! Operate Format

MAXxxx

Ra.rq,Rb.rq,Rc.wq
Ra.rq,#b.ib,Rc.wq

! Operate Format

Operation:
CASE
MINUB8:
FOR i FROM 0 TO 7
Rcv<i*8+7:i*8> = MINU(Rav<i*8+7:i*8>,Rbv<i*8+7:i*8>)
END
MINSB8:
FOR i FROM 0 TO 7
Rcv<i*8+7:i*8> = MINS(Rav<i*8+7:i*8>,Rbv<i*8+7:i*8>)
END
MINUW4:
FOR i FROM 0 TO 3
Rcv<i*16+15:i*16> = MINU(Rav<i*16+15:i*16>,Rbv<i*16+15:i*16>)
END
MINSW4:
FOR i FROM 0 TO 3
Rcv<i*16+15:i*16> = MINS(Rav<i*16+15:i*16>,Rbv<i*16+15:i*16>)
END
MAXUB8:
FOR i FROM 0 TO 7
Rcv<i*8+7:i*8> = MAXU(Rav<i*8+7:i*8>,Rbv<i*8+7:i*8>)
END
MAXSB8:
FOR i FROM 0 TO 7
Rcv<i*8+7:i*8> = MAXS(Rav<i*8+7:i*8>,Rbv<i*8+7:i*8>)
END
MAXUW4:
FOR i FROM 0 TO 3
Rcv<i*16+15:i*16> = MAXU(Rav<i*16+15:i*16>,Rbv<i*16+15:i*16>)
END
MAXSW4:
FOR i FROM 0 TO 3
Rcv<i*16+15:i*16> = MAXS(Rav<i*16+15:i*16>,Rbv<i*16+15:i*16>)
END
ENDCASE:

Exceptions:
None

4–152 Alpha Architecture Handbook

Instruction mnemonics:
MINUB8
MINSB8
MINUW4
MINSW4
MAXUB8
MAXSB8
MAXUW4
MAXSW4

Vector Unsigned Byte Minimum
Vector Signed Byte Minimum
Vector Unsigned Word Minimum
Vector Signed Word Minimum
Vector Unsigned Byte Maximum
Vector Signed Byte Maximum
Vector Unsigned Word Maximum
Vector Signed Word Maximum

Qualifiers:
None

Description:
For MINxB8, each byte of Rc is written with the smaller of the corresponding bytes of Ra or
Rb. The bytes may be interpreted as signed or unsigned values.
For MINxW4, each word of Rc is written with the smaller of the corresponding words of Ra or
Rb. The words may be interpreted as signed or unsigned values.
For MAXxB8, each byte of Rc is written with the larger of the corresponding bytes of Ra or
Rb. The bytes may be interpreted as signed or unsigned values.
For MAXxW4, each word of Rc is written with the larger of the corresponding words of Ra or
Rb. The words may be interpreted as signed or unsigned values.

Instruction Descriptions 4–153

4.13.2 Pixel Error
Format:
PERR

Ra.rq,Rb.rq,Rc.wq

! Operate Format

Operation:
temp = 0
FOR i FROM 0 TO 7
IF { Rav<i*8+7:i*8> GEU Rbv<i*8+7:i*8>} THEN
temp ← temp + (Rav<i*8+7:i*8> - Rbv<i*8+7:i*8>)
ELSE
temp ← temp + (Rbv<i*8+7:i*8> - Rav<i*8+7:i*8>)
END
Rc ← temp

Exceptions:
None

Instruction mnemonics:
PERR

Pixel Error

Qualifiers:
None

Description:
The absolute value of the difference between each of the bytes in Ra and Rb is calculated. The
sum of the resulting bytes is written to Rc.

4–154 Alpha Architecture Handbook

4.13.3 Pack Bytes
Format:
PKxB

Rb.rq,Rc.wq

! Operate Format

Operation:
CASE
PKLB:
BEGIN
Rc<07:00> ← Rbv<07:00>
Rc<15:08> ← Rbv<39:32>
Rc<63:16> ← 0
END
PKWB:
BEGIN
Rc<07:00> ← Rbv<07:00>
Rc<15:08> ← Rbv<23:16>
Rc<23:16> ← Rbv<39:32>
Rc<31:24> ← Rbv<55:48>
Rc<63:32> ← 0
END
ENDCASE

Exceptions:
None

Instruction mnemonics:
PKLB
PKWB

Pack Longwords to Bytes
Pack Words to Bytes

Qualifiers:
None

Description:
For PKLB, the component longwords of Rb are truncated to bytes and written to the lower two
byte positions of Rc. The upper six bytes of Rc are written with zero.
For PKWB, the component words of Rb are truncated to bytes and written to the lower four
byte positions of Rc. The upper four bytes of Rc are written with zero.

Instruction Descriptions 4–155

4.13.4 Unpack Bytes
Format:
UNPKBx

Rb.rq,Rc.wq

! Operate Format

Operation:
temp = 0
CASE
UNPKBL:
BEGIN
temp<07:00> = Rbv<07:00>
temp<39:32> = Rbv<15:08>
END
UNPKBW:
BEGIN
temp<07:00> = Rbv<07:00>
temp<23:16> = Rbv<15:08>
temp<39:32> = Rbv<23:16>
temp<55:48> = Rbv<31:24>
END
ENDCASE
Rc ← temp

Exceptions:
None

Instruction mnemonics:
UNPKBL

Unpack Bytes to Longwords

UNPKBW

Unpack Bytes to Words

Qualifiers:
None

Description:
For UNPKBL, the lower two component bytes of Rb are zero-extended to longwords. The
resulting longwords are written to Rc.
For UNPKBW, the lower four component bytes of Rb are zero-extended to words. The resulting words are written to Rc.

4–156 Alpha Architecture Handbook

Chapter 5

System Architecture and Programming
Implications

5.1 Introduction
Portions of the Alpha architecture have implications for programming, and the system structure, of both uniprocessor and multiprocessor implementations. Architectural implications
considered in the following sections are:

•

Physical address space behavior

•

Caches and write buffers

•

Translation buffers and virtual caches

•

Data sharing

•

Read/write ordering

•

Arithmetic traps

To meet the requirements of the Alpha architecture, software and hardware implementors need
to take these issues into consideration.

5.2 Physical Address Space Characteristics
Alpha physical address space is divided into four equal-size regions. The regions are delineated by the two most significant, implemented, physical address bits. Each region’s
characteristics are distinguished by the coherency, granularity, and width of memory accesses,
and whether the region exhibits memory-like behavior or non-memory-like behavior.

5.2.1 Coherency of Memory Access
Alpha implementations must provide a coherent view of memory, in which each write by a
processor or I/O device (hereafter, called "processor") becomes visible to all other processors.
No distinction is made between coherency of "memory space" and "I/O space."

System Architecture and Programming Implications 5–1

Memory coherency may be provided in different ways for each of the four physical address
regions.
Possible per-region policies include, but are not restricted to:

•

No caching
No copies are kept of data in a region; all reads and writes access the actual data
location (memory or I/O register), but a processor may elide multiple accesses to the
same data (see Section 5.2.3).

•

Write-through caching
Copies are kept of any data in the region; reads may use the copies, but writes update
the actual data location and either update or invalidate all copies.

•

Write-back caching
Copies are kept of any data in the region; reads and writes may use the copies, and
writes use additional state to determine whether there are other copies to invalidate or
update.

Software/Hardware Note:
To produce separate and distinct accesses to a specific location, the location must be a
region with no caching and a memory barrier instruction must be inserted between
accesses. See Section 5.2.3.
Part of the coherency policy implemented for a given physical address region may include
restrictions on excess data transfers (performing more accesses to a location than is necessary
to acquire or change the location’s value) or may specify data transfer widths (the granularity
used to access a location).
Independent of coherency policy, a processor may use different hardware or different hardware resource policies for caching or buffering different physical address regions.

5.2.2 Granularity of Memory Access
For each region, an implementation must support aligned quadword access and may optionally
support aligned longword access or byte access. If byte access is supported in a region, aligned
word access and aligned longword access are also supported.
For a quadword access region, accesses to physical memory must be implemented such that
independent accesses to adjacent aligned quadwords produce the same results regardless of the
order of execution. Further, an access to an aligned quadword must be done in a single atomic
operation.
For a longword access region, accesses to physical memory must be implemented such that
independent accesses to adjacent aligned longwords produce the same results regardless of the
order of execution. Further, an access to an aligned longword must be done in a single atomic
operation, and an access to an aligned quadword must also be done in a single atomic
operation.

5–2 Alpha Architecture Handbook

For a byte access region, accesses to physical memory must be implemented such that independent accesses to adjacent bytes or adjacent aligned words produce the same results, regardless
of the order of execution. Further, an access to a byte, an aligned word, an aligned longword,
or an aligned quadword must be done in a single atomic operation.
In this context, "atomic" means that the following is true if different processors do simultaneous reads and writes of the same data:

•

The result of any set of writes must be the same as if the writes had occurred sequentially in some order, and

•

Any read that observes the effect of a write on some part of memory must observe the
effect of that write (or of a later write or writes) on the entire part of memory that is
accessed by both the read and the write.

When a write accesses only part of a given word, longword, or quadword, a read of the entire
structure may observe the effect of that partial write without observing the effect of an earlier
write of another byte or bytes to the same structure. See Sections 5.6.1.5 and 5.6.1.6.

5.2.3 Width of Memory Access
Subject to the granularity, ordering, and coherency constraints given in Sections 5.2.1, 5.2.2,
and 5.6, accesses to physical memory may be freely cached, buffered, and prefetched.
A processor may read more physical memory data (such as a full cache block) than is actually
accessed, writes may trigger reads, and writes may write back more data than is actually
updated. A processor may elide multiple reads and/or writes to the same data.

5.2.4 Memory-Like and Non-Memory-Like Behavior
Memory-like regions obey the following rules:

•

Each page frame in the region either exists in its entirety or does not exist in its entirety;
there are no holes within a page frame.

•

All locations that exist are read/write.

•

A write to a location followed by a read from that location returns precisely the bits
written; all bits act as memory.

•

A write to one location does not change any other location.

•

Reads have no side effects.

•

Longword access granularity is provided, and if the byte/word extension is implemented, byte access granularity is provided.

•

Instruction-fetch is supported.

•

Load-locked and store-conditional are supported.

Non-memory-like regions may have much more arbitrary behavior:

•

Unimplemented locations or bits may exist anywhere.

•

Some locations or bits may be read-only and others write-only.

System Architecture and Programming Implications 5–3

•

Address ranges may overlap, such that a write to one location changes the bits read
from a different location.

•

Reads may have side effects, although this is strongly discouraged.

•

Longword granularity need not be supported and, even if the byte/word extension is
implemented, byte access granularity need not be implemented.

•

Instruction-fetch need not be supported.

•

Load-locked and store-conditional need not be supported.

Hardware/Software Coordination Note:
The details of such behavior are outside the scope of the Alpha architecture. Specific
processor and I/O device implementations may choose and document whatever behavior
they need. It is the responsibility of system designers to impose enough consistency to
allow processors successfully to access matching non-memory devices in a coherent way.

5.3 Translation Buffers and Virtual Caches
A system may choose to include a virtual instruction cache (virtual I-cache) or a virtual data
cache (virtual D-cache). A system may also choose to include either a combined data and
instruction translation buffer (TB) or separate data and instruction TBs (DTB and ITB). The
contents of these caches and/or translation buffers may become invalid, depending on what
operating system activity is being performed.
Whenever a non-software field of a valid page table entry (PTE) is modified, copies of that
PTE must be made coherent. PALcode mechanisms are available to clear all TBs, both DTB
and ITB entries for a given VA, either DTB or ITB entries for a given VA, or all entries with
the address space match (ASM) bit clear. Virtual D-cache entries are made coherent whenever
the corresponding DTB entry is requested to be cleared by any of the appropriate PALcode
mechanisms. Virtual I-cache entries can be made coherent via the IMB instruction.
If a processor implements address space numbers (ASNs), and the old PTE has the Address
Space Match (ASM) bit clear (ASNs in use) and the Valid bit set, then entries can also effectively be made coherent by assigning a new, unused ASN to the currently running process and
not reusing the previous ASN before calling the appropriate PALcode routine to invalidate the
translation buffer (TB).
In a multiprocessor environment, making the TBs and/or caches coherent on only one processor is not always sufficient. An operating system must arrange to perform the above actions on
each processor that could possibly have copies of the PTE or data for any affected page.

5.4 Caches and Write Buffers
A hardware implementation may include mechanisms to reduce memory access time by making local copies of recently used memory contents (or those expected to be used) or by
buffering writes to complete at a later time. Caches and write buffers are examples of these
mechanisms. They must be implemented so that their existence is transparent to software
(except for timing, error reporting/control/recovery, and modification to the I-stream).

5–4 Alpha Architecture Handbook

The following requirements must be met by all cache/write-buffer implementations. All processors must provide a coherent view of memory.

•

Write buffers may be used to delay and aggregate writes. From the viewpoint of another
processor, buffered writes appear not to have happened yet. (Write buffers must not
delay writes indefinitely. See Section 5.6.1.9.)

•

Write-back caches must be able to detect a later write from another processor and invalidate or update the cache contents.

•

A processor must guarantee that a data store to a location followed by a data load from
the same location reads the updated value.

•

Cache prefetching is allowed, but virtual caches must not prefetch from invalid pages.
See Sections 5.6.1.3, 5.6.4.3, and 5.6.4.4.

•

A processor must guarantee that all of its previous writes are visible to all other processors before a HALT instruction completes. A processor must guarantee that its caches
are coherent with the rest of the system before continuing from a HALT.

•

If battery backup is supplied, a processor must guarantee that the memory system
remains coherent across a powerfail/recovery sequence. Data that was written by the
processor before the powerfail may not be lost, and any caches must be in a valid state
before (and if) normal instruction processing is continued after power is restored.

•

Virtual instruction caches are not required to notice modifications of the virtual
I-stream (they need not be coherent with the rest of memory). Software that creates or
modifies the instruction stream must execute a CALL_PAL IMB before trying to execute the new instructions.
In this context, to "modify the virtual I-stream" means either:
–

any Store to the same physical address that is subsequently fetched as an instruction
by some corresponding (virtual address, ASN) pair, or

–

any change to the virtual-to-physical address mapping so that different values are
fetched.

For example, if two different virtual addresses, VA1 and VA2, map to the same page
frame, a store to VA1 modifies the virtual I-stream fetched by VA2.
However, the following sequence does not modify the virtual I-stream (this might
happen in soft page faults).
1. Change the mapping of an I-stream page from valid to invalid.
2. Copy the corresponding page frame to a new page frame.
3. Change the original mapping to be valid and point to the new page frame.

•

Physical instruction caches are not required to notice modifications of the physical
I-stream (they need not be coherent with the rest of memory), except for certain paging
activity. (See Section 5.6.4.4.) Software that creates or modifies the instruction stream
must execute a CALL_PAL IMB before trying to execute the new instructions.
In this context, to "modify the physical I-stream" means any Store to the same physical
address that is subsequently fetched as an instruction.

System Architecture and Programming Implications 5–5

5.5 Data Sharing
In a multiprocessor environment, writes to shared data must be synchronized by the
programmer.

5.5.1 Atomic Change of a Single Datum
The ordinary STL and STQ instructions can be used to perform an atomic change of a shared
aligned longword or quadword. ("Change" means that the new value is not a function of the old
value.) In particular, an ordinary STL or STQ instruction can be used to change a variable that
could be simultaneously accessed via an LDx_L/STx_C sequence.

5.5.2 Atomic Update of a Single Datum
The load-locked/store-conditional instructions may be used to perform an atomic update of a
shared aligned longword or quadword. ("Update" means that the new value is a function of the
old value.)
The following sequence performs a read-modify-write operation on location x. Only register-to-register operate instructions and branch fall-throughs may occur in the sequence:
try_again:
LDQ_L R1,x
<modify R1>
STQ_C R1,x
BEQ
R1,no_store
:
no_store:
<code to check for excessive iterations>
BR
try_again

If this sequence runs with no exceptions or interrupts, and no other processor writes to location x (more precisely, the locked range including x) between the LDQ_L and STQ_C
instructions, then the STQ_C shown in the example stores the modified value in x and sets R1
to 1. If, however, the sequence encounters exceptions or interrupts that eventually continue the
sequence, or another processor writes to x, then the STQ_C does not store and sets R1 to 0. In
this case, the sequence is repeated by the branches to no_store and try_again. This repetition
continues until the reasons for exceptions or interrupts are removed and no interfering store is
encountered.
To be useful, the sequence must be constructed so that it can be replayed an arbitrary number
of times, giving the same result values each time. A sufficient (but not necessary) condition is
that, within the sequence, the set of operand destinations and the set of operand sources are
disjoint.

Note:
A sufficiently long instruction sequence between LDx_L and STx_C will never complete,
because periodic timer interrupts will always occur before the sequence completes. The
rules in Section A.5 describe sequences that will eventually complete in all Alpha
implementations.
5–6 Alpha Architecture Handbook

This load-locked/store-conditional paradigm may be used whenever an atomic update of a
shared aligned quadword is desired, including getting the effect of atomic byte writes.

5.5.3 Atomic Update of Data Structures
Before accessing shared writable data structures (those that are not a single aligned longword
or quadword), the programmer can acquire control of the data structure by using an atomic
update to set a software lock variable. Such a software lock can be cleared with an ordinary
store instruction.
A software-critical section, therefore, may look like the sequence:
stq_c_loop:
spin_loop:
LDQ R1,lock_variable
BLBS R1,already_set
LDQ_L R1,lock_variable
BLBS R1,already_set
OR R1,#1,R2
STQ_C R2,lock_variable
BEQ R2,stq_c_fail

; This optional spin-loop code
; should be used unless the
; lock is known to be low-contention.
; \
; \
;
> Set lock bit
; /
; /

MB
<critical section: updates various data structures>
MB
; Second MB
STQ R31,lock_variable
; Clear lock bit
:
:
already_set:
<code to block or reschedule or test for too many iterations>
BR spin_loop
stq_c_fail:
<code to test for too many iterations>
BR stq_c_loop

This code has a number of subtleties:

•

If the lock_variable is already set, the spin loop is done without doing any stores. This
avoidance of stores improves memory subsystem performance and avoids the deadlock
described below. The loop uses an ordinary load. This code sequence is preferred unless
the lock is known to be low-contention, because the sequence increases the probability
that the LDQ_L hits in the cache and the LDQ_L/STQ_C sequence complete quickly
and successfully.

•

If the lock_variable is actually being changed from 0 to 1, and the STQ_C fails (due to
an interrupt, or because another processor simultaneously changed lock_variable), the
entire process starts over by reading the lock_variable again.

•

Only the fall-through path of the BLBS instructions does a STx_C; some implementations may not allow a successful STx_C after a branch-taken.

•

Only register-to-register operate instructions are used to do the modify.

System Architecture and Programming Implications 5–7

•

Both conditional branches are forward branches, so they are properly predicted not to
be taken (to match the common case of no contention for the lock).

•

The OR writes its result to a second register; this allows the OR and the BLBS to be
interchanged if that would give a faster instruction schedule.

•

Other operate instructions (from the critical section) may be scheduled into the
LDQ_L..STQ_C sequence, so long as they do not fault or trap and they give correct
results if repeated; other memory or operate instructions may be scheduled between the
STQ_C and BEQ.

•

The memory barrier instructions are discussed in Section 5.5.4. It is correct to substitute
WMB for the second MB only if:
–

All data locations that are read or written in the critical section are accessed only
after acquiring a software lock by using lock_variable (and before releasing the
software lock).

–

For each read u of shared data in the critical section, there is a write v such that:
1. v is BEFORE the WMB
2. v follows u in processor issue sequence (see Section 5.6.1.1)
3. v either depends on u (see Section 5.6.1.7) or overlaps u (see Section 5.6.1), or
both.

–

Both lock_variable and all the shared data are in memory-like regions (or
lock_variable and all the shared data are in non-memory-like regions). If the
lock_variable is in a non-memory-like region, the atomic lock protocol must use
some implementation-specific hardware support.

Generally, the substitution of a WMB for the second MB increases performance.

•

An ordinary STQ instruction is used to clear the lock_variable.

It would be a performance mistake to spin-wait by repeating the full LDQ_L..STQ_C sequence
(to move the BLBS after the BEQ) because that sequence may repeatedly change the software
lock_variable from "locked" to "locked," with each write causing extra access delays in all
other caches that contain the lock_variable. In the extreme, spin-waits that contain writes may
deadlock as follows:
If, when one processor spins with writes, another processor is modifying (not changing)
the lock_variable, then the writes on the first processor may cause the STx_C of the
modify on the second processor always to fail.
This deadlock situation is avoided by:

•

Having only one processor execute a store (no STx_C), or

•

Having no write in the spin loop, or

•

Doing a write only if the shared variable actually changes state (1 → 1 does not change
state).

5–8 Alpha Architecture Handbook

5.5.4 Ordering Considerations for Shared Data Structures
A critical section sequence, such as shown in Section 5.5.3, is conceptually only three steps:
1. Acquire software lock
2. Critical section — read/write shared data
3. Clear software lock
In the absence of explicit instructions to the contrary, the Alpha architecture allows reads and
writes to be reordered. While this may allow more implementation speed and overlap, it can
also create undesired side effects on shared data structures. Normally, the critical section just
described would have two instructions added to it:
<acquire software lock>
MB (memory barrier #1)
<critical section – read/write shared data>
MB (memory barrier #2)
<clear software lock>
<endcode_example>

The first memory barrier prevents any reads (from within the critical section) from being
prefetched before the software lock is acquired; such prefetched reads would potentially contain stale data.
The second memory barrier prevents any writes and reads in the critical section being delayed
past the clearing of the software lock. Such delayed accesses could interact with the next user
of the shared data, defeating the purpose of the software lock entirely. It is correct to substitute
WMB for the second MB only if:
1. All data locations that are read or written in the critical section are accessed only after
acquiring a software lock by using lock_variable (and before releasing the software
lock).
2. For each read u of shared data in the critical section, there is a write v such that:
a. v is BEFORE the WMB
b. v follows u in processor issue sequence (see Section 5.6.1.1)
c. v either depends on u (see Section 5.6.1.7) or overlaps u (see Section 5.6.1), or both.
3. Both lock_variable and all the shared data are in memory-like regions (or lock_variable
and all the shared data are in non-memory-like regions). If the lock_variable is in a
non-memory-like region, the atomic lock protocol must use some implementation-specific hardware support.
Generally, the substitution of a WMB for the second MB increases performance.

Software Note:
In the VAX architecture, many instructions provide noninterruptable read-modify-write
sequences to memory variables. Most programmers never regard data sharing as an issue.
In the Alpha architecture, programmers must pay more attention to synchronizing access to
shared data; for example, to AST routines. In the VAX architecture, a programmer can use

System Architecture and Programming Implications 5–9

an ADDL2 to update a variable that is shared between a "MAIN" routine and an AST
routine, if running on a single processor. In the Alpha architecture, a programmer must
deal with AST shared data by using multiprocessor shared data sequences.

5.6 Read/Write Ordering
This section applies to programs that run on multiple processors or on one or more processors
that are interacting with DMA I/O devices. To a program running on a single processor and not
interacting with DMA I/O devices, all memory accesses appear to happen in the order specified by the programmer. This section deals with predictable read/write ordering across multiple
processors and/or DMA I/O devices.
The order of reads and writes done in an Alpha implementation may differ from that specified
by the programmer.
For any two memory accesses A and B, either A must occur before B in all Alpha implementations, B must occur before A, or they are UNORDERED. In the last case, software cannot
depend upon one occurring first: the order may vary from implementation to implementation,
and even from run to run or moment to moment on a single implementation.
If two accesses cannot be shown to be ordered by the rules given, they are UNORDERED and
implementations are free to do them in any order that is convenient. Implementations may take
advantage of this freedom to deliver substantially higher performance.
The discussion that follows first defines the architectural issue sequence of memory accesses
on a single processor, then defines the (partial) ordering on this issue sequence that all Alpha
implementations are required to maintain.
The individual issue sequences on multiple processors are merged into access sequences at
each shared memory location. The discussion defines the (partial) ordering on the individual
access sequences that all Alpha implementations are required to maintain.
The net result is that for any code that executes on multiple processors, one can determine
which memory accesses are required to occur before others on all Alpha implementations and
hence can write useful shared-variable software.
Software writers can force one access to occur before another by inserting a memory barrier
instruction (MB, WMB, or CALL_PAL IMB) between the accesses.

5.6.1 Alpha Shared Memory Model
An Alpha system consists of a collection of processors, I/O devices (and possibly a bridge to
connect remote I/O devices), and shared memories that are accessible by all processors.

Note:
An example of an unshared location is a physical address in I/O space that refers to a CSR
that is local to a processor and not accessible by other processors.
A processor is an Alpha CPU.
5–10 Alpha Architecture Handbook

In most systems, DMA I/O devices or other agents can read or write shared memory locations.
The order of accesses by those agents is not completely specified in this document. It is possible in some systems for read accesses by I/O devices or other agents to give results indicating
some reordering of accesses. However, there are guarantees that apply in all systems. See Section 5.6.4.7.
A shared memory is the primary storage place for one or more locations.
A location is a byte, specified by its physical address. Multiple virtual addresses may map to
the same physical address. Ordering considerations are based only on the physical address.
This definition of location specifically includes locations and registers in memory mapped I/O
devices and bridges to remote I/O (for example, Mailbox Pointer Registers, or MBPRs).

Implementation Note:
An implementation may allow a location to have multiple physical addresses, but the rules
for accesses via mixtures of the addresses are implementation-specific and outside the
scope of this section. Accesses via exactly one of the physical addresses follow the rules
described next.
Each processor may generate accesses to shared memory locations. There are six types of
accesses:
1. Instruction fetch by processor i to location x, returning value a, denoted Pi:I<4>(x,a).
2. Data read (including load-locked) by processor i to location x, returning value a,
denoted Pi:R<size>(x,a).
3. Data write (including successful store-conditional) by processor i to location x, storing
value a, denoted Pi:W<size>(x,a).
4. Memory barrier issued by processor i, denoted Pi:MB.
5. Write memory barrier issued by processor i, denoted Pi:WMB.
6. I-stream memory barrier issued by processor i, denoted Pi:IMB.
The first access type is also called an I-stream access or I-fetch. The next two are also called
D-stream accesses. The first three types are collectively called read/write accesses, denoted
Pi:Op<m>(x,a), where m is the size of the access in bytes, x is the (physical) address of the
access, and a is a value representable in m bytes; for any k in the range 0..m–1, byte k of value
a (where byte 0 is the low-order byte) is the value written to or read from location x+k by the
access. This relationship reflects little-endian addressing; big-endian addressing representation
is as described in Chapter 2.
The last three types collectively are called barriers or memory barriers.
The size of a read/write access is 8 for a quadword access, 4 for a longword access (including
all instruction fetches), 2 for a word access, or 1 for a byte access. All read/write accesses in
this chapter are naturally aligned. That is, they have the form Pi:Op<m>(x,a), where the
address x is divisible by size m.
The word "access" is also used as a verb; a read/write access Pi:Op<m>(x,a) accesses byte z if
x ≤ z < x+m. Two read/write accesses Op1<m>(x,a) and Op2<n>(y,b) are defined to overlap if

System Architecture and Programming Implications 5–11

there is at least one byte that is accessed by both, that is, if max(x,y) < min(x+m,y+n).

5.6.1.1 Architectural Definition of Processor Issue Sequence
The issue sequence for a processor is architecturally defined with respect to a hypothetical simple implementation that contains one processor and a single shared memory, with no caches or
buffers. This is the instruction execution model:
1. I-fetch: An Alpha instruction is fetched from memory.
2. Read/Write: That instruction is executed and runs to completion, including a single data
read from memory for a Load instruction or a single data write to memory for a Store
instruction.
3. Update: The PC for the processor is updated.
4. Loop: Repeat the above sequence indefinitely.
If the instruction fetch step gets a memory management fault, the I-fetch is not done and the
PC is updated to point to a PALcode fault handler. If the read/write step gets a memory management fault, the read/write is not done and the PC is updated to point to a PALcode fault
handler.

5.6.1.2 Definition of Before and After
The ordering relation BEFORE (⇐ ) is a partial order on memory accesses. It is further defined
in Sections 5.6.1.3 through 5.6.1.9.
The ordering relation BEFORE (⇐ ), being a partial order, is acyclic.
The BEFORE order cannot be observed directly, nor fully predicted before an actual execution, nor reproduced exactly from one execution to another. Nonetheless, some useful ordering
properties must hold in all Alpha implementations.
If u ⇐ v, then v is said to be AFTER u.

5.6.1.3 Definition of Processor Issue Constraints
Processor issue constraints are imposed on the processor issue sequence defined in Section
5.6.1.1, as shown in Table 5–1:

5–12 Alpha Architecture Handbook

Table 5–1: Processor Issue Constraints
1st↓ 2nd →

Pi:I<n=4>(y,b)

Pi:I<m=4>(x,a)

⇐ if overlap

Pi:R<n>(y,b)

Pi:W<n>(y,b)

Pi:MB

Pi:IMB

⇐ if overlap

⇐

⇐ if overlap

⇐

⇐ if overlap

⇐

⇐ if overlap

Pi:R<m>(x,a)
Pi:W<m>(x,a)
Pi:MB
Pi:IMB

⇐

Where "overlap" denotes the condition max(x,y) < min(x+m,y+n).
For two accesses u and v issued by processor Pi, if u precedes v by processor issue constraint,
then u precedes v in BEFORE order. u and v on Pi are ordered by processor issue constraint if
any of the following applies:
1. The entry in Table 5–1 indicated by the access type of u (1st) and v (2nd) indicates the
accesses are ordered.
2. u and v are both writes to memory-like regions and there is a WMB between u and v in
processor issue sequence.
3. u and v are both writes to non-memory-like regions and there is a WMB between u and
v in processor issue sequence.
4. u is a TB fill that updates a PTE, for example, a PTE read in order to satisfy a TB miss,
and v is an I- or D-stream access using that PTE (see Sections 5.6.4.3 and 5.6.4.4).
In Table 5–1, 1st and 2nd refer to the ordering of accesses in the processor issue sequence.
Note that Table 5–1 imposes no direct constraint on the ordering relationship between nonoverlapping read/write accesses, though there may be indirect constraints due to the transitivity
of BEFORE (⇐ ). Conditions 2 through 4, above, impose ordering constraints on some pairs of
nonoverlapping read/write accesses.
Table 5–1 permits a read access Pi:R<n>(y,b) to be ordered BEFORE an overlapping write
access Pi:W<m>(x,a) that precedes the read access in processor issue order. This asymmetry
for reads allows reads to be satisfied by using data from an earlier write in processor issue
sequence by the same processor (for example, by hitting in a write buffer) before the write
completes. The write access remains "visible" to the read access; "visibility" is described in
Sections 5.6.1.5 and 5.6.1.6 and illustrated in Litmus Test 11 in Section 5.6.2.11.
An I-fetch Pi:I<4>(y,b) may also be ordered BEFORE an overlapping write Pi:W<m>(x,a) that
precedes it in processor issue sequence. In that case, the write may, but need not, be visible to
the I-fetch. This asymmetry in Table 5–1 allows writes to the I-stream to be incoherent until a
CALL_PAL IMB is executed.
Implementations are free to perform memory accesses from a single processor in any sequence
that is consistent with processor issue constraints.

System Architecture and Programming Implications 5–13

5.6.1.4 Definition of Location Access Constraints
Location access constraints are imposed on overlapping read/write accesses. If u and v are
overlapping read/write accesses, at least one of which is a write, then u and v must be comparable in the BEFORE (⇐ ) ordering, that is, either u ⇐ v or v ⇐ u.
There is no direct requirement that nonoverlapping accesses be comparable in the BEFORE
(⇐ ) ordering.
All writes accessing any given byte are totally ordered, and any read or I-fetch accessing a
given byte is ordered with respect to all writes accessing that byte.

5.6.1.5 Definition of Visibility
If u is a write access Pi:W<m>(x,a) and v is an overlapping read access Pj:R<n>(y,b), u is visible to v only if:
u ⇐ v, or
u precedes v in processor issue sequence (possible only if Pi=Pj).
If u is a write access Pi:W<m>(x,a) and v is an overlapping instruction fetch Pj:I<4>(y,b),
there are the following rules for visibility:
1. If u ⇐ v, then u is visible to v.
2. If u precedes v in processor issue sequence, then:
a. If there is a write w such that:
u overlaps w and precedes w in processor issue sequence, and
w is visible to v,
then u is visible to v.
b. If there is an instruction fetch w such that:
u is visible to w, and
w overlaps v and precedes v in processor issue sequence,
then u is visible to v.
3. If u does not precede v in either processor issue sequence or BEFORE order, then u is
not visible to v.
Note that the rules of visibility for reads and instruction fetches are slightly different. If a write
u precedes an overlapping instruction fetch v in processor issue sequence, but u is not
BEFORE v, then u may or may not be visible to v.

5.6.1.6 Definition of Storage
The property of storage applies only to memory-like regions.
The value read from any byte by a read access or instruction fetch v, is the value written by the
latest (in BEFORE order) write u to that byte that is visible to v. More formally:
If u is Pi:W<m>(x,a), and v is either Pj:I<4>(y,b) or Pj:R<n>(y,b), and z is a byte accessed
by both u and v, and u is visible to v; and there is no write that is AFTER u, is visible to v,
5–14 Alpha Architecture Handbook

and accesses byte z; then the value of byte z read by v is exactly the value written by u. In
this situation, u is a source of v.
The only way to communicate information between different processors is for one to write a
shared location and the other to read the shared location and receive the newly written value.
(In this context, the sending of an interrupt from processor Pi to Pj is modeled as Pi writing to a
location INTij, and Pj reading from INTij.)

5.6.1.7 Definition of Dependence Constraint
The depends relation (DP) is defined as follows. Given u and v issued by processor Pi, where u
is a read or an instruction fetch and v is a write, u precedes v in DP order (written u DP v, that
is, v depends on u) in either of the following situations:

•

u determines the execution of v, the location accessed by v, or the value written by v.

•

u determines the execution or address or value of another memory access z that precedes v or might precede v (that is, would precede v in some execution path depending
on the value read by u) by processor issue constraint (see Section 5.6.1.3).

Note that the DP relation does not directly impose a BEFORE (⇐) ordering between accesses
u and v.
The dependence constraint requires that the union of the DP relation and the "is a source of"
relation (see Section 5.6.1.6) be acyclic. That is, there must not exist reads and/or I-fetches R1,
…, Rn, and writes W1, …, Wn, such that:
1. n ≥ 1,
2. For each i, 1 ≤ i ≤ n, Ri DP Wi,
3. For each i, 1 ≤ i < n, Wi is a source of Ri + 1, and
4. Wn is a source of R1.
That constraint eliminates the possibility of "causal loops." A simple example of a "causal
loop" is when the execution of a write on Pi depends on the execution of a write on Pj and vice
versa, creating a circular dependence chain. The following simple example of a "causal loop"
is written in the style of the litmus tests in Section 5.6.2, where initially x and y are 1:
Processor Pi executes:
LDQ
STQ

R1,x
R1,y

Processor Pj executes:
LDQ
STQ

R1,y
R1,x

System Architecture and Programming Implications 5–15

Representing those code sequences in the style of the litmus tests in Section 5.6.2, it is impossible for the following sequence to result:
Pi

[U1] Pi:R<8>(x,0)

[V1] Pj:R<8>(y,0)

[U2] Pi:W<8>(y,0)

[V2] Pj:W<8>(x,0)

Analysis:

<1>

By the definitions of storage and visibility, U2 is the source of V1, and V2 is the
source of U1.

<2>

By the definition of DP and examination of the code, U1 DP U2, and V1 DP V2.

<3>

Thus, U1 DP U2, U2 is the source of V1, V1 DP V2, and V2 is the source of U1.
This circular chain is forbidden by the dependence constraint.

Given the initial condition x, y = 1, the access sequence above would also be impossible if the
code were:
Processor Pi’s program:
LDQ
BNE
STQ

R1,x
R1,done
R31,y

done:

Processor Pj’s program:
LDQ
BNE
STQ

R1,y
R1,done
R31,x

done:

5.6.1.8 Definition of Load-Locked and Store-Conditional
The property of load-locked and store-conditional applies only to memory-like regions.
For each successful store-conditional v, there exists a load-locked u such that the following are
true:
1. u precedes v in the processor issue sequence.
2. There is no load-locked or store-conditional between u and v in the processor issue
sequence.
3. If u and v access within the same naturally aligned 16-byte physical and virtual block in
memory, then for every write w by a different processor that accesses within u’s lock
range (where w is either a store or a successful store conditional), it must be true that w
⇐ u or v ⇐ w.
u’s lock range contains the region of physical memory that u accesses. See Sections 4.2.4 and
4.2.5, which define the lock range and conditions for success or failure of a store conditional.

5–16 Alpha Architecture Handbook

5.6.1.9 Timeliness
Even in the absence of a barrier after the write, no write by a processor may be delayed indefinitely in the BEFORE ordering.

5.6.2 Litmus Tests
Many issues about writing and reading shared data can be cast into questions about whether a
write is before or after a read. These questions can be answered by rigorously checking
whether any ordering satisfies the rules in Sections 5.6.1.3 through 5.6.1.8.
In litmus tests 1–9 below, all initial quadword memory locations contain 1. In all these litmus
tests, it is assumed that initializations are performed by a write or writes that are BEFORE all
the explicitly listed accesses, that all relevant writes other than the initializations are explicitly
shown, and that all accesses shown are to memory-like regions (so the definition of storage
applies).

5.6.2.1 Litmus Test 1 (Impossible Sequence)
Initially, location x contains 1:

[U1]Pi:W<8>(x,2)

[V1]Pj:R<8>(x,2)
[V2]Pj:R<8>(x,1)

Analysis:

<1>

By the definition of storage (Section 5.6.1.6), V1 reading 2 implies that U1 is visible
to V1.

<2>

By the rules for visibility (Section 5.6.1.5), U1 being visible to V1, but being issued
by a different processor, implies that U1 ⇐ V1.

<3>

By the processor issue constraints (Section 5.6.1.3), V1 ⇐ V2.

<4>

By the transitivity of the partial order ⇐, it follows from <2> and <3> that U1 ⇐
V2.

<5>

By the rules for visibility, it follows from U1 ⇐ V2 that U1 is visible to V2.

<6>

Since U1 is AFTER the initialization of x, U1 is the latest (in the ⇐ ordering) write
to x that is visible to V1.

<7>

By the definition of storage, it follows that V2 should read the value written by U1,
in contradiction to the stated result.

Thus, once a processor reads a new value from a location, it must never see an old value – time
must not go backward. V2 must read 2.

System Architecture and Programming Implications 5–17

5.6.2.2 Litmus Test 2 (Impossible Sequence)
Initially, location x contains 1:

[U1]Pi:W<8>(x,2)

[V1]Pj:W<8>(x,3)
[V2]Pj:R<8>(x,2)
[V3]Pj:R<8>(x,3)

Analysis:

<1>

Since V1 precedes V2 in processor issue sequence, V1 is visible to V2.

<2>

V2 reading 2 implies U1 is the latest (in ⇐ order) write to x visible to V2.

<3>

From <1> and <2>, V1 ⇐ U1.

<4>

Since U1 is visible to V2, and they are issued by different processors, U1 ⇐ V2.

<5>

By the processor issue constraints, V2 ⇐ V3.

<6>

From <4> and <5>, U1 ⇐ V3.

<7>

From <6> and the visibility rules, U1 is visible to V3.

<8>

Since both V1 and the initialization of x are BEFORE U1, U1 is the latest write to x
that is visible to V3.

<9>

By the definition of storage, it follows that V3 should read the value written by U1,
in contradiction to the stated result.

Thus, once processor Pj reads a new value written by U1, any other writes that must precede
the read must also precede U1. V3 must read 2.

5.6.2.3 Litmus Test 3 (Impossible Sequence)
Initially, location x contains 1:

[U1]Pi:W<8>(x,2)

[V1]Pj:W<8>(x,3)

[W1]Pk:R<8>(x,3)

[U2]Pi:R<8>(x,3)

[W2]Pk:R<8>(x,2)

Analysis:

<1>

U2 reading 3 implies V1 is the latest write to x visible to U2, therefore U1 ⇐ V1.

<2>

W1 reading 3 implies V1 is visible to W1, so V1 ⇐ W1 ⇐ W, therefore V1 is also
visible to W2.

<3>

W2 reading 2 implies U1 is the latest write to x visible to W2, therefore V1 ⇐ U1.

<4>

From <1> and <3>, U1 ⇐ V1 ⇐ U1.

Again, time cannot go backwards. If V1 is ordered before U1, then processor Pk cannot read
first the later value 3 and then the earlier value 2. Alternatively, if V1 is ordered before U1, U2
must read 2.

5–18 Alpha Architecture Handbook

5.6.2.4 Litmus Test 4 (Sequence Okay)
Initially, locations x and y contain 1:

[U1]Pi:W<8>(x,2)

[V1]Pj:R<8>(y,2)

[U2]Pi:W<8>(y,2)

[V2]Pj:R<8>(x,1)

Analysis:

<1>

V1 reading 2 implies U2 ⇐ V1, by storage and visibility.

<2>

Since V2 does not read 2, there cannot be U1 ⇐ V2.

<3>

By the access order constraints, it follows from <2> that V2 ⇐ U1.

There are no conflicts in the sequence. There are no violations of the definition of BEFORE.

5.6.2.5 Litmus Test 5 (Sequence Okay)
Initially, locations x and y contain 1:

[U1]Pi:W<8>(x,2)

[V1]Pj:R<8>(y,2)
[V2]Pj:MB

[U2]Pi:W<8>(y,2)

[V3]Pj:R<8>(x,1)

Analysis:

<1>

V1 reading 2 implies U2 ⇐ V1, by storage and visibility.

<2>

V1 ⇐ V2 ⇐ V3, by processor issue constraints.

<3>

V3 reading 1 implies V3 ⇐ U1, by storage and visibility.

There is U2 ⇐ V1 ⇐ V2 ⇐ V3 ⇐ U1. There are no conflicts in this sequence. There are no
violations of the definition of BEFORE.

5.6.2.6 Litmus Test 6 (Sequence Okay)
Initially, locations x and y contain 1:

[U1]Pi:W<8>(x,2)

[V1]Pj:R<8>(y,2)

[U2]Pi:MB
[U3]Pi:W<8>(y,2)

[V2]Pj:R<8>(x,1)

Analysis:

<1>

U1 ⇐ U2 ⇐ U3, by processor issue constraints.

<2>

V1 reading 2 implies U3 ⇐ V1, by storage and visibility.

<3>

V2 reading 1 implies V2 ⇐ U1, by storage and visibility.

System Architecture and Programming Implications 5–19

There is V2 ⇐ U1 ⇐ U2 ⇐ U3 ⇐ V1. There are no conflicts in this sequence. There are no
violations of the definition of BEFORE.
In litmus tests 4, 5, and 6, writes to two different locations x and y are observed (by another
processor) to occur in the opposite order than that in which they were performed. An update to
y propagates quickly to Pj, but the update to x is delayed, and Pi and Pj do not both have MBs.

5.6.2.7 Litmus Test 7 (Impossible Sequence)
Initially, locations x and y contain 1:

[U1]Pi:W<8>(x,2)

[V1]Pj:R<8>(y,2)

[U2]Pi:MB

[V2]Pj:MB

[U3]Pi:W<8>(y,2)

[V3]Pj:R<8>(x,1)

Analysis:

<1>

V3 reading 1 implies V3 ⇐ U1, by storage and visibility.

<2>

V1 reading 2 implies U3 ⇐ V1, by storage and visibility.

<3>

U1 ⇐ U2 ⇐ U3, by processor issue constraints.

<4>

V1 ⇐ V2 ⇐ V3, by processor issue constraints.

<5>

By <2>, <3>, and <4>, U1 ⇐ U2 ⇐ U3 ⇐ V1 ⇐ V2 ⇐ V3.

Both <1> and <5> cannot be true, so if V1 reads 2, then V3 must also read 2.
If both x and y are in memory-like regions, the sequence remains impossible if U2 is changed
to a WMB. Similarly, if both x and y are in non-memory-like regions, the sequence remains
impossible if U2 is changed to a WMB.

5.6.2.8 Litmus Test 8 (Impossible Sequence)
Initially, locations x and y contain 1:

[U1]Pi:W<8>(x,2)

[V1]Pj:W<8>(y,2)

[U2]Pi:MB

[V2]Pj:MB

[U3]Pi:R<8>(y,1)

[V3]Pj:R<8>(x,1)

Analysis:

<1>

V3 reading 1 implies V3 ⇐ U1, by storage and visibility.

<2>

U3 reading 1 implies U3 ⇐ V1, by storage and visibility.

<3>

U1 ⇐ U2 ⇐ U3, by processor issue constraints.

<4>

V1 ⇐ V2 ⇐ V3, by processor issue constraints.

<5>

By <2>, <3>, and <4>, U1 ⇐ U2 ⇐ U3 ⇐ V1 ⇐ V2 ⇐ V3.

Both <1> and <5> cannot be true, so if U3 reads 1, then V3 must read 2, and vice versa.
5–20 Alpha Architecture Handbook

5.6.2.9 Litmus Test 9 (Impossible Sequence)
Initially, location x contains 1:

[U1]Pi:W<8>(x,2)

[V1]Pj:W<8>(x,3)

[U2]Pi:R<8>(x,2)

[V2]Pj:R<8>(x,3)

[U3]Pi:R<8>(x,3)

[V3]Pj:R<8>(x,2)

Analysis:

<1>

V3 reading 2 implies U1 is the latest write to x visible to V3, therefore V1 ⇐ U1.

<2>

U3 reading 3 implies V1 is the latest write to x visible to U3, therefore U1 ⇐ V1.

Both <1> and <2> cannot be true. Time cannot go backwards. If V3 reads 2, then U3 must read
2. Alternatively, if U3 reads 3, then V3 must read 3.

5.6.2.10 Litmus Test 10 (Sequence Okay)
For an aligned quadword location, x, initially 10000000116:

[U1]Pi:W<4>(x,2)

[V1]Pj:W<4>(x+4,2)

[U2]Pi:R<8>(x,10000000216)

[V2]Pj:R<8>(x,20000000116)

Analysis:

<1>

Since U2 reads 1 from x+4, V1 is not visible to U2. Thus U2 ⇐ V1.

<2>

Similarly, V2 ⇐ U1.

<3>

U1 is visible to U2, but since they are issued by the same processor, it is not necessarily the case that U1 ⇐ U2.

<4>

Similarly, it is not necessarily the case that V1 ⇐ V2.

There is no ordering cycle, so the sequence is permitted.

5.6.2.11 Litmus Test 11 (Impossible Sequence)
For an aligned quadword location, x, initially 10000000116:

[U1]Pi:W<4>(x,2)

[V1]Pj:R<8>(x,20000000116)

[U2]Pi:MB or WMB
[U3]Pi:W<4>(x+4,2)
Analysis:

<1>

V1 reading 20000000116 implies U3 ⇐ V1 ⇐ U1 by storage and visibility.

<2>

U1 ⇐ U2 ⇐ U3, by processor issue constraints.

Both <1> and <2> cannot be true.

System Architecture and Programming Implications 5–21

5.6.3 Implied Barriers
There are no implied barriers in Alpha. If an implied barrier is needed for functionally correct
access to shared data, it must be written as an explicit instruction. (Software must explicitly
include any needed MB, WMB, or CALL_PAL IMB instructions.)
Alpha transitions such as the following have no built-in implied memory barriers:

•

Entry to PALcode

•

Sending and receiving interrupts

•

Returning from exceptions, interrupts, or machine checks

•

Swapping context

•

Invalidating the Translation Buffer (TB)

Depending on implementation choices for maintaining cache coherency, some PALcode/cache
implementations may have an implied CALL_PAL IMB in the I-stream TB fill routine, but
this is transparent to the non-PALcode programmer.

5.6.4 Implications for Software
Software must explicitly include MB, WMB, or CALL_PAL IMB instructions according to the
following circumstances.

5.6.4.1 Single Processor Data Stream
No barriers are ever needed. A read to physical address x will always return the value written
by the immediately preceding write to x in the processor issue sequence.

5.6.4.2 Single Processor Instruction Stream
An I-fetch from virtual or physical address x does not necessarily return the value written by
the immediately preceding write to x in the issue sequence. To make the I-fetch reliably get the
newly written instruction, a CALL_PAL IMB is needed between the write and the I-fetch.

5.6.4.3 Multiprocessor Data Stream (Including Single Processor with DMA I/O)
Generally, the only way to reliably communicate shared data is to write the shared data on one
processor or DMA I/O device, execute an MB (or the logical equivalent1 if it is a DMA I/O
device), then write a flag (equivalently, send an interrupt) signaling the other processor that the
shared data is ready. Each receiving processor must read the new flag (equivalently, receive the
interrupt), execute an MB, then read or update the shared data. In the special case in which data
1 In this context, the logical equivalent of an MB for a DMA device is whatever is necessary under the
applicable I/O subsystem architecture to ensure that preceding writes will be BEFORE (see Section
5.6.1.2) the subsequent write of a flag or transmission of an interrupt. Not all I/O devices behave
exactly as required by the Alpha architecture. To interoperate properly with those devices, some special action might be required by the program executing on the CPU. For example, PCI bus devices
require that after the CPU has received an interrupt, the CPU must read a CSR location on the PCI
device, execute an MB, then read or update the shared data. From the perspective of the Alpha architecture, this CSR read can be regarded as a necessary assist to help the DMA I/O device complete its
logical equivalent of an MB.
5–22 Alpha Architecture Handbook

is communicated through just one location in memory, memory barriers are not necessary.

Software Note:
Note that this section does not describe how to reliably communicate data from a processor
to a DMA device. See Section 5.6.4.7.
Leaving out the first MB removes the assurance that the shared data is written before the flag is
written.
Leaving out the second MB removes the assurance that the shared data is read or updated only
after the flag is seen to change; in this case, an early read could see an old value, and an early
update could be overwritten.
This implies that after a DMA I/O device has written some data to memory (such as paging in
a page from disk), the DMA device must logically execute an MB1 before posting a completion interrupt, and the interrupt handler software must execute an MB before the data is
guaranteed to be visible to the interrupted processor. Other processors must also execute MBs
before they are guaranteed to see the new data.
An important special case occurs when a write is done (perhaps by an I/O device) to some
physical page frame, then an MB is executed, and then a previously invalid PTE is changed to
be a valid mapping of the physical page frame that was just written. In this case, all processors
that access virtual memory by using the newly valid PTE must guarantee to deliver the newly
written data after the TB miss, for both I-stream and D-stream accesses.

5.6.4.4 Multiprocessor Instruction Stream (Including Single Processor with DMA I/O)
The only way to update the I-stream reliably is to write the shared I-stream on one processor or
DMA I/O device, then execute a CALL_PAL IMB (or an MB if the processor is not going to
execute the new I-stream, or the logical equivalent of an MB if it is a DMA I/O device), then
write a flag (equivalently, send an interrupt) signaling the other processor that the shared
I-stream is ready. Each receiving processor must read the new flag (equivalently, receive the
interrupt), execute a CALL_PAL IMB, then fetch the shared I-stream.

Software Note:
Note that this section does not describe how to reliably communicate I-stream from a
processor to a DMA device. See Section 5.6.4.7.
Leaving out the first CALL_PAL IMB (or MB) removes the assurance that the shared I-stream
is written before the flag.
Leaving out the second CALL_PAL IMB removes the assurance that the shared I-stream is
read only after the flag is seen to change; in this case, an early read could see an old value.

1 See Footnote 1 on page 5-22.

System Architecture and Programming Implications 5–23

This implies that after a DMA I/O device has written some I-stream to memory (such as paging in a page from disk), the DMA device must logically execute an MB 1 before posting a
completion interrupt, and the interrupt handler software must execute a CALL_PAL IMB
before the I-stream is guaranteed to be visible to the interrupted processor. Other processors
must also execute CALL_PAL IMB instructions before they are guaranteed to see the new
I-stream.
An important special case occurs under the following circumstances:
1. A write (perhaps by an I/O device) is done to some physical page frame.
2. A CALL_PAL IMB (or MB) is executed.
3. A previously invalid PTE is changed to be a valid mapping of the physical page frame
that was written in step 1.
In this case, all processors that access virtual memory by using the newly valid PTE must guarantee to deliver the newly written I-stream after the TB miss.

5.6.4.5 Multiprocessor Context Switch
If a process migrates from executing on one processor to executing on another, the context
switch operating system code must include a number of barriers.
A process migrates by having its context stored into memory, then eventually having that context reloaded on another processor. In between, some shared mechanism must be used to
communicate that the context saved in memory by the first processor is available to the second
processor. This could be done by using an interrupt, by using a flag bit associated with the
saved context, or by using a shared-memory multiprocessor data structure, as follows:

First Processor

Second Processor

:
Save state of current process.
MB [1]
Pass ownership of process context data structure memory.

⇒

Pick up ownership of process context data
structure memory.
MB [2]
Restore state of new process context data structure memory.
Make I-stream coherent [3].
Make TB coherent [4].
:
Execute code for new process that accesses
memory that is not common to all processes.

1 See Footnote 1 on page 5-22.
5–24 Alpha Architecture Handbook

MB [1] ensures that the writes done to save the state of the current process happen before
the ownership is passed.
MB [2] ensures that the reads done to load the state of the new process happen after the
ownership is picked up and hence are reliably the values written by the processor saving
the old state. Leaving this MB out makes the code fail if an old value of the context
remains in the second processor’s cache and invalidates from the writes done on the first
processor are not delivered soon enough.
The TB on the second processor must be made coherent with any write to the page tables
that may have occurred on the first processor just before the save of the process state. This
must be done with a series of TB invalidate instructions to remove any nonglobal page
mapping for this process, or by assigning an ASN that is unused on the second processor to
the process. One of these actions must occur sometime before starting execution of the
code for the new process that accesses memory (instruction or data) that is not common to
all processes. A common method is to assign a new ASN after gaining ownership of the
new process and before loading its context, which includes its ASN.
The D-cache on the second processor must be made coherent with any write to the
D-stream that may have occurred on the first processor just before the save of process
state. This is ensured by MB [2] and does not require any additional instructions.
The I-cache on the second processor must be made coherent with any write to the I-stream
that may have occurred on the first processor just before the save of process state. This can
be done with a CALL_PAL IMB sometime before the execution of any code that is not
common to all processes, More commonly, this can be done by forcing a TB miss (via the
new ASN or via TB invalidate instructions) and using the TB-fill rule (see Section 5.6.4.3).
This latter approach does not require any additional instruction.
Combining all these considerations gives the following, where, on a single processor, there is
no need for the barriers:

System Architecture and Programming Implications 5–25

First Processor

Second Processor

:
Pick up ownership of process context data structure memory.
MB
Assign new ASN or invalidate
TBs.
Save state of current process.
Restore state of new process.
MB
Pass ownership of process context
data structure memory.
:

⇒

:
Pickup ownership of new process context
data structure memory.
MB
Assign new ASN or invalidate TBs.
Save state of current process.
Restore state of new process.
MB
Pass ownership of old process context data
structure memory.
:
Execute code for new process that accesses
memory that is not common to all processes.

5.6.4.6 Multiprocessor Send/Receive Interrupt
If one processor writes some shared data, then sends an interrupt to a second processor, and
that processor receives the interrupt, then accesses the shared data, the sequence from Section
5.6.4.3 must be used:

5–26 Alpha Architecture Handbook

First Processor

Second Processor

:
Write data
MB
Send interrupt

⇒

Receive interrupt
MB
Access data
:

Leaving out the MB at the beginning of the interrupt-receipt routine causes the code to fail if
an old value of the context remains in the second processor’s cache, and invalidates from the
writes done on the first processor are not delivered soon enough.

5.6.4.7 Implications for Memory Mapped I/O
Sections 5.6.4.3 and 5.6.4.4 describe methods for communicating data from a processor or
DMA I/O device to another processor that work reliably in all Alpha systems. Special considerations apply to the communication of data or I-stream from a processor to a DMA I/O
device. These considerations arise from the use of bridges to connect to I/O buses with devices
that are accessible by memory accesses to non-memory-like regions of physical memory.
The following communication method works in all Alpha systems.
To reliably communicate shared data from a processor to an I/O device:
1. Write the shared data to a memory-like physical memory region on the processor.
2. Execute an MB instruction.
3. Write a flag (equivalently, send an interrupt or write a register location implemented in
the I/O device).
The receiving I/O device must:
1. Read the flag (equivalently, detect the interrupt or detect the write to the register location implemented in the I/O device).
2. Execute the equivalent of an MB1
3. Read the shared data.
As shown in Section 5.6.4.3, leaving out the memory barrier removes the assurance that the
shared data is written before the flag is. Unlike the case in Section 5.6.4.3, writing the shared
data to a non-memory-like physical memory region removes the assurance that the I/O device
1 In this context, the logical equivalent of an MB for a DMA device is whatever is necessary under the
applicable I/O subsystem architecture to ensure that preceding writes will be BEFORE (see Section
5.6.1.2) the subsequent reads of shared data. Typically, this action is defined to be present between
every read and write access done by the I/O device, according to the applicable I/O subsystem architecture.

System Architecture and Programming Implications 5–27

will detect the writes of the shared data before detecting the flag write, interrupt, or device register write.
This implies that after a processor has prepared a data buffer to be read from memory by a
DMA I/O device (such as writing a buffer to disk), the processor must execute an MB before
starting the I/O. The I/O device, after receiving the start signal, must logically execute an MB
before reading the data buffer, and the buffer must be located in a memory-like physical memory region.
There are methods of communicating data that may work in some systems but are not guaranteed in all systems. Two notable examples are:
1. If an Alpha processor writes a location implemented in a component located on an I/O
bus in the system, then executes a memory barrier, then writes a flag in some memory
location (in a memory-like or non-memory-like region), a device on the I/O bus may be
able to detect (via read access) the result of the flag in memory write and the write of
the location on the I/O bus out of order (that is, in a different order than the order in
which the Alpha processor wrote those locations).
2. If an Alpha processor writes a location that is a control register within an I/O device,
then executes a memory barrier, then writes a location in memory (in a memory-like or
non-memory-like region), the I/O device may be able to detect (via read access) the
result of the memory write before receiving and responding to the write of its own control register.
In almost every case, a mechanism that ensures the completion of writes to control register
locations within I/O devices is provided. The normal and strongly recommended mechanism is
to read a location after writing it, which guarantees that the write is complete. In any case, all
systems that use a particular I/O device should provide the same mechanism for that device.

5.6.4.8 Multiple Processors Writing to a Single I/O Device
Generally, for multiple processors to cooperate in writing to a single I/O device, the first processor must write to the device, execute an MB, then notify other processors. Another
processor that intends to write the same I/O device after the first processor must receive the
notification, execute an MB, and then write to the I/O device. For example:

First Processor
:
Write CSR_A
MB
Write flag (in memory)

5–28 Alpha Architecture Handbook

Second Processor

⇒

Read flag (in memory)
MB
Write CSR_B
:

The MB on the first processor guarantees that the write to CSR_A precedes the write to flag in
memory, as perceived on other processors. (The MB does not guarantee that the write to
CSR_A has completed. See Section 5.6.4.7 for a discussion of how a processor can guarantee
that a write to an I/O device has completed at that device.) The MB on the second processor
guarantees that the write to CSR_B will reach the I/O device after the write to CSR_A.

5.6.5 Implications for Hardware
The coherency point for physical address x is the place in the memory subsystem at which
accesses to x are ordered. It may be at a main memory board, or at a cache containing x exclusively, or at the point of winning a common bus arbitration.
The coherency point for x may move with time, as exclusive access to x migrates between
main memory and various caches.
MB and CALL_PAL IMB force all preceding writes to at least reach their respective coherency points. This does not mean that main-memory writes have been done, just that the order
of the eventual writes is committed. For example, on the XMI with retry, this means getting the
writes acknowledged as received with good parity at the inputs to memory board queues; the
actual RAM write happens later.
MB and CALL_PAL IMB also force all queued cache invalidates to be delivered to the local
caches before starting any subsequent reads (that may otherwise cache hit on stale data) or
writes (that may otherwise write the cache, only to have the write effectively overwritten by a
late-delivered invalidate).
WMB ensures that the final order of writes to memory-like regions is committed and that the
final order of writes to non-memory-like regions is committed. This does not imply that the
final order of writes to memory-like regions relative to writes to non-memory-like regions is
committed. It also prevents writes that precede the WMB from merging with writes that follow the WMB. For example, an implementation with a write buffer might implement WMB by
closing all valid write buffer entries from further merging and then drain the write buffer
entries in order.
Implementations may allow reads of x to hit (by physical address) on pending writes in a write
buffer, even before the writes to x reach the coherency point for x. If this is done, it is still true
that no earlier value of x may subsequently be delivered to the processor that took the hit on the
write buffer value.
Virtual data caches are allowed to deliver data before doing address translation, but only if
there cannot be a pending write under a synonym virtual address. Lack of a write-buffer match
on untranslated address bits is sufficient to guarantee this.
Virtual data caches must invalidate or otherwise become coherent with the new value whenever a PALcode routine is executed that affects the validity, fault behavior, protection
behavior, or virtual-to-physical mapping specified for one or more pages. Becoming coherent
can be delayed until the next subsequent MB instruction or TB fill (using the new mapping) if
the implementation of the PALcode routine always forces a subsequent TB fill.

System Architecture and Programming Implications 5–29

5.7 Arithmetic Traps
Alpha implementations are allowed to execute multiple instructions concurrently and to forward results from one instruction to another. Thus, when an arithmetic trap is detected, the PC
may have advanced an arbitrarily large number of instructions past the instruction T (calculating result R) whose execution triggered the trap.
When the trap is detected, any or all of these subsequent instructions may run to completion
before the trap is actually taken. The set of instructions subsequent to T that complete before
the trap is taken are collectively called the trap shadow of T. The PC pushed on the stack when
the trap is taken is the PC of the first instruction past the trap shadow.
The instructions in the trap shadow of T may use the UNPREDICTABLE result R of T, they
may generate additional traps, and they may completely change the PC (branches, JSR).
Thus, by the time a trap is taken, the PC pushed on the stack may bear no useful relationship to
the PC of the trigger instruction T, and the state visible to the programmer may have been
updated using the UNPREDICTABLE result R. If an instruction in the trap shadow of T uses
R to calculate a subsequent register value, that register value is UNPREDICTABLE, even
though there may be no trap associated with the subsequent calculation. Similarly:

•

If an instruction in the trap shadow of T stores R or any subsequent UNPREDICTABLE result, the stored value is UNPREDICTABLE.

•

If an instruction in the trap shadow of T uses R or any subsequent UNPREDICTABLE
result as the basis of a conditional or calculated branch, the branch target is UNPREDICTABLE.

•

If an instruction in the trap shadow of T uses R or any subsequent UNPREDICTABLE
result as the basis of an address calculation, the memory address actually accessed is
UNPREDICTABLE.

Software can follow the rules in Section 4.7.7.3 to reliably bound how far the PC may advance
before taking a trap, how far an UNPREDICTABLE result may propagate or continue from a
trap by supplying a well-defined result R within an arithmetic trap handler. Arithmetic instructions that do not use the /S exception completion qualifier can reliably produce that behavior
by inserting TRAPB instructions at appropriate points.

5–30 Alpha Architecture Handbook

Chapter 6

Common PALcode Architecture

6.1 PALcode
In a family of machines, both users and operating system developers require functions to be
implemented consistently. When functions conform to a common interface, the code that uses
those functions can be used on several different implementations without modification.
These functions range from the binary encoding of the instruction and data to the exception
mechanisms and synchronization primitives. Some of these functions can be implemented cost
effectively in hardware, but others are impractical to implement directly in hardware. These
functions include low-level hardware support functions such as Translation Buffer miss fill
routines, interrupt acknowledge, and vector dispatch. They also include support for privileged
and atomic operations that require long instruction sequences.
In the VAX, these functions are generally provided by microcode. This is not seen as a problem because the VAX architecture lends itself to a microcoded implementation.
One of the goals of Alpha architecture is to implement functions consistently without microcode. However, it is still desirable to provide an architected interface to these functions that
will be consistent across the entire family of machines. The Privileged Architecture Library
(PALcode) provides a mechanism to implement these functions without microcode.

6.2 PALcode Instructions and Functions
PALcode is used to implement the following functions:

•
•
•
•
•
•
•
•
•

Instructions that require complex sequencing as an atomic operation
Instructions that require VAX style interlocked memory access
Privileged instructions
Memory management control, including translation buffer (TB) management
Context swapping
Interrupt and exception dispatching
Power-up initialization and booting
Console functions
Emulation of instructions with no hardware support

Common PALcode Architecture 6–1

The Alpha architecture lets these functions be implemented in standard machine code that is
resident in main memory. PALcode is written in standard machine code with some implementation-specific extensions to provide access to low-level hardware. This lets an Alpha
implementation make various design trade-offs based on the hardware technology being used
to implement the machine. The PALcode can abstract these differences and make them invisible to system software.
For example, in a MOS VLSI implementation, a small (32-entry) fully associative TB can be
the right match to the media, given that chip area is a costly resource. In an ECL version, a
large (1024 entry) direct-mapped TB can be used because it will use RAM chips and does not
have fast associative memories available. This difference would be handled by implementation-specific versions of the PALcode on the two systems, both versions providing transparent
TB miss service routines. The operating system code would not need to know there were any
differences.
An Alpha Privileged Architecture Library (PALcode) of routines and environments is supplied
by Compaq. Other systems may use a library supplied by Compaq or architect and implement a
different library of routines. Alpha systems are required to support the replacement of PALcode defined by Compaq with an operating system-specific version.

6.3 PALcode Environment
The PALcode environment differs from the normal environment in the following ways:

•

Complete control of the machine state.

•

Interrupts are disabled.

•

Implementation-specific hardware functions are enabled, as described below.

•

I-stream memory management traps are prevented (by disabling I-stream mapping,
mapping PALcode with a permanent TB entry, or by other mechanisms).

Complete control of the machine state allows all functions of the machine to be controlled.
Disabling interrupts allows the system to provide multi-instruction sequences as atomic operations. Enabling implementation-specific hardware functions allows access to low-level system
hardware. Preventing I-stream memory management traps allows PALcode to implement
memory management functions such as translation buffer fill.

6.4 Special Functions Required for PALcode
PALcode uses the Alpha instruction set for most of its operations. A small number of additional functions are needed to implement the PALcode. Five opcodes are reserved to
implement PALcode functions: PAL19, PAL1B, PAL1D, PAL1E, and PAL1F. These instructions produce an trap if executed outside the PALcode environment.

•

PALcode needs a mechanism to save the current state of the machine and dispatch into
PALcode.

•

PALcode needs a set of instructions to access hardware control registers.

6–2 Alpha Architecture Handbook

•

PALcode needs a hardware mechanism to transition the machine from the PALcode
environment to the non-PALcode environment. This mechanism loads the PC, enables
interrupts, enables mapping, and disables PALcode privileges.

An Alpha implementation may also choose to provide additional functions to simplify or
improve performance of some PALcode functions. The following are some examples:

•

An Alpha implementation may include a read/write virtual function that allows PALcode to perform mapped memory accesses using the mapping hardware rather than providing the virtual-to-physical translation in PALcode routines. PALcode may provide a
special function to do physical reads and writes and have the Alpha loads and stores
continue to operate on virtual address in the PALcode environment.

•

An Alpha implementation may include hardware assists for various functions, such as
saving the virtual address of a reference on a memory management error rather than
having to generate it by simulating the effective address calculation in PALcode.

•

An Alpha implementation may include private registers so it can function without having to save and restore the native general registers.

6.5 PALcode Effects on System Code
PALcode will have one effect on system code. Because PALcode may reside in main memory
and maintain privileged data structures in main memory, the operating system code that allocates physical memory cannot use all of physical memory.
The amount of memory PALcode requires is small, so the loss to the system is negligible.

6.6 PALcode Replacement
Alpha systems are required to support the replacement of PALcode supplied by Compaq with
an operating system-specific version. The following functions must be implemented in PALcode, not directly in hardware, to facilitate replacement with different versions.

•

Translation Buffer fill. Different operating systems will want to replace the Translation
Buffer (TB) fill routines. The replacement routines will use different data structures.
Page tables will not be present in these systems. Therefore, no portion of the TB fill
flow that would change with a change in page tables may be placed in hardware, unless
it is placed in a manner that can be overridden by PALcode.

•

Process structure. Different operating systems might want to replace the process context switch routines. The replacement routines will use different data structures. The
HWPCB or PCB will not be present in these systems. Therefore, no portion of the context switching flows that would change with a change in process structure may be
placed in hardware.

PALcode can be viewed as consisting of the following somewhat intertwined components:

•

Chip/architecture component

•

Hardware platform component

•

Operating system component

Common PALcode Architecture 6–3

PALcode should be written modularly to facilitate the easy replacement or conditional building of each component. Such a practice simplifies the integration of CPU hardware, system
platform hardware, console firmware, operating system software, and compilers.
PALcode subsections that are commonly subject to modification include:

•

Translation Buffer fill

•

Process structure and context switch

•

Interrupt and exception frame format and routine dispatch

•

Privileged PALcode instructions

•

Transitions to and from console I/O mode

•

Power-up reset

6.7 Required PALcode Instructions
The PALcode instructions listed in Table 6–1 and Section C.11 must be recognized by mnemonic and opcode in all operating system implementations, but the effect of each instruction is
dependent on the implementation. Compaq defines the operation of these PALcode instructions for operating system implementations supplied by Compaq.

Table 6–1: PALcode Instructions that Require Recognition
Mnemonic

Name

BPT

Breakpoint trap

BUGCHK

Bugcheck trap

CSERVE

Console service

GENTRAP

Generate trap

RDUNIQUE

Read unique value

SWPPAL

Swap PALcode

WRUNIQUE

Write unique value

6–4 Alpha Architecture Handbook

The PALcode instructions listed in Table 6–2 and described in the following sections must be
supported by all Alpha implementations:

Table 6–2: Required PALcode Instructions
Mnemonic

Type

Operation

DRAINA

Privileged

Drain aborts

HALT

Privileged

Halt processor

IMB

Unprivileged

I-stream memory barrier

Common PALcode Architecture 6–5

6.7.1 Drain Aborts
Format:
CALL_PAL

!PALcode format

DRAINA

Operation:
IF PS<literal>(<)CM> NE 0 THEN
{privileged instruction exception}
{Stall instruction issuing until all prior
instructions are guaranteed to complete
without incurring aborts.}

Exceptions:
Privileged Instruction

Instruction mnemonics:
CALL_PAL DRAINA

Drain Aborts

Description:
If aborts are deliberately generated and handled (such as nonexistent memory aborts while sizing memory or searching for I/O devices), the DRAINA instruction forces any outstanding
aborts to be taken before continuing.
Aborts are necessarily implementation dependent. DRAINA stalls instruction issue at least
until all previously issued instructions have completed and any associated aborts have been
signaled, as follows:

•

For operate instructions, this usually means stalling until the result register has been
written.

•

For branch instructions, this usually means stalling until the result register and PC have
been written.

•

For load instructions, this usually means stalling until the result register has been written.

•

For store instructions, this usually means stalling until at least the first level in a potentially multilevel memory hierarchy has been written.

For load instructions, DRAINA does not necessarily guarantee that the unaccessed portions of
a cache block have been transferred error free before continuing.
For store instructions, DRAINA does not necessarily guarantee that the ultimate target location of the store has received error-free data before continuing. An implementation-specific
technique must be used to guarantee the ultimate completion of a write in implementations that
have multilevel memory hierarchies or store-and-forward bus adapters.
6–6 Alpha Architecture Handbook

6.7.2 Halt
Format:
CALL_PAL

!PALcode format

HALT

Operation:
IF PS<literal>(<)CM> NE 0 THEN
{privileged instruction exception}
CASE {halt_action} OF
! Operating System or Platform dependent choice
halt:
{halt}
restart/boot/halt:
{restart/boot/halt}
boot/halt:
{boot/halt}
debugger/halt:
{debugger/halt}
restart/halt:
{restart/halt}
ENDCASE

Exceptions:
Privileged Instruction

Instruction mnemonics:
CALL_PAL HALT

Halt Processor

Description:
The HALT instruction stops normal instruction processing and initiates some other operating
system or platform-specific behavior, depending on the HALT action setting. The choice of
behavior typically includes the initiation of a restart sequence, a system bootstrap, or entry into
console mode. See Console Interface (III), Chapter 3, in the Alpha Architecture Reference
Manual.

Common PALcode Architecture 6–7

6.7.3 Instruction Memory Barrier
Format:
CALL_PAL

!PALcode format

IMB

Operation:
{Make instruction stream coherent with data stream}

Exceptions:
None

Instruction mnemonics:
CALL_PAL IMB

I-stream Memory Barrier

Description:
An IMB instruction must be executed after software or I/O devices write into the instruction
stream or modify the instruction stream virtual address mapping, and before the new value is
fetched as an instruction. An implementation may contain an instruction cache that does not
track either processor or I/O writes into the instruction stream. The instruction cache and memory are made coherent by an IMB instruction.
If the instruction stream is modified and an IMB is not executed before fetching an instruction
from the modified location, it is UNPREDICTABLE whether the old or new value is fetched.

Software Note:
In a multiprocessor environment, executing an IMB on one processor does not affect
instruction caches on other processors. Thus, a single IMB on one processor is
insufficient to guarantee that all processors see a modification of the instruction stream.
The cache coherency and sharing rules are described in Console Interface (III), Chapter 2, in
the Alpha Architecture Reference Manual.

6–8 Alpha Architecture Handbook

Chapter 7

Console Subsystem Overview

On an Alpha system, underlying control of the system platform hardware is provided by a console subsystem. The console subsystem:

•

Initializes, tests, and prepares the system platform hardware for Alpha system software.

•

Bootstraps (loads into memory and starts the execution of) system software.

•

Controls and monitors the state and state transitions of each processor in a multiprocessor system.

•

Provides services to system software that simplify system software control of and
access to platform hardware.

•

Provides a means for a console operator to monitor and control the system.

The console subsystem interacts with system platform hardware to accomplish the first three
tasks. The actual mechanisms of these interactions are specific to the platform hardware; however, the net effects are common to all systems.
The console subsystem interacts with system software once control of the system platform
hardware has been transferred to that software.
The console subsystem interacts with the console operator through a virtual display device or
console terminal. The console operator may be a person or a management application.

Console Subsystem Overview 7–1

Chapter 8

Input/Output Overview

Conceptually, Alpha systems can consist of processors, memory, a processor-memory interconnect (PMI), I/O buses, bridges, and I/O devices.
Figure 8–1 shows the Alpha system overview.

Figure 8–1: Alpha System Overview
Processor-Memory Interconnect

I/O Device

Processor

Memory

Bridge

I/O Bus

I/O Device

As shown in Figure 8–1, processors, memory, and possibly I/O devices, are connected by a
PMI.
A bridge connects an I/O bus to the system, either directly to the PMI or through another I/O
bus. The I/O bus address space is available to the processor either directly or indirectly. Indirect access is provided through either an I/O mailbox or an I/O mapping mechanism. The I/O
mapping mechanism includes provisions for mapping between PMI and I/O bus addresses and
access to I/O bus operations.
Alpha I/O operations can include:

•

Accesses between the processor and an I/O device across the PMI

•

Accesses between the processor and an I/O device across an I/O bus

•

DMA accesses — I/O devices initiating reads and writes to memory

•

Processor interrupts requested by devices

•

Bus-specific I/O accesses

Chapter 9

OpenVMS Alpha

The following sections specify the Privileged Architecture Library (PALcode) instructions, that are
required to support an OpenVMS Alpha system.

9.1 Unprivileged OpenVMS Alpha PALcode
The unprivileged PALcode instructions provide support for system operations to all modes of operation (kernel, executive, supervisor, and user).
Table 9–1 describes the unprivileged OpenVMS Alpha PALcode instructions.
Table 9–1 : Unprivileged OpenVMS Alpha PALcode Instruction Summary
Mnemonic

Operation and Description

BPT

Breakpoint
The BPT instruction is provided for program debugging. It switches the processor to kernel mode and pushes R2..R7, the updated PC, and PS on the kernel stack. It then dispatches to the address in the Breakpoint vector, stored in a
control block.

BUGCHK

Bugcheck
The BUGCHK instruction is provided for error reporting. It switches the processor to kernel mode and pushes R2..R7, the updated PC, and PS on the kernel stack. It then dispatches to the address in the bugcheck vector, stored in a
control block. The value in R16 specifies the particular bugcheck type.

9–1

Table 9–1 : Unprivileged OpenVMS Alpha PALcode Instruction Summary (Continued)
Mnemonic

Operation and Description

CHME

Change mode to executive
The CHME instruction allows a process to change its mode in a controlled
manner. A change in mode also results in a change of stack pointers: the old
pointer is saved, the new pointer is loaded. Registers R2..R7, PS, and PC are
pushed onto the selected stack. The saved PC addresses the instruction following the CHME instruction. The value in R16 specifies the particular
exception type.

CHMK

Change mode to kernel
CHMK allows a process to change its mode to kernel in a controlled manner.
A change in mode also results in a change of stack pointers: the old pointer is
saved, the new pointer is loaded. R2..R7, PS, and PC are pushed onto the kernel stack. The saved PC addresses the instruction following the CHMK
instruction. The value in R16 specifies the particular exception type.

CHMS

Change mode to supervisor
CHMS allows a process to change its mode in a controlled manner. A change
in mode also results in a change of stack pointers: the old pointer is saved, the
new pointer is loaded. R2..R7, PS, and PC are pushed onto the selected stack.
The saved PC addresses the instruction following the CHMS instruction. The
value in R16 specifies the particular exception type.

CHMU

Change mode to user
CHMU allows a process to call a routine via the change mode mechanism.R2..R7, PS, and PC are pushed onto the current stack. The saved PC
addresses the instruction following the CHMU instruction. The value in R16
specifies the particular exception type.

CLRFEN

Clear floating-point enable
CLRFEN writes a zero to the floating-point enable register.

GENTRAP

Generate trap
GENTRAP is provided for reporting runtime software conditions. It switches
the processor to kernel mode and pushes registers R2..R7, the updated PC,
and the PS on the kernel stack. It then dispatches to the address of the GENTRAP vector, stored in a control block.

9–2 Alpha Architecture Handbook

Table 9–1 : Unprivileged OpenVMS Alpha PALcode Instruction Summary (Continued)
Mnemonic

Operation and Description

IMB

I-Stream memory barrier
IMB ensures that the contents of an instruction cache are coherent after the
instruction stream has been modified by software or I/O devices.If the instruction stream is modified and an IMB is not executed before fetching an instruction from the modified location, it is UNPREDICTABLE whether the old or
new value is fetched.

INSQHIL

Insert into longword queue at header, interlocked
The entry specified in R17 is inserted into the self-relative queue following
the header specified in R16. The insertion is a noninterruptible operation.
The insertion is interlocked to prevent concurrent interlocked insertions or
removals at the head or tail of the same queue by another process, in a multiprocessor environment.

INSQHILR

Insert into longword queue at header, interlocked resident
The entry specified in R17 is inserted into the self-relative queue following
the header specified in R16. The insertion is a noninterruptible operation.
The insertion is interlocked to prevent concurrent interlocked insertions or
removals at the head or tail of the same queue by another process, in a multiprocessor environment. This instruction requires that the queue be memoryresident and that the queue header and elements are quadword-aligned.

INSQHIQ

Insert into quadword queue at header, interlocked
The entry specified in R17 is inserted into the self-relative queue following
the header specified in R16. The insertion is a noninterruptible operation.
The insertion is interlocked to prevent concurrent interlocked insertions or
removals at the head or tail of the same queue by another process, in a multiprocessor environment.

INSQHIQR

Insert into quadword queue at header, interlocked resident
The entry specified in R17 is inserted into the self-relative queue following
the header specified in R16. The insertion is a noninterruptible operation.
The insertion is interlocked to prevent concurrent interlocked insertions or
removals at the head or tail of the same queue by another process, in a multiprocessor environment.This instruction requires that the queue be memoryresident and that the queue header and elements are octaword-aligned.

INSQTIL

Insert into longword queue at tail, interlocked
The entry specified in R17 is inserted into the self-relative queue preceding
the header specified in R16. The insertion is a noninterruptible operation.
The insertion is interlocked to prevent concurrent interlocked insertions or
removals at the head or tail of the same queue by another process, in a multiprocessor environment.
9–3

Table 9–1 : Unprivileged OpenVMS Alpha PALcode Instruction Summary (Continued)
Mnemonic

Operation and Description

INSQTILR

Insert into longword queue at tail, interlocked resident
The entry specified in R17 is inserted into the self-relative queue preceding
the header specified in R16. The insertion is a noninterruptible operation.
The insertion is interlocked to prevent concurrent interlocked insertions or
removals at the head or tail of the same queue by another process, in a multiprocessor environment. This instruction requires that the queue be memoryresident and that the queue header and elements are quadword-aligned.

INSQTIQ

Insert into quadword queue at tail, interlocked
The entry specified in R17 is inserted into the self-relative queue preceding
the header specified in R16. The insertion is a noninterruptible operation.
The insertion is interlocked to prevent concurrent interlocked insertions or
removals at the head or tail of the same queue by another process, in a multiprocessor environment.

INSQTIQR

Insert into quadword queue at tail, interlocked resident
The entry specified in R17 is inserted into the self-relative queue preceding
the header specified in R16. The insertion is a noninterruptible operation.
The insertion is interlocked to prevent concurrent interlocked insertions or
removals at the head or tail of the same queue by another process, in a multiprocessor environment. This instruction requires that the queue be memoryresident and that the queue header and elements are octaword-aligned.

INSQUEL

Insert into longword queue
The entry specified in R17 is inserted into the absolute queue following the
entry specified by the predecessor addressed by R16 for INSQUEL, or following the entry specified by the contents of the longword addressed by R16
for INSQUEL/D. The insertion is a noninterruptible operation.

INSQUEQ

Insert into quadword queue
The entry specified in R17 is inserted into the absolute queue following the
entry specified by the predecessor addressed by R16 for INSQUEQ, or following the entry specified by the contents of the quadword addressed by R16
for INSQUEQ/D. The insertion is a noninterruptible operation.

PROBE

Probe read/write access
PROBE checks the read (PROBER) or write (PROBEW) accessibility of the
first and last byte specified by the base address and the signed offset; the bytes
in between are not checked. System software must check all pages between
the two bytes if they are to be accessed. PROBE is only intended to check
a single datum for accessibility.

9–4 Alpha Architecture Handbook

Table 9–1 : Unprivileged OpenVMS Alpha PALcode Instruction Summary (Continued)
Mnemonic

Operation and Description

RD_PS

Read processor status
RD_PS writes the Processor Status (PS) to register R0.

READ_UNQ

Read unique context
READ_UNQ reads the hardware process (thread) unique context value, if
previously written by WRITE_UNQ, and places that value in R0.

REI

Return from exception or interrupt
The PS, PC, and saved R2..R7 are popped from the current stack and held in
temporary registers. The new PS is checked for validity and consistency. If it
is valid and consistent, the current stack pointer is then saved and a new stack
pointer is selected. Registers R2 through R7 are restored by using the saved
values held in the temporary registers. A check is made to determine if an
AST or interrupt is pending.If the enabling conditions are present for an interrupt or AST at the completion of this instruction, the interrupt or AST occurs
before the next instruction.

REMQHIL

Remove from longword queue at header, interlocked
The self-relative queue entry following the header, pointed to by R16, is
removed from the queue, and the address of the removed entry is returned in
R1. The removal is interlocked to prevent concurrent interlocked insertions or
removals at the head or tail of the same queue by another process, in a multiprocessor environment. The removal is a noninterruptible operation.

REMQHILR

Remove from longword queue at header, interlocked resident
The queue entry following the header, pointed to by R16, is removed from the
self-relative queue, and the address of the removed entry is returned in R1.
The removal is interlocked to prevent concurrent interlocked insertions or
removals at the head or tail of the same queue by another process, in a multiprocessor environment. The removal is a noninterruptible operation. This
instruction requires that the queue be memory-resident and that the queue
header and elements are quadword-aligned.

REMQHIQ

Remove from quadword queue at header, interlocked
The self-relative queue entry following the header, pointed to by R16, is
removed from the queue and the address of the removed entry is returned in
R1. The removal is interlocked to prevent concurrent interlocked insertions or
removals at the head or tail of the same queue by another process, in a multiprocessor environment. The removal is a noninterruptible operation.

9–5

Table 9–1 : Unprivileged OpenVMS Alpha PALcode Instruction Summary (Continued)
Mnemonic

Operation and Description

REMQHIQR

Remove from quadword queue at header, interlocked resident
The queue entry following the header, pointed to by R16, is removed from the
self-relative queue and the address of the removed entry is returned in R1. The
removal is interlocked to prevent concurrent interlocked insertions or removals at the head or tail of the same queue by another process, in a multiprocessor environment. The removal is a noninterruptible operation. This instruction
requires that the queue be memory-resident and that the queue header and elements are octaword-aligned.

REMQTIL

Remove from longword queue at tail, interlocked
The queue entry preceding the header, pointed to by R16, is removed from the
self-relative queue and the address of the removed entry is returned in R1. The
removal is interlocked to prevent concurrent interlocked insertions or removals at the head or tail of the same queue by another process, in a multiprocessor environment. The removal is a noninterruptible operation.

REMQTILR

Remove from longword queue at tail, interlocked resident
The queue entry preceding the header, pointed to by R16, is removed from the
self-relative queue and the address of the removed entry is returned in R1. The
removal is interlocked to prevent concurrent interlocked insertions or removals at the head or tail of the same queue by another process, in a multiprocessor environment. The removal is a noninterruptible operation. This instruction
requires that the queue be memory-resident and that the queue header and elements are quadword-aligned.

REMQTIQ

Remove from quadword queue at tail, interlocked
The self-relative queue entry preceding the header, pointed to by R16, is
removed from the queue and the address of the removed entry is returned in
R1. The removal is interlocked to prevent concurrent interlocked insertions or
removals at the head or tail of the same queue by another process, in a multiprocessor environment. The removal is a noninterruptible operation.

REMQTIQR

Remove from quadword queue at tail, interlocked resident
The queue entry preceding the header, pointed to by R16, is removed from the
self-relative queue and the address of the removed entry is returned in R1. The
removal is interlocked to prevent concurrent interlocked insertions or removals at the head or tail of the same queue by another process, in a multiprocessor environment. The removal is a noninterruptible operation. This instruction
requires that the queue be memory-resident and that the queue header and elements are octaword-aligned.

9–6 Alpha Architecture Handbook

Table 9–1 : Unprivileged OpenVMS Alpha PALcode Instruction Summary (Continued)
Mnemonic

Operation and Description

REMQUEL

Remove from longword queue
The queue entry addressed by R16 for REMQUEL or the entry addressed by
the longword addressed by R16 for REMQUEL/D is removed from the longword absolute queue, and the address of the removed entry is returned in R1.
The removal is a noninterruptible operation.

REMQUEQ

Remove from quadword queue
The queue entry addressed by R16 for REMQUEQ or the entry addressed by
the quadword addressed by R16 for REMQUEL/D is removed from the quadword absolute queue, and the address of the removed entry removed is
returned in R1. The removal is a noninterruptible operation.

RSCC

Read system cycle counter
Register R0 is written with the value of the system cycle counter. This counter
is an unsigned 64-bit integer that increments at the same rate as the process
cycle counter. The system cycle counter is suitable for timing a general range
of intervals to within 10% error and may be used for detailed performance
characterization.

SWASTEN

Swap AST enable
SWASTEN swaps the AST enable bit for the current mode. The new state for
the enable bit is supplied in register R16<0> and previous state of the enable
bit is returned, zero-extended, in R0. A check is made to determine if an AST
is pending. If the enabling conditions are present for an AST at the completion
of this instruction, the AST occurs before the next instruction.

WRITE_UNQ

Write unique context
WRITE_UNQ writes the hardware process (thread) unique context value
passed in R16 to internal storage or to the hardware privileged context block.

WR_PS_SW

Write processor status software field
WR_PS_SW writes the Processor Status software field (PS<SW>) with the
low-order three bits of R16<2:0>.

9–7

9.2 Privileged OpenVMS Alpha Palcode
The privileged PALcode instructions can be called in kernel mode only.
Table 9–2 describes the privileged OpenVMS Alpha PALcode instructions.
Table 9–2 : Privileged OpenVMS Alpha PALcode Instructions Summary
Mnemonic

Operation and Description

CFLUSH

Cache flush
At least the entire physical page specified by a page frame number in R16 is
flushed from any data caches associated with the current processor. After doing
a CFLUSH, the first subsequent load on the same processor to an arbitrary
address in the target page is fetched from physical memory.

CSERVE

Console service
CSERVE is specific to each PALcode and console implementation and is not
intended for operating system use.

DRAINA

Drain aborts
DRAINA stalls instruction issuing until all prior instructions are guaranteed to
complete without incurring aborts.

HALT

Halt processor
HALT stops normal instruction processing.

LDQP

Load quadword physical
The quadword-aligned memory operand, whose physical address is in R16, is
fetched and written to R0. If the operand address in R16 is not quadwordaligned, the result is UNPREDICTABLE.

MFPR

Move from processor register
The internal processor register specified by the PALcode function field is written to R0.

MTPR

Move to processor register
The source operands in integer registers R16 (and R17, reserved for future use)
are written to the internal processor register specified by the PALcode function
field. The effect of loading a processor register is guaranteed to be active on the
next instruction.

9–8 Alpha Architecture Handbook

Table 9–2 : Privileged OpenVMS Alpha PALcode Instructions Summary (Continued)
Mnemonic

Operation and Description

STQP

Store quadword physical
The quadword contents of R17 are written to the memory location whose physical address is in R16. If the operand address in R16 is not quadword-aligned,
the result is UNPREDICTABLE.

SWPCTX

Swap privileged context
SWPCTX returns ownership of the data structure that contains the current
hardware privileged context (the HWPCB) to the operating system and passes
ownership of the new HWPCB to the processor.

SWPPAL

Swap PALcode image
SWPPAL causes the current PALcode to be replaced by the specified new
PALcode image. Intended for use by operating systems only during bootstraps
and by consoles during transitions to console I/O mode.

WTINT

Wait for interrupt
WTINT requests that, if possible, the PALcode wait for the first of either of the
following conditions before returning: any interrupt other than a clock tick; or,
the first clock tick after a specified number of clock ticks has been skipped.

9–9

Chapter 10

Digital UNIX

The following sections specifiy the Privileged Architecture Library (PALcode) instructions that are
required to support a Digital UNIX system.

10.1 Unprivileged Digital UNIX PALcode
Table 10–1 describes the unprivileged Digital UNIX PALcode instructions.
Table 10–1 : Unprivileged Digital UNIX PALcode Instruction Summary

Mnemonic

Operation and Description

bpt

Break point trap
The bpt instruction switches mode to kernel, builds a stack frame on the kernel
stack, and dispatches to the breakpoint code.

bugchk

Bugcheck
The bugchk instruction switches mode to kernel, builds a stack frame on the
kernel stack, and dispatches to the breakpoint code.

callsys

System call
The callsys instruction switches mode to kernel, builds a callsys stack frame,
and dispatches to the system call code.

clrfen

Clear floating-point enable
The clrfen instruction writes a zero to the floating-point enable register.

gentrap

Generate trap
The gentrap instruction switches mode to kernel, builds a stack frame on the
kernel stack, and dispatches to the gentrap code.

imb

I-stream memory barrier
The imb instruction makes the I-cache coherent with main memory.

10–1

Table 10–1 : Unprivileged Digital UNIX PALcode Instruction Summary (Continued)

Mnemonic

Operation and Description

rdunique

Read unique
The rdunique instruction returns the process unique value.

urti

Return from user mode trap
The urti instruction pops from the user stack the registers a0 through a2, the
global pointer, the new user assembler temporary register, the stack pointer, the
program counter, and the processor status register.

wrunique

Write unique
The wrunique instruction sets the process unique register.

10.2 Privileged Digital UNIX PALcode
The privileged PALcode instructions can be called only from kernel mode. They provide an interface
to control the privileged state of the machine.
Table 10–2 describes the privileged Digital UNIX PALcode instructions.
Table 10–2 : Privileged Digital UNIX PALcode Instruction Summary

Mnemonic

Operation and Description

cflush

Cache flush The cflush instruction flushes an entire physical page pointed to by
the specified page frame number (PFN) from any data caches associated with
the current processor. All processors must implement this instruction.

cserve

Console service This instruction is specific to each PALcode and console
implementation and is not intended for operating system use.

draina

Drain aborts The draina instruction stalls instruction issuing until all prior
instructions are guaranteed to complete without incurring aborts.

halt

Halt processor The halt instruction stops normal instruction processing.
Depending on the halt action setting, the processor can either enter console
mode or the restart sequence.

rdmces

Read machine check error summary The rdmces instruction returns the MCES
register in v0.

rdps

Read processor status The rdps instruction returns the current PS.

rdusp

Read user stack pointer The rdusp instruction reads the user stack pointer while
in kernel mode and returns it.

rdval

Read system value The rdval instruction reads a 64-bit per-processor value and
returns it.

10–2 Alpha Architecture Handbook

Table 10–2 : Privileged Digital UNIX PALcode Instruction Summary (Continued)

Mnemonic

Operation and Description

retsys

Return from system call The retsys instruction pops the return address, the user
stack pointer, and the user global pointer from the kernel stack. It then saves the
kernel stack pointer, sets mode to user, enables interrupts, and jumps to the
address popped off the stack.

rti

Return from trap, fault or interrupt The rti instruction pops certain registers
from the kernel stack. If the new mode is user, the kernel stack is saved and the
user stack restored.

swpctx

Swap privileged context The swpctx instruction saves the current process data
in the current process control block (PCB). Then swpctx switches to the PCB
and loads the new process context.

swpipl

Swap IPL The swpipl instruction returns the current value IPL and sets the IPL.

swppal

Swap PALcode image The swppal instruction causes the current PALcode to
be replaced by the specified new PALcode image. Intended only for use by
operating systems during bootstraps and by consoles during transitions to console I/O mode.

tbi

TB invalidate The tbi instruction removes entries from the instruction and data
translation buffers when the mapping entries change.

whami

Who_Am_I The whami instruction returns the processor number for the current processor. The processor number is in the range 0 to the number of processors minus one (0..numproc–1) that can be configured in the system.

wrent

Write system entry address The wrent instruction sets the virtual address of the
system entry points.

wrfen

Write floating-point enable The wrfen instruction writes a bit to the floatingpoint enable register.

wripir

Write interprocessor interrupt request The wripr instruction generates an interprocessor interrupt on the processor number passed as an input parameter. The
interrupt request is recorded on the target processor and initiated when the
proper enabling conditions are present.

wrkgp

Write kernel global pointer The wrkgp instruction writes the kernel global
pointer internal register.

wrmces

Write machine check error summary The wrmces instructions clears the
machine check in progress bit and clears the processor- or system-correctable
error in progress bit in the MCES register. The instruction also sets or clears the
processor- or system-correctable error reporting enable bit in the MCES register.

wrperfmon

Performance monitoring function The wrperfmon instruction alerts any performance monitoring software/hardware in the system to monitor the performance
of this process.

wrusp

Write user stack pointer The wrusp instruction writes a value to the user stack
pointer while in kernel mode.
10–3

Table 10–2 : Privileged Digital UNIX PALcode Instruction Summary (Continued)

Mnemonic

Operation and Description

wrval

Write system value The wrval instruction writes a 64-bit per-processor value.

wrvptptr

Write virtual page table pointer The wrvptptr instruction writes a pointer to the
virtual page table pointer (vptptr).

wtint

Wait for interrupt The wtint instruction requests that, if possible, the PALcode
wait for the first of either of the following conditions before returning: any
interrupt other than a clock tick; or, the first clock tick after a specified number
of clock ticks has been skipped.

10–4 Alpha Architecture Handbook

Chapter 11

Windows NT Alpha

The following sections specify the Privileged Architecture Library (PALcode) instructions that are
required to support a Windows NT Alpha system.

11.1 Unprivileged Windows NT Alpha PALcode
The unprivileged PALcode instuctions provide support for system operations and may be called from
both kernel and user modes.

Table 11–1 : Unprivileged Windows NT Alpha PALcode Instruction Summary
Mnemonic

Operation and description

bpt

Breakpoint trap (standard user-mode breakpoint)
The bpt instruction raises a breakpoint general exception to the kernel, setting a
USER_BREAKPOINT breakpoint type.

callkd

Call kernel debugger
The callkd instruction raises a breakpoint general exception to the kernel, setting the breakpoint type with the value supplied as an input parameter.

callsys

System service call
The callsys instruction raises a system service call exception to the kernel. Callsys switches to kernel mode if necessary, builds a trap frame on the kernel
stack, and then enters the kernel at the kernel system service exception handler.

gentrap

Generate a trap
The gentrap instruction generates a software general exception that raises an
exception code to the current thread. The exception code is generated from a
trap number that is specified as an input parameter. Gentrap is used to raise
software-detected exceptions such as bound check errors or overflow conditions.

11–1

Table 11–1 : Unprivileged Windows NT Alpha PALcode Instruction Summary
Mnemonic

Operation and description

imb

Instruction memory barrier
The imb instruction guarantees that all subsequent instruction stream fetches
are coherent with respect to main memory. Imb must be issued before executing code in memory that has been modified (either by stores from the processor
or DMA from an I/O processor). User-mode code that modifies the I-stream
must call the appropriate Windows NT API to ensure I-cache coherency.

kbpt

Kernel breakpoint trap
The kbpt instruction raises a breakpoint general exception to the kernel, setting
a KERNEL_BREAKPOINT breakpoint type.

rdteb

Read thread environment block pointer
The rdteb instruction returns the contents of the TEB internal processor register
for the currently executing thread (the base address of the thread environment
block).

11.2 Privileged Windows NT Alpha PALcode
The privileged PALcode instuctions provide support for system operations and may be called from
only kernel mode.

Table 11–2 : Privileged Windows NT Alpha PALcode Instruction Summary
Mnemonic

Operation and description

csir

Clear software interrupt request
The csir instruction clears the specified bit in the SIRR internal processor register.

dalnfix

Disable alignment fixups
The dalnfix instruction disables alignment fixups in PALcode and generates
alignment fault exceptions whenever an alignment fault occurs. After dalnfix is
executed on a processor, all alignment faults on that processor are not fixed-up
by PALcode and alignment fault exceptions are dispatched to the kernel until
the ealnfix instruction is executed on that processor.

Disable all interrupts
The di instruction disables all interrupts by clearing the interrupt enable (IE) bit
in the PSR internal processor register. The IRQL field is unaffected. Interrupts
may be re-enabled via the ei instruction.

11–2 Alpha Architecture Handbook

Table 11–2 : Privileged Windows NT Alpha PALcode Instruction Summary (Continued)
Mnemonic

Operation and description

draina

Drain all aborts including machine checks
The draina instruction drains all aborts, including machine checks, from the
current processor. Draina guarantees that no abort is signaled for an instruction
issued before the draina while any instruction issued subsequent to the draina is
executing.

dtbis

Data translation buffer invalidate single
The dtbis instruction invalidates a single data stream translation. The translation for the virtual address must be invalidated in all data translation buffers
and in all virtual data caches.

ealnfix

Enable alignment fixups
The ealnfix instruction enables alignment fixups in PALcode and prevents
alignment fault exceptions. After ealnfix is executed on a processor, all alignment faults on that processor are fixed-up by PALcode and no alignment fault
exceptions are dispatched to the kernel until the dalnfix instruction is executed
on that processor.

Enable interrupts
The ei instruction enables interrupts for the IRQL set in the PSR internal processor register by setting the interrupt enable (IE) bit in the PSR.

halt

Halt the operating system by forcing illegal instruction trap
The halt instruction forces an illegal instruction exception.

initpal

Initialize PALcode data structures with operating system values
The initpal instruction is called early in the kernel initialization sequence to
establish values for internal processor registers (IPRs) that are needed for trap
and fault handling. The KGP and PCR registers are initialized once and persist
throughout the run time of the operating system.

initpcr

Initialize processor control region data
The initpcr instruction caches process-specific information, including parts of
the interrupt level table (ILT), for use by the PALcode.

rdcounters

Read the software event counters
The rdcounters instruction is only used with debug PALcode. With production
PALcode, rdcounters returns a status value of zero, indicating that it is not
implemented in the current PALcode image.

11–3

Table 11–2 : Privileged Windows NT Alpha PALcode Instruction Summary (Continued)
Mnemonic

Operation and description

rdirql

Read the current IRQL from the PSR
The rdirql instruction returns the contents of the interrupt request level (IRQL)
field of the PSR internal processor register.

rdksp

Read initial kernel stack pointer for the current thread
The rdksp instruction returns the contents of the IKSP (initial kernel stack
pointer) internal processor register for the currently executing thread.

rdmces

Read the machine check error summary register
The rdmces instruction returns the contents of the machine check error summary (MCES) internal processor register.

rdpcr

Read the processor control region base address
The rdpcr instruction returns the contents of the PCR internal processor register
(the base address value of the processor control region).

rdpsr

Read the current processor status register (PSR)
The rdpsr instruction returns the contents of the current PSR (Processor Status
Register) internal processor register.

rdstate

Read the current internal processor state
The rdstate instruction returns the internal processor state to an internal buffer.

rdthread

Read the thread value for the current thread
The rdthread instruction returns the contents of the THREAD internal processor register (the value of the currently executing thread).

reboot

Transfer to console firmware
The reboot instruction stops the operating system from executing and returns
execution to the boot environment. Reboot is responsible for completing the
ARC restart block before returning to the boot environment.

restart

Restart the operating system from the restart block
The restart instruction restores saved processor state and resumes execution of
the operating system.

11–4 Alpha Architecture Handbook

Table 11–2 : Privileged Windows NT Alpha PALcode Instruction Summary (Continued)
Mnemonic

Operation and description

retsys

Return from system service call exception
The retsys instruction returns from a system service call exception by unwinding the trap frame and returning to the code stream that was executing when the
original exception was initiated. In addition, retsys accepts a parameter to set
software interrupt requests that became pending while the exception was handled.

rfe

Return from trap or interrupt
The rfe instruction returns from exceptions by unwinding the trap frame and
returning to the code stream that was executing when the original exception
was initiated. In addition, rfe accepts a parameter to set software interrupt
requests that became pending while the exception was handled.

ssir

Set software interrupt request
The ssir instruction sets software interrupt requests by setting the appropriate
bits in the SIRR internal processor register.

swpctx

Swap thread context
The swpctx instruction swaps the privileged portions of thread context. Thread
context is swapped by establishing the new IKSP, THREAD, and TEB internal
processor register values.

swpirql

Swap the current interrupt request level
The swpirql instruction swaps the current IRQL field in the PSR internal processor register by setting the processor so that only permitted interrupts are
enabled for the new IRQL. Swpirql updates the IRQL field and returns the previous IRQL.

swpksp

Swap the initial kernel stack pointer for the current thread
The swpksp instruction returns the value of the previous IKSP internal processor register and writes a new IKSP for the currently executing thread.

swppal

Swap the currently executing PALcode
The swppal instruction swaps the currently executing PALcode by transferring
to the base address of the new PALcode image in the PALcode environment.

swpprocess

Swap process context (swap address space)
The swpprocess instruction swaps the privileged process context by changing
the address space for the currently executing thread.

11–5

Table 11–2 : Privileged Windows NT Alpha PALcode Instruction Summary (Continued)
Mnemonic

Operation and description

tbia

Translation buffer invalidate all
The tbia instruction invalidates all translations and virtual cache blocks within
the processor.

tbim

Translation buffer invalidate multiple
The tbim instruction invalidates multiple virtual translations for the current
ASN. The translation for the virtual address must be invalidated in all processor
translation buffers and virtual caches.

tbimasn

Translation buffer invalidate multiple for ASN
The tbimasn instruction invalidates multiple virtual translations for a specified
ASN. The translation for the virtual addresses must be invalidated in all processor translation buffers and virtual caches.

tbis

Translation buffer invalidate single
The tbis instruction invalidates a single virtual translation. The translation for
the passed virtual address must be invalidated in all processor translation buffers and virtual caches.

tbisasn

Translation buffer invalidate single for ASN
The tbisasn instruction invalidates a single virtual translation for a specified
address space. The translation for the passed virtual address must be invalidated in all processor translation buffers and virtual caches.

wrentry

Write kernel exception entry routine
The wrentry instruction provides the registry of exception handling routines for
the exception classes. The kernel must use wrentry to register an exception handler for each of the exception classes.

wrmces

Write the machine check error summary register
The wrmces instruction writes new values for the MCES internal processor
register and returns the previous values of that register.

wrperfmon

Write performance counter interrupt control information
The wrperfmon instruction writes control information for the two processor
performance counters. One parameter identifies the selected performance
counter, while another controls whether the selected performance counter is
enabled or disabled. The instruction returns the previous enable state for the
selected performance counter.

Appendix A

Software Considerations

A.1 Hardware-Software Compact
The Alpha architecture, like all RISC architectures, depends on careful attention to data alignment and instruction scheduling to achieve high performance.
Since there will be various implementations of the Alpha architecture, it is not obvious how
compilers can generate high-performance code for all implementations. This chapter gives
some scheduling guidelines that, if followed by all compilers and respected by all implementations, will result in good performance. As such, this section represents a good-faith compact
between hardware designers and software writers. It represents a set of common goals, not a
set of architectural requirements. Thus, an Appendix, not a Chapter.
Many of the performance optimizations discussed below provide an advantage only for frequently executed code. For rarely executed code, they may produce a bigger program that is
not any faster. Some of the branching optimizations also depend on good prediction of which
path from a conditional branch is more frequently executed. These optimizations are best determined by using an execution profile, either an estimate generated by compiler heuristics, or a
real profile of a previous run, such as that gathered by PC-sampling in PCA.
Each computer architecture has a "natural word size." For the PDP-11, it is 16 bits; for VAX,
32 bits; and for Alpha, 64 bits. Other architectures also have a natural word size that varies
between 16 and 64 bits. Except for very low-end implementations, ALU data paths, cache
access paths, chip pin buses, and main memory data paths are all usually the natural word size.
As an architecture becomes commercially successful, high-end implementations inevitably
move to double-width data paths that can transfer an aligned (at an even natural word address)
pair of natural words in one cycle. For Alpha, this means 128-bit wide data paths will eventually be implemented. It is difficult to get much speed advantage from paired transfers unless
the code being executed has instructions and data appropriately aligned on aligned octaword
boundaries. Since this is difficult to retrofit to old code, the following sections sometimes
encourage "over-aligning" to octaword boundaries in anticipation of high-speed Alpha
implementations.

A–1

In some cases, there are performance advantages to aligning instructions or data to cache-block
boundaries, or putting data whose use is correlated into the same cache block, or trying to
avoid cache conflicts by not having data whose use is correlated placed at addresses that are
equal modulo the cache size. Since the Alpha architecture will have many implementations, an
exact cache design cannot be outlined here.
In each case below, the performance implication is given by an order-of-magnitude number: 1,
3, 10, 30, or 100. A factor of 10 means that the performance difference being discussed will
likely range from 3 to 30 across all Alpha implementations.

A.2 Instruction-Stream Considerations
The following sections describe considerations for the instruction stream.

A.2.1 Instruction Alignment
Code PSECTs should be octaword aligned. Targets of frequently taken branches should be at
least quadword aligned, and octaword aligned for very frequent loops. Compilers could use
execution profiles to identify frequently taken branches.
Quadword I-fetch implementors should give first priority to executing aligned quadwords
quickly. Octaword-fetch implementors should give first priority to executing aligned octawords quickly, and second priority to executing aligned quadwords quickly. Dual-issue
implementations should give first priority to issuing both halves of an aligned quadword in one
cycle, and second priority to buffering and issuing other combinations.

A.2.2 Branch Prediction and Minimizing Branch-Taken — Factor of 3
In many Alpha implementations, an unexpected change in I-stream address will result in about
10 lost instruction times. "Unexpected" may mean any branch-taken or may mean a mispredicted branch. In many implementations, even a correctly predicted branch to a quadword
target address will be slower than straight-line code.
Compilers should follow these rules to minimize unexpected branches:
1. Branch prediction is implementation specific. Based on execution profiles, compilers
should physically rearrange code so that it has matching behavior.
2. Make basic blocks as big as possible. A good goal is 20 instructions on average
between branch-taken. This requires unrolling loops so that they contain at least 20
instructions, and putting subroutines of less than 20 instructions directly in line. It also
requires using execution profiles to rearrange code so that the frequent case of a conditional branch falls through. For very high-performance loops, it will be profitable to
move instructions across conditional branches to fill otherwise wasted instruction issue
slots, even if the instructions moved will not always do useful work. Note that using the
Conditional Move instructions can sometimes avoid breaking up basic blocks.
3. In an if-then-else construct whose execution profile is skewed even slightly away from
50%-50% (51-49 is enough), put the infrequent case completely out of line, so that the
frequent case encounters zero branch-takens, and the infrequent case encounters two

A–2 Alpha Architecture Handbook

branch-takens. If the infrequent case is rare (5%), put it far enough away that it never
comes into the I-cache. If the infrequent case is extremely rare (error message code),
put it on a page of rarely executed code and expect that page never to be paged in.
4. There are two functionally identical branch-format opcodes, BSR and BR, as shown in
Figure A–1.

Figure A–1: Branch-Format BSR and BR Opcodes
31

26 25

21 20

BSR

Displacement

Branch Format

Displacement

Branch Format

Compilers should use the first one for subroutine calls, and the second for GOTOs.
Some implementations may push a stack of predicted return addresses for BSR and not
push the stack for BR. Failure to compile the correct opcode will result in mispredicted
return addresses, and hence make subroutine returns slow.
5. The memory-format JSR instruction, shown in Figure A–2, has 16 unused bits. These
should be used by the compilers to communicate a hint about expected branch-target
behavior (see Section 4.3).

Figure A–2: Memory-Format JSR Instruction
31

16 15

JSR

Memory Format

If the JSR is used for a computed GOTO or a CASE statement, compile bits <15:14>
as 00, and bits <13:0> such that (updated PC+Instr<13:0>*4) <15:0> equals
(likely_target_addr) <15:0>. In other words, pick the low 14 bits so that a normal
PC+displacement*4 calculation will match the low 16 bits of the most likely target
longword address. (Implementations will likely prefetch from the matching cache
block.)
If the JSR is used for a computed subroutine call, compile bits <15:14> as 01, and bits
<13:0> as above. Some implementations will prefetch the call target using the
prediction and also push updated PC on a return-prediction stack.
If the JSR is used as a subroutine return, compile bits <15:14> as 10. Some
implementations will pop an address off a return-prediction stack.
If the JSR is used as a coroutine linkage, compile bits <15:14> as 11. Some
implementations will pop an address off a return-prediction stack and also push
updated PC on the return-prediction stack.
Implementors should give first priority to executing straight-line code with no branch-takens as
Software Considerations A–3

quickly as possible, second priority to predicting conditional branches based on the sign of the
displacement field (backward taken, forward not-taken), and third priority to predicting subroutine return addresses by running a small prediction stack. (VAX traces show a stack of two
to four entries correctly predicts most branches.)

A.2.3 Improving I-Stream Density — Factor of 3
Compilers should try to use profiles to make sure almost 100% of the bytes brought into an
I-cache are actually executed. This requires aligning branch targets and putting rarely executed
code out of line.

A.2.4 Instruction Scheduling — Factor of 3
The performance of Alpha programs is sensitive to how carefully the code is scheduled to minimize instruction-issue delays.
"Result latency" is defined as the number of CPU cycles that must elapse between an instruction that writes a result register and one that uses that register, if execution-time stalls are to be
avoided. Thus, with a latency of zero, the instruction writes a result register and the instruction
that uses that register can be multiple-issued in the same cycle. With a latency of 2, if the writing instruction is issued at cycle N, the reading instruction can issue no earlier than cycle N+2.
Latency is implementation specific.
Most Alpha instructions have a non-zero result latency. Compilers should schedule code so
that a result is not used too soon, at least in frequently executed code (inner loops, as identified
by execution profiles). In general, this will require unrolling loops and inlining short
procedures.
Compilers should try to schedule code to match the above latency rules and also to match the
multiple-issue rules. If doing both is impractical for a particular sequence of code, the latency
rules are more important (since they apply even in single-issue implementations).
Implementors should give first priority to minimizing the latency of back-to-back integer operations, of address calculations immediately followed by load/store, of load immediately
followed by branch, and of compare immediately followed by branch. Give second priority to
minimizing latencies in general.

A.3 Data-Stream Considerations
The following sections describe considerations for the data stream.

A.3.1 Data Alignment — Factor of 10
Data PSECTs should be at least octaword aligned, so that aggregates (arrays, some records,
subroutine stack frames) can be allocated on aligned octaword boundaries to take advantage of
any implementations with aligned octaword data paths, and to decrease the number of cache
fills in almost all implementations.
Aggregates (arrays, records, common blocks, and so forth) should be allocated on at least

A–4 Alpha Architecture Handbook

aligned octaword boundaries whenever language rules allow. In some implementations, a
series of writes that completely fill a cache block may be a factor of 10 faster than a series of
writes that partially fill a cache block, when that cache block would give a read miss. This is
true of write-back caches that read a partially filled cache block from memory, but optimize
away the read for completely filled blocks.
For such implementations, long strings of sequential writes will be faster if they start on a
cache-block boundary (a multiple of 128 bytes will do well for most, if not all, Alpha implementations). This applies to array results that sweep through large portions of memory, and to
register-save areas for context switching, graphics frame buffer accesses, and other places
where exactly 8, 16, 32, or more quadwords are stored sequentially. Allocating the targets at
multiples of 8, 16, 32, or more quadwords, respectively, and doing the writes in order of
increasing address will maximize the write speed.
Items within aggregates that are forced to be unaligned (records, common blocks) should generate compile-time warning messages and inline byte extract/insert code. Users must be
educated that the warning message means that they are taking a factor of 30 performance hit.
Compiled code for parameters should assume that the parameters are aligned. Unaligned actuals will cause run-time alignment traps and very slow fixups. The fixup routine, if invoked,
should generate warning messages to the user, preferably giving the first few statement numbers that are doing unaligned parameter access, and at the end of a run the total number of
alignment traps (and perhaps an estimate of the performance improvement if the data were
aligned). Users must be educated that the trap routine warning message means they are taking a
factor of 30 performance hit.
Frequently used scalars should reside in registers. Each scalar datum allocated in memory
should normally be allocated an aligned quadword to itself, even if the datum is only a byte
wide. This allows aligned quadword loads and stores and avoids partial-quadword writes
(which may be half as fast as full-quadword writes, due to such factors as read-modify-write a
quadword to do quadword ECC calculation).
Implementors should give first priority to fast reads of aligned octawords and second priority
to fast writes of full cache blocks.

A.3.2 Shared Data in Multiple Processors — Factor of 3
Software locks are aligned quadwords and should be allocated to large cache blocks that either
contain no other data or read-mostly data whose usage is correlated with the lock.
Whenever there is high contention for a lock, one processor will have the lock and be using the
guarded data, while other processors will be in a read-only spin loop on the lock bit. Under
these circumstances, any write to the cache block containing the lock will likely cause excess
bus traffic and cache fills, thus affecting performance on all processors that are involved and
the buses between them. In some decomposed FORTRAN programs, refills of the cache blocks
containing one or two frequently used locks can account for a third of all the bus bandwidth the
program consumes.
Whenever there is almost no contention for a lock, one processor will have the lock and be
using the guarded data. Under these circumstances, it might be desirable to keep the guarded
Software Considerations A–5

data in the same cache block as the lock.
For the high-sharing case, compilers should assume that almost all accesses to shared data
result in cache misses all the way back to main memory, for each distinct cache block used.
Such accesses will likely be a factor of 30 slower than cache hits. It is helpful to pack correlated shared data into a small number of cache blocks. It is helpful also to segregate blocks
written by one processor from blocks read by others.
Therefore, accesses to shared data, including locks, should be minimized. For example, a
four-processor decomposition of some manipulation of a 1000-row array should avoid accessing lock variables every row, but instead might access a lock variable every 250 rows.
Array manipulation should be partitioned across processors so that cache blocks do not thrash
between processors. Having each of four processors work on every fourth array element
severely impairs performance on any implementation with a cache block of four elements or
larger. The processors all contend for copies of the same cache blocks and use only one quarter of the data in each block. Writes in one processor severely impair cache performance on all
processors.
A better decomposition is to give each processor the largest possible contiguous chunk of data
to work on (N/4 consecutive rows for four processors and row-major array storage; N/4 columns for column-major storage). With the possible exception of three cache blocks at the
partition boundaries, this decomposition will result in each processor caching data that is
touched by no other processor.
Operating-system scheduling algorithms should attempt to minimize process migration from
one processor to another. Any time migration occurs, there are likely to be a large number of
cache misses on the new processor.
Similarly, operating-system scheduling algorithms should attempt to enforce some affinity
between a given device’s interrupts and the processor on which the interrupt-handler runs. I/O
control data structures and locks for different devices should be disjoint. Observing these
guidelines allows higher cache hit rates on the corresponding I/O control data structures.
Implementors should give first priority to an efficient (low-bandwidth) way of transferring isolated lock values and other isolated, shared write data between processors.
Implementors should assume that the amount of shared data will continue to increase, so over
time the need for efficient sharing implementations will also increase.

A.3.3 Avoiding Cache/TB Conflicts — Factor of 1
Occasionally, programs that run with a direct-mapped cache or TB will thrash, taking excessive cache or TB misses. With some work, thrashing can be minimized at compile time.

Note:
No Alpha processor through and including the 21264 has implemented a direct-mapped
TB.

A–6 Alpha Architecture Handbook

In a frequently executed loop, compilers could allocate the data items accessed from memory
so that, on each loop iteration, all of the memory addresses accessed are either in exactly the
same aligned 64-byte block or differ in bits VA<10:6>. For loops that go through arrays in a
common direction with a common stride, this requires allocating the arrays, checking that the
first-iteration addresses differ, and if not, inserting up to 64 bytes of padding between the
arrays. This rule will avoid thrashing in small direct-mapped data caches with block sizes up to
64 bytes and total sizes of 2K bytes or more.
Example:
REAL*4 A(1000),B(1000)
DO 60 i=1,1000
60 A( i ) = f(B( i ))

Figures A–3, A–4, and A–5 show bad, better, and best allocation in cache, respectively.
BAD allocation (A and B thrash in 8 KB direct-mapped cache):

Figure A–3: Bad Allocation in Cache

12K

16K

BETTER allocation (A and B offset by 64 mod 2 KB, so 16 elements of A and 16 of B can be
in cache simultaneously):

Figure A–4: Better Allocation in Cache

8K+64

12K

16K

BEST allocation (A and B offset by 64 mod 2 KB, so 16 elements of A and 16 of B can be in
cache simultaneously, and both arrays fit entirely in 8 KB or bigger cache):

Figure A–5: Best Allocation in Cache

A
0

B
4K-64

12K

16K

tion addresses differ, and if they do not, inserting up to 8K bytes of padding between the
arrays. This rule will avoid thrashing in direct-mapped TBs and in some large direct-mapped
data caches with total sizes of 32 pages (256 KB) or more.
Usually, this padding will mean zero extra bytes in the executable image, just a skip in virtual
address space to the next-higher page boundary.
For large caches, the rule above should be applied to the I-stream, in addition to all the
D-stream references. Some implementations will have combined I-stream/D-stream large
caches.
Both of the rules above can be satisfied simultaneously, thus often eliminating thrashing in all
anticipated direct-mapped cache/TB implementations.

A.3.4 Sequential Read/Write — Factor of 1
All other things being equal, sequences of consecutive reads or writes should use ascending
(rather than descending) memory addresses. Where possible, the memory address for a block
of 2**Kbytes should be on a 2**K boundary, since this minimizes the number of different
cache blocks used and minimizes the number of partially written cache blocks.
To avoid overrunning memory bandwidth, sequences of more than eight quadword load or
store instructions should be broken up with intervening instructions (if there is any useful work
to be done).
For consecutive reads, implementors should give first priority to prefetching ascending cache
blocks and second priority to absorbing up to eight consecutive quadword load instructions
(aligned on a 64-byte boundary) without stalling.
For consecutive writes, implementors should give first priority to avoiding read overhead for
fully written aligned cache blocks and second priority to absorbing up to eight consecutive
quadword store instructions (aligned on a 64-byte boundary) without stalling.

A.3.5 Prefetching — Factor of 3
Prefetching can be directed toward a cache block (a cache line) in the primary cache.
Alpha hardware, beginning with the 21164 (EV5) and subsequent, supports cache block
prefetching. Cache block prefetching is performed by the following load operations to the R31
or F31 register:

Table A–1: Cache Block Prefetching
Type

Instructions

Operation

Normal Prefetch

LDL R31, xxx (Rn)

If the load operation hits in the Dcache, the
instruction is dismissed; otherwise, the
addressed cache block is allocated into the
Dcache.

A–8 Alpha Architecture Handbook

Table A–1: Cache Block Prefetching
Type

Instructions

Operation

Prefetch
with
Modify Intent

LDS F31, xxx (Rn)

If the load operation hits a dirty, modified,
Dcache block, the instruction is dismissed. Otherwise, the addressed cache block is allocated
into the Dcache for write access — its dirty and
modified bits are set.

Prefetch,
Next

LDQ R31, xxx (Rn)

Prefetch a cache block and mark that block in an
associated cache to be evicted on the next cache
fill to an associated address. (This operation is
useful to prefetch data that is not to be repeatedly referenced.)

Evict

A.4 Code Sequences
The following section describes code sequences.

A.4.1 Aligned Byte/Word (Within Register) Memory Accesses
The instruction sequences given in Section 4.6 for byte-within-register accesses are worst-case
code. More importantly, they do not reflect the instructions available with the BWX extension,
described in the Sections 4.2.2, 4.2.6, and 4.6.5, and in Section D.3. If the BWX extension
instructions are available, it is wise to consider them rather than the sequences that follow.
The following sequences are appropriate if the BWX extension instructions are not available.
In the common case of accessing a byte or aligned word field at a known offset from a pointer
that is expected to be at least longword aligned, the common-case code is much shorter.
"Expected" means that the code should run fast for a longword-aligned pointer and trap for
unaligned. The trap handler may at its option fix up the unaligned reference.
For access at a known offset D from a longword-aligned pointer Rx, let D.lw be D rounded
down to a multiple of 4 ((D div 4)*4), and let D.mod be D mod 4.
In the common case, the intended sequence for loading and zero-extending an aligned word is:
LDL
R1,D.lw(Rx)
EXTWL R1,#D.mod,R1

! Traps if unaligned
! Picks up word at byte 0 or byte 2

In the common case, the intended sequence for loading and sign-extending an aligned word is:
LDL
SLL
SRA

R1,D.lw(Rx)
R1,#48-8*D.mod,R1
R1,#48,R1

! Traps if unaligned
! Aligns word at high end of R1
! SEXT to low end of R1

Software Considerations A–9

Note:
The shifts often can be combined with shifts that might surround subsequent arithmetic
operations (for example, to produce word overflow from the high end of a register).
In the common case, the intended sequence for loading and zero-extending a byte is:
LDL
EXTBL

R1,D.lw(Rx)
R1,#D.mod,R1

!
!

In the common case, the intended sequence for loading and sign-extending a byte is:
LDL
SLL
SRA

R1,D.lw(Rx)
R1,#56-8*D.mod,R1
R1,#56,R1

!
!
!

In the common case, the intended sequence for storing an aligned word R5 is:
LDL
R1,D.lw(Rx)
!
INSWL
R5,#D.mod,R3
!
MSKWL
R1,#D.mod,R1
!
BIS
R3,R1,R1
!
STL
R1,D.lw(Rx)
!
In the common case, the intended sequence for storing a byte R5 is:
LDL
INSBL
MSKBL
BIS
STL

R1,D.lw(Rx)
R5,#D.mod,R3
R1,#D.mod,R1
R3,R1,R1
R1,D.lw(Rx)

!
!
!
!
!

A.4.2 Division
In all implementations, floating-point division is likely to have a substantially longer result
latency than floating-point multiply. In addition, in many implementations multiplies will be
pipelined and divides will not.
Thus, any division by a constant power of two should be compiled as a multiply by the exact
reciprocal, if it is representable without overflow or underflow. If language rules or surrounding context allow, multiplication by the reciprocal can closely approximate other divisions by
constants.
Integer division does not exist as a hardware opcode. Division by a constant can always be
done via UMULH of another appropriate constant, followed by a right shift. A subroutine can
do general quadword division by true variables. The subroutine could test for small divisors
(less than about 1000 in absolute value) and for those, do a table lookup on the exact constant
and shift count for an UMULH/shift sequence. For the remaining cases, a table lookup on
about a 1000-entry table and a multiply can give a linear approximation to 1/divisor that is
accurate to 16 bits.
Using this approximation, a multiply and a back-multiply and a subtract can generate one

A–10 Alpha Architecture Handbook

16-bit quotient digit plus a 48-bit new partial dividend. Three more such steps can generate the
full quotient. Having prior knowledge of the possible sizes of the divisor and dividend, normalizing away leading bytes of zeros, and performing an early-out test can reduce the average
number of multiplies to about five (compared to a best case of one and a worst case of nine).

A.4.3 Byte Swap
When it is necessary to swap all the bytes of a datum, perhaps because the datum originated on
a machine of the opposite byte numbering convention, the simplest sequence is to use the VAX
floating-point load instruction to swap words, followed by an integer sequence to swap four
pairs of bytes. Assume as shown below that an aligned quadword datum is in memory at location X and is to be left in R1 after byte-swapping; temp is an aligned quadword temporary, and
"." (period) in the comments stands for a byte of zeros. Similar sequences can be used for data
in registers, sometimes doing the byte swaps first and word swap second:

LDG
STT
LDQ
SLL
SRL
ZAP
ZAP
OR

; X = ABCD EFGH
; F0 = GHEF CDAB

F0,X
F0,temp
R1,temp
R1,#8,R2
R1,#8,R1
R2,#55(hex),R2
R1,#AA(hex),R1
R1,R2,R1

; R1 = GHEF CDAB
; R2 = HEFC DAB.
; R1 = .GHE FCDA
; R2 = H.F. D.B.
; R1 = .G.E .C.A
; R1 = HGFE DCBA

For bulk swapping of arrays, this sequence can be usefully unrolled about four times and
scheduled, using four different aligned quadword memory temps.

A.4.4 Stylized Code Forms
Using the same stylized code form for a common operation improves the readability of compiler output and increases the likelyhood that an implementation will speed up the stylized
form.

A.4.4.1 NOP
The universal NOP form is:
UNOP

LDQ_U

R31,0(Rx)

In most implementations, UNOP should encounter no operand issue delays, no destination
issue delay, and no functional unit issue delays. (In some implementations, it may encounter an
operand issue delay for Rx.) Implementations are free to optimize UNOP into no action and
zero execution cycles.
If the actual instruction is encoded as LDQ_U Rn,0(Rx), where n is other than 31, and such an
instruction generates a memory-management exception, it is UNPREDICTABLE whether
UNOP would generate the same exception. On most implementations, UNOP does not generate memory management exceptions.

Software Considerations A–11

The standard NOP forms are:
NOP
FNOP

==
==

BIS
CPYS

R31,R31,R31
F31,F31,F31

These generate no exceptions. In most implementations, they should encounter no operand
issue delays and no destination issue delay. Implementations are free to optimize these into no
action and zero execution cycles.

A.4.4.2 Clear a Register
The standard clear register forms are:
CLR
FCLR

==
==

BIS
CPYS

R31,R31,Rx
F31,F31,Fx

These generate no exceptions. In most implementations, they should encounter no operand
issue delays and no functional unit issue delay.

A.4.4.3 Load Literal
The standard load integer literal (ZEXT 8-bit) form is:
MOV #lit8,Ry

BIS R31, lit8, Ry

The Alpha literal construct in Operate instructions creates a canonical longword constant for
values 0..255.
A longword constant stored in an Alpha 64-bit register is in canonical form when bits
<63:32>=bit <31>.
A canonical 32-bit literal can usually be generated with one or two instructions, but sometimes
three instructions are needed. Use the following procedure to determine the offset fields of the
instructions:
val = <sign-extended, 32-bit value>
low = val <15:0>
tmp1 = val - SEXT(low)
! Account for LDA instruction
high = tmp1 <31:16>
tmp2 = tmp1 - SHIFT_LEFT( SEXT(high,16) )
if tmp2 NE 0 then
! original val was in range 7FFF800016..7FFFFFFF16
extra = 400016
tmp1 = tmp1 - 4000000016
high = tmp1 <31:16>
else
extra = 0
endif

A–12 Alpha Architecture Handbook

The general sequence is:
LDA Rdst, low(R31)
LDAH Rdst, extra(Rdst)
LDAH Rdst, high(Rdst)

! Omit if extra=0
! Omit if high=0

A.4.4.4 Register-to-Register Move
The standard register move forms are:
MOV RX,RY
FMOV FX,FY

==
==

BIS
CPYS

RX,RX,RY
FX,FX,FY

These move forms generate no exceptions. In most implementations, these should encounter no
functional unit issue delay.

A.4.4.5 Negate
The standard register negate forms are:
NEGz Rx,Ry
NEGz Fx,Fy
FNEGz Fx,Fy

==
==
==

SUBz
SUBz
CPYSN

R31,Rx,Ry
F31,Fx,Fy
Fx,Fx,Fy

! z = L or Q
! z = F G S or T
! z = F G S or T

The integer subtract generates no Integer Overflow trap if Rx contains the largest negative
number (SUBz/V would trap). The floating subtract generates a floating-point exception for a
non-finite value in Fx. The CPYSN form generates no exceptions.

A.4.4.6 NOT
The standard integer register NOT form is:
NOT Rx,Ry

ORNOT

R31,Rx,Ry

This generates no exceptions. In most implementations, this should encounter no functional
unit issue delay.

A.4.4.7 Booleans
The standard alternative to BIS is:
OR Rx,Ry,Rz

BIS

Rx,Ry,Rz

BIC

Rx,Ry,Rz

EQV

Rx,Ry,Rz

The standard alternative to BIC is:
ANDNOT Rx,Ry,Rz ==
The standard alternative to EQV is:
XORNOT Rx,Ry,Rz ==

Software Considerations A–13

A.4.5 Exceptions and Trap Barriers
The EXCB instruction allows software to guarantee that in a pipelined implementation, all previous instructions have completed any behavior that is related to exceptions or rounding modes
before any instructions after the EXCB are issued. In particular, all changes to the floating-point control register (FPCR) are guaranteed to have been made, whether or not there is an
associated exception. Also, all potential floating-point exceptions and integer overflow exceptions are guaranteed to have been taken.
The TRAPB instruction guarantees that it and any following instructions do not issue until all
possible preceding traps have been signaled. This does not mean that all preceding instructions
have necessarily run to completion (for example, a Load instruction may have passed all the
fault checks but not yet delivered data from a cache miss).
EXCB is thus a superset of TRAPB.

A.4.6 Pseudo-Operations (Stylized Code Forms)
This section summarizes the pseudo-operations for the Alpha architecture that may be used by
various software components in an Alpha system. Most of these forms are discussed in preceding sections.
In the context of this section, pseudo-operations all represent a single underlying machine
instruction. Each pseudo-operation represents a particular instruction with either replicated
fields (such as FMOV), or hard-coded zero fields. Since the pattern is distinct, these
pseudo-operations can be decoded by instruction decode mechanisms.
In Table A–2, the pseudo-operation codes can be viewed as macros with parameters. The formal form is listed in the left column, and the expansion in the code stream is listed in the right
column.
Some instruction mnemonics have synonyms. These differ from pseudo-operations in that each
synonym represents the same underlying instruction with no special encoding of operand
fields. As a result, synonyms cannot be distinquished from each other. They are not listed in
the table. Examples of synonyms are: BIC/ANDNOT, BIS/OR, and EQV/XORNOT.

Table A–2: Decodable Pseudo-Operations (Stylized Code Forms)
Pseudo-Operation
in Listing

Meaning

Actual Instruction
Encoding

target

Branch to target (21-bit signed
displacement)

R31, target

CLR

Clear integer register

BIS

R31, R31, Rx

FABS

Fx, Fy

No-exception generic floating
absolute value

CPYS

F31, Fx, Fy

FCLR

Clear a floating-point register

CPYS

F31, F31, Fx

FMOV

Fx, Fy

Floating-point move

CPYS

Fx, Fx, Fy

A–14 Alpha Architecture Handbook

Table A–2: Decodable Pseudo-Operations (Stylized Code Forms) (Continued)
Pseudo-Operation
in Listing
FNEG

Fx, Fy

FNOP

Meaning

Actual Instruction
Encoding

No-exception generic floating
negation

CPYSN

Fx, Fx, Fy

Floating-point no-op

CPYS

F31, F31, F31

MOV

Lit, Rx

Move 16-bit sign-extended
literal to Rx

LDA

Rx,lit(R31)

MOV

{Rx/Lit8}, Ry

Move Rx/8-bit zero-extended
literal to Ry

BIS

R31,{Rx/Lit8},Ry

MF_FPCR

Move from FPCR

MF_FPCR

Fx, Fx, Fx

MT_FPCR

Move to FPCR

MT_FPCR

Fx, Fx, Fx

NEGF

Fx, Fy

Negate F_floating

SUBF

F31, Fx, Fy

NEGF/S

Fx, Fy

Negate F_floating, semi-precise

SUBF/S

F31, Fx, Fy

NEGG

Fx, Fy

Negate G_floating

SUBG

F31, Fx, Fy

NEGG/S

Fx, Fy

Negate G_floating,
semi-precise

SUBG/S

F31, Fx, Fy

NEGL

{Rx/Lit8}, Ry

Negate longword

SUBL

R31,{Rx/Lit},Ry

NEGL/V

{Rx/Lit8}, Ry

Negate longword with
overflow detection

SUBL/V

R31, {Rx/Lit}, Ry

NEGQ

{Rx/Lit8}, Ry

Negate quadword

SUBQ

R31, {Rx/Lit}, Ry

NEGQ/V

{Rx/Lit8}, Ry

Negate quadword with
overflow detection

SUBQ/V

R31, {Rx/Lit}, Ry

NEGS

Fx, Fy

Negate S_floating

SUBS

F31, Fx, Fy

NEGS/SU

Fx, Fy

Negate S_floating, software
with underflow detection

SUBS/SU

F31, Fx, Fy

NEGS/SUI

Fx, Fy

Negate S_floating, software
with underflow and inexact
result detection

SUBS/SUI

F31, Fx, Fy

NEGT

Fx, Fy

Negate T_floating

SUBT

F31, Fx, Fy

NEGT/SU

Fx, Fy

Negate T_floating, software
with underflow detection

SUBT/SU

F31, Fx, Fy

NEGT/SUI

Negate T_floating, software
with underflow and inexact
result detection

SUBT/SUI

F31,Fx, Fy

NOP

Integer no-op

BIS

R31, R31, R31

Logical NOT of Rx/8-bit
zero-extended literal storing
results in Ry

ORNOT

R31, {Rx/Lit}, Ry

NOT

{Rx/Lit8}, Ry

Software Considerations A–15

Table A–2: Decodable Pseudo-Operations (Stylized Code Forms) (Continued)
Pseudo-Operation
in Listing
SEXTL

{Rx/Lit8}, Ry

UNOP

Meaning

Actual Instruction
Encoding

Longword sign-extension of Rx
storing results in Ry

ADDL

R31, {Rx/Lit}, Ry

Universal NOP for both integer
and floating-point code

LDQ_U

R31,0(Rx)

A.5 Timing Considerations: Atomic Sequences
A sufficiently long instruction sequence between LDx_L and STx_C will never complete,
because periodic timer interrupts will always occur before the sequence completes. The following rules describe sequences that will eventually complete in all Alpha implementations:

•

At most 40 operate or conditional-branch (not taken) instructions executed in the
sequence between LDx_L and STx_C.

•

At most two I-stream TB-miss faults. Sequential instruction execution guarantees this.

•

No other exceptions triggered during the last execution of the sequence.

Implementation Note:
On all expected implementations, this allows for about 50 µsec of execution time, even
with 100 percent cache misses. This should satisfy any requirement for a 1-msec timer
interrupt rate.

A–16 Alpha Architecture Handbook

Appendix B

IEEE Floating-Point Conformance

A subset of IEEE Standard for Binary Floating-Point Arithmetic (ANSI/IEEE Standard
754-1985) is provided in the Alpha floating-point instructions. This appendix describes how to
construct a complete IEEE implementation.
The order of presentation parallels the order of the IEEE specification.

B.1 Alpha Choices for IEEE Options
Alpha supports IEEE single, double, and optionally (in software) extended double formats.
There is no hardware support for the optional extended double format.
Alpha hardware supports normal and chopped IEEE rounding modes. IEEE plus infinity and
minus infinity rounding modes can be implemented in hardware or software.
Alpha hardware does not support optional IEEE software trap enable/disable modes. See the
following discussion about software support.
Alpha hardware supports add, subtract, multiply, divide, convert between floating formats,
convert between floating and integer formats, compare, and square root. Software routines support remainder, round to integer in floating-point format, and convert binary to/from decimal.
In the Alpha architecture, copying without change of format is not considered an operation.
(LDx, CPYSx, and STx do not check for non-finite numbers; an operation would.) Compilers
may generate ADDx F31,Fx,Fy to get the opposite effect.
Optional operations for differing formats are not provided.
The Alpha choice is that the accuracy provided by conversions between decimal strings and
binary floating-point numbers will meet or exceed IEEE standard requirements. It is implementation dependent whether the software binary/decimal conversions beyond 9 or 17 digits
treat any excess digits as zeros.

B–1

Overflow and underflow, NaNs, and infinities encountered during software binary to decimal
conversion return strings that specify the conditions.
Alpha hardware supports comparisons of same-format numbers. Software supports comparisons of different-format numbers.
In the Alpha architecture, results are true-false in response to a predicate.
Alpha hardware supports the required six predicates and the optional unordered predicate. The
other 19 optional predicates can be constructed from sequences of two comparisons and two
branches.
Alpha hardware supports infinity arithmetic with the compare instructions (CMPTyy). When a
/S qualifier is included, Alpha hardware may optionally support infinity arithmetic when infinity operands are encountered and, together with overflow disable (OVFD) and division by zero
disable (DZED), when infinity is to be generated from finite operands. Otherwise, Alpha hardware supports infinity arithmetic by trapping. That is the case when an infinity operand is
encountered and when an infinity is to be created from finite operands by overflow or division
by zero. An OS completion handler (interposed between the hardware and the IEEE user) provides correct infinity arithmetic.
When a /S qualifier is included, Alpha hardware may optionally support NaNs and invalid
operations, controlled by the INVD option. Otherwise, Alpha hardware supports NaNs and
invalid operations by trapping when a NaN operand is encountered and when a NaN is to be
created. An OS completion handler (interposed between the hardware and the IEEE user) provides correct Signaling and Quiet NaN behavior.
In the Alpha architecture, Quiet NaNs do not afford retrospective diagnostic information.
In the Alpha architecture, copying a Signaling NaN without a change of format does not signal
an invalid exception (LDx, CPYSx, and STx do not check for non-finite numbers). Compilers
may generate ADDx F31,Fx,Fy to get the opposite effect.
Alpha hardware fully supports negative zero operands and follows the IEEE rules for creating
negative zero results except for underflow. When a /S qualifier is included, Alpha hardware
may optionally support underflow and denormalized numbers, controlled by the UNFD option.
Otherwise, Alpha hardware supports underflow and denormalized numbers by trapping when a
denormalized operand is encountered, when a denormalized result is created, and when an
underflow occurs. An OS completion handler (interposed between the hardware and the IEEE
user) provides correct denormalized and underflow arithmetic.
Except for the optional trap disable bits in the FPCR, Alpha hardware does not supply IEEE
exception trap behavior; the hardware traps are a superset of the IEEE-required conditions. An
OS completion handler (interposed between the hardware and the IEEE user) provides correct
IEEE exception behavior.
In the Alpha architecture, tininess is detected by hardware after rounding, and loss of accuracy
is detected by software as an inexact result.

B–2 Alpha Architecture Handbook

In the Alpha architecture, user signal handlers are supported by compilers and an OS completion handler (interposed between the hardware and the IEEE user), as described in the next
section.

B.2 Alpha Support for OS Completion Handlers
Alpha floating-point trap behavior is statically controlled by the /S, /U, and /I mode qualifiers
on floating-point instructions. Changing these options usually requires recompiling. Instructions with any valid qualifier combination that includes the /S qualifier can be dynamically
controlled by the optional trap disable bits and denormal control bits in the FPCR.
Each Alpha implementation may choose how to distribute support for the completion modes
(/S, /SU, /SV, /SUI, and /SVI), between hardware and software. An implementation may
minimize hardware complexity by trapping to implementation software for support of exceptions and non-finites. An implementation may choose increased floating-point performance at
the cost of increased hardware complexity by providing hardware support for exceptions and
non-finites.
However completion mode support is distributed, application software on any system that
meets the Alpha architecture specification will see consistent floating-point semantics because
Alpha implementation software provides support for any floating-point feature that is not
directly supported by the hardware.
Each Alpha operating system must include an OS completion handler that does software completion of instructions that have any valid qualifier combination that includes the /S qualifier,
and that finishes the computation of any floating-point operation that is not completed by the
hardware. The OS completion handler is responsible for providing the result specified by the
architecture. The handler either continues execution of the application program or signals an
exception to the application.
If the exception summary parameter of an arithmetic trap indicates that an instruction requiring software completion caused the trap, the operating system must finish the operation. An
OS completion handler uses the register write mask parameter to ignore instructions in the trap
shadow and to locate the trigger instruction of the arithmetic trap. The handler then uses the
trigger instruction input register values to compute the result in the output register and to
record any appropriate signal status. The handler then continues execution with the instruction
following the trigger instruction, unless the application has requested execution of an optional
signal handler.
It is recommended that the OS completion handler report an enabled IEEE exception to the
user application as a fault, rather than as a trap. When reported as a fault, the reported PC
points to the trigger instruction, rather than after the trigger instruction. Regardless of whether
an enabled fault occurs, it is recommended that the completion trap handler set the result register and status flags to the IEEE standard nontrapping results, as defined in the IEEE Standard
section in Section 4.7.10. That behavior makes it possible for the user application to continue
from a fault by stepping over the trigger instruction.
The Floating-Point Control Register (FPCR) contains several trap disable bits and denormal
control bits. Implementation of these bits in the FPCR is optional. A system that includes
IEEE Floating-Point Conformance B–3

these bits may choose to complete computations involving non-finite values without the assistance of software completion. Operating systems use these FPCR bits to enable hardware
completion of instructions with any valid qualifier combination that includes /S in those cases
where the operating system does not require a trap to do exception signaling.
To get the optional full IEEE user trap handler behavior, an OS completion handler must be
provided that implements the exception status flags, dynamic user trap handler disabling, handler saving and restoring, default behavior for disabled user trap handlers, and linkages that
allow a user handler to return a substitute result. OS completion handlers can use the
FP_Control quadword, along with the floating-point control register (FPCR), to provide various levels of IEEE-compliant behavior.
OS completion handlers provide two options for special handling of denormal numbers in
instructions that are compiled with any valid qualifier combination that includes the /S qualifier. These options are controlled by bits defined by implementation software in the IEEE
Floating-Point Control (FP_C) Quadword.

•

The first option maps all denormal results to a true zero value. That option is useful for
improving the performance of IEEE compliant code that does not need gradual underflow and for mixing IEEE instructions that both include and do not include the /S qualifier.

•

A second option treats all denormal input operands as if they were signed zeros. That
option is useful for improving the performance of IEEE compliant code that encounters
spurious denormal values in uninitialized data.

The optional UNDZ and DNZ (denormal control) bits in the FPCR can assist hardware to
improve the performance of these denormal handling options.

B.2.1 IEEE Floating-Point Control (FP_C) Quadword
Operating system implementations provide the following support for an IEEE floating-point
control quadword (FP_C), illustrated in Figure B–1 and described in Table B–1.

Figure B–1: IEEE Floating-Point Control (FP_C) Quadword
7 6 5 4 3 2 1 0

23 22 21 20 19 18 17 16

Reserved

D I UOD I
NNNV Z N
OE F F E V
SS S S S S

Reserved

D I UOD I
NNNV Z N
OE F F E V
EE E E E E

•

The operating system software completion mechanism maintains the FP_C. Therefore,
the FP_C affects (and is affected by) only those instructions with any valid qualifier
combination that includes the /S qualifier.

•

The FP_C quadword is context switched when the operating system switches the thread
context. (The FP_C can be placed in a currently switched data structure.)

•

Although the operating system can keep the FP_C in a user mode memory location,
user code may not directly access the FP_C.

B–4 Alpha Architecture Handbook

•

Integer overflow (IOV) exceptions are controlled by the INVE enable mask bit
(FP_C<1>), as allowed by the IEEE standard. Implementation software is responsible
for setting the INVS status bit (FP_C<17>) when a CVTTQ or CVTQL instruction
traps into the software completion mechanism for integer overflow .

•

At process creation, all trap enable flags in the FP_C are clear. The settings of other
FP_C bits, defined in Table B–1 as reserved for implementation software, are defined
by operating system software.

At other events such as forks or thread creation, and at asynchronous routine calls such as traps
and signals, the operating system controls all assigned FP_C bits and those defined as reserved
for implementation software.

Table B–1: Floating-Point Control (FP_C) Quadword Bit Summary
Bit

Description

63–48

Reserved for implementation software.

47–23

Reserved for future architecture definition.

Denormal operand status (DNOS)
A floating arithmetic or conversion operation used a denormal operand value.
This status field is left unchanged if the system is treating denormal operand values as if they were signed zero values. If an operation with a denormal operand
causes other exceptions, all appropriate status bits are set.

Inexact result status (INES)
A floating arithmetic or conversion operation gave a result that differed from the
mathematically exact result.

Underflow status (UNFS)
A floating arithmetic or conversion operation underflowed the destination exponent.

Overflow status (OVFS)
A floating arithmetic or conversion operation overflowed the destination exponent.

Division by zero status (DZES)
An attempt was made to perform a floating divide operation with a divisor of zero.

Invalid operation status (INVS)
An attempt was made to perform a floating arithmetic, conversion, or comparison
operation, and one or more of the operand values were illegal.

16–12

Reserved for implementation software.

11–7

Reserved for future architecture definition.

Denormal operand exception enable (DNOE)
Initiate an INV exception if a floating arithmetic or conversion operation involves
a denormal operand value. This exception does not signal if the system is treating
denormal operand values as if they were signed zero values. If an operation can
initiate more than one enabled exception, the denormal operand exception has priority.

IEEE Floating-Point Conformance B–5

Table B–1: Floating-Point Control (FP_C) Quadword Bit Summary (Continued)
Bit

Description

Inexact result enable (INEE)
Initiate an INE exception if the result of a floating arithmetic or conversion operation differs from the mathematically exact result.

Underflow enable (UNFE)
Initiate a UNF exception if a floating arithmetic or conversion operation underflows the destination exponent.

Overflow enable (OVFE)
Initiate an OVF exception if a floating arithmetic or conversion operation overflows the destination exponent.

Division by zero enable (DZEE)
Initiate a DZE exception if an attempt is made to perform a floating divide operation with a divisor of zero.

Invalid operation enable (INVE)
Initiate an INV exception if an attempt is made to perform a floating arithmetic,
conversion, or comparison operation, and one or more of the operand values is
illegal.

Reserved for implementation software.

B.3 Mapping to IEEE Standard
There are five IEEE exceptions, each of which can be "IEEE software trap-enabled" or disabled (the default condition). Implementing the IEEE software trap-enabled mode is optional
in the IEEE standard.
The assumption, therefore, is that the only access to IEEE-specified software trap-enabled
results will be generated in assembly language code. The following design allows this, but only
if such assembly language code has TRAPB instructions after each floating-point instruction,
and generates the IEEE-specified scaled result in a trap handler by emulating the instruction
that was trapped by hardware overflow/underflow detection, using the original operands.
There is a set of detailed IEEE-specified result values, both for operations that are specified to
raise IEEE traps and those that do not. This behavior is created on Alpha by four layers of
hardware, PALcode, the operating-system completion handler, and the user signal handler, as
shown in Figure B–2.

B–6 Alpha Architecture Handbook

Figure B–2: IEEE Trap Handling Behavior
Hardware
Traps to PALcode
PALcode
Traps to Operating System
Operating System
Traps to User IEEE Trap Handler
(IEEE Standard)
User Signal Handler

The IEEE-specified trap behavior occurs only with respect to the user signal handler (the last
layer in Figure B–2); any trap-and-fixup behavior in the first three layers is outside the scope
of the IEEE standard.
The IEEE number system is divided into finite and non-finite numbers:
The finites are normal numbers:

•

–MAX..–MIN, –0, 0, +MIN..+MAX

•

The non-finites are:

•

Denormals, +/– Infinity, Signaling NaN, Quiet NaN

Alpha hardware must treat minus zero operands and results as special cases, as required by the
IEEE standard.
If the DNZ (denormal operands to zero) bit in the FPCR is set or if the OS completion handler
is treating denormal operands as zero, then IEEE trap handling is done as if each denormal
operand had the corresponding signed zero value.
Table B–2 specifies, for the IEEE /S qualifier modes, which layer does each piece of trap handling. The table describes where the hardware and PALcode can trap to the OS completion
handler. However, for IEEE operations with any valid qualifier combination that includes the
/S qualifier, the system may choose not to trap to the OS completion handler, provided that any
applicable exception is disabled by the trap disable bits in the FPCR and the hardware and
PALcode can produce the expected IEEE result as modified by the denormal control bits in the
FPCR. See Section 4.7.8 for more detail on the hardware instruction descriptions.

IEEE Floating-Point Conformance B–7

Table B–2: IEEE Floating-Point Trap Handling

PALCode

Alpha Instructions

Hardware1

FBEQ FBNE FBLT FBLE FBGT
FBGE
LDS LDT
STS STT
CPYS CPYSN
FCMOVx

Bits Only – No Exceptions

OS
Completion
Handler

User
Signal
Handler

Bits Only—No Exceptions
Bits Only—No Exceptions
Bits Only—No Exceptions
Bits Only—No Exceptions

ADDx SUBx INPUT Exceptions:
Denormal operand

Trap

Supply sum

+/-Inf operand
QNaN operand
SNaN operand
+Inf + –Inf

Trap
Trap
Trap
Trap

Supply sum
Supply QNaN
Supply QNaN
Supply QNaN

[Denormal Op2]
–
–
[Invalid Op]
[Invalid Op]

ADDx SUBx OUTPUT Exceptions:
Exponent overflow

Trap

Supply
+/–Inf
+/–MAX

[Overflow3]
Scale by bias
adjust

Exponent underflow and disabled

Supply +0

–

–4

Exponent underflow and enabled

Supply +0
and trap

Trap

[Underflow3]
Scale by bias
adjust

Inexact and disabled
Inexact and enabled

–
Supply sum
and trap

–
Trap

Supply
+/–MIN
denorm
+/–0
–
–

Denormal operand

Trap

Supply prod.

+/-Inf operand
QNaN operand
SNaN operand
0 * Inf

Trap
Trap
Trap
Trap

Supply prod.
Supply QNaN
Supply QNaN
Supply QNaN

–
[Inexact]

MULx INPUT Exceptions:

B–8 Alpha Architecture Handbook

[Denormal Op2]
–
–
[Invalid Op]
[Invalid Op]

Table B–2: IEEE Floating-Point Trap Handling (Continued)

Alpha Instructions

Hardware

PALCode

OS
Completion
Handler

User
Signal
Handler
[Overflow3]
Scale by bias
adjust
–

MULx OUTPUT Exceptions:
Exponent overflow

Trap

Supply
+/–Inf
+/–MAX

Exponent underflow and disabled
Exponent underflow and enabled

Supply +0
Supply +0
and Trap

–
Trap

Inexact and disabled
Inexact and enabled

–
Supply prod.
and trap

–
Trap

–
Supply
+/–MIN
denorm
+/–0
–
–

Denormal operand

Trap

Supply quot.

+/-Inf operand
QNaN operand
SNaN operand
0/0 or Inf/Inf
A/0

Trap
Trap
Trap
Trap
Trap

Supply quot.
Supply QNaN
Supply QNaN
Supply QNaN
Supply
+/– Inf

Exponent overflow

Trap

Supply
+/–Inf
+/– MAX

Exponent underflow and disabled
Exponent underflow and enabled

Supply +0
Supply +0
and trap

–
Trap

Inexact and disabled
Inexact and enabled

–
Supply quot.
and trap

–
Trap

–
Supply
+/– MIN
denorm
+/–0
–
–

[Underflow3]
Scale by bias
adjust
–
[Inexact]

DIVx INPUT Exceptions:
[Denormal Op2]
–
–
[Invalid Op]
[Invalid Op]
[Div. Zero]

DIVx OUTPUT Exceptions:
[Overflow3]
Scale by bias
adjust
–
[Underflow3]
Scale by bias
adjust
–
[Inexact]

CMPTEQ CMPTUN INPUT Exceptions:
Denormal operand

Trap

Supply (=)

QNaN operand

Trap

SNaN operand

Trap

Supply False
for EQ, True
for UN
Supply
False/ True

[Denormal Op2]
–

[Invalid Op]

IEEE Floating-Point Conformance B–9

Table B–2: IEEE Floating-Point Trap Handling (Continued)

Alpha Instructions

Hardware

PALCode

OS
Completion
Handler

User
Signal
Handler
[Denormal Op2]
[Invalid Op]
[Invalid Op]

CMPTLT CMPTLE INPUT Exceptions:
Denormal operand

Trap

Supply ≤ or <

QNaN operand
SNaN operand

Trap
Trap

Supply False
Supply False

Denormal operand

Trap

Supply Cvt

+/-Inf operand
QNaN operand
SNaN operand

Trap
Trap
Trap

Supply 0
Supply 0
Supply 0

[Denormal Op2]
[Invalid Op]
–
[Invalid Op]

–
Supply Cvt
and trap
Supply Trunc.
result and trap
if enabled

–
Trap

–
–

–
[Inexact]

Trap

–

[Invalid Op5]

–
Supply Cvt
and trap

–
Trap

–
–

–
[Inexact]

Denormal operand

Trap

Supply Cvt

+/-Inf operand
QNaN operand
SNaN operand

Trap
Trap
Trap

Supply Cvt
Supply QNaN
Supply QNaN

[Denormal Op2]
–
–
[Invalid Op]

CVTfi INPUT Exceptions:

CVTfi OUTPUT Exceptions:
Inexact and disabled
Inexact and enabled
Integer overflow

CVTif OUTPUT Exceptions:
Inexact and disabled
Inexact and enabled

CVTff INPUT Exceptions:

CVTff OUTPUT Exceptions:
Exponent overflow

Trap

Supply
+/–Inf
+/–MAX

Exponent underflow and disabled
Exponent underflow and enabled

Supply +0
Supply +0
and trap

–
Trap

Inexact and disabled
Inexact and enabled

–
Supply Cvt
and trap

–
Trap

–
Supply
+/– MIN
denorm
+/–0
–
–

B–10 Alpha Architecture Handbook

[Overflow3]
Scale by bias
adjust
–
[Underflow3]
Scale by bias
adjust
–
[Inexact]

Table B–2: IEEE Floating-Point Trap Handling (Continued)

Alpha Instructions

Hardware

PALCode

OS
Completion
Handler

User
Signal
Handler
[Invalid Op]
–

SQRTx INPUT Exceptions
Negative nonzero operand
+/–0
+ Denormal operand

Trap
Supply +/–0
Trap

Trap
–
Trap

Supply QNan
–
Supply SQRT

– Denormal operand

Trap

Supply QNaN

+ Infinity operand
– Infinity operand
QNaN operand
SNaN operand

Trap
Trap
Trap
Trap

Supply +Inf
Supply QNaN
Supply QNaN
Supply QNaN

[Denormal Op2]
[Denormal Op/
Invalid Op]
–
[Invalid Op]
–
[Invalid Op]

–
Trap

–
–

–
[Inexact]

SQRTx OUTPUT Exceptions
Exponent overflow
Exponent underflow
Inexact and disabled
Inexact and enabled

Not possible
Not possible
–
Supply SQRT

This column describes the minimum necessary hardware support.
[Denormal Op] signals have priority over all other signals.
3
[Overflow] and [Underflow] signals have priority over [Inexact] signals.
2

4
5

An implementation could choose instead to trap to PALcode and have the PALcode
supply a zero result on all underflows.
An implementation could choose instead to trap to PALcode on extreme values and
have the PALcode supply a truncated result on all overflows.

Other IEEE operations (software subroutines or sequences of instructions) are listed here for
completeness:
Remainder
Round float to integer-valued float
Convert binary to/from decimal
Compare, other combinations than the four above

IEEE Floating-Point Conformance B–11

Table B–3 shows the IEEE standard charts. In the charts, the second column is the result when
the user signal handler is disabled; the third column is the result when that handler is enabled.
The OS completion handler supplies the IEEE default that is specified in the second column.
The contents of the Alpha registers contain sufficient information for an enabled user handler
to compute the value in the third column.

Table B–3: IEEE Standard Charts
Exception

User Signal Handler
Disabled (IEEE Default)

User Signal Handler
Enabled (Optional)

Invalid Operation
(1) Input signaling NaN

Quiet NaN

(2) Mag. subtract Inf.

Quiet NaN

(3) 0 * Inf.

Quiet NaN

(4) 0/0 or Inf/Inf

Quiet NaN

(5) x REM 0 or Inf REM y

Quiet NaN

(6) SQRT(negative non-zero)

Quiet NaN

(7) Cvt to int(ovfl)

Low-order bits

(8) Cvt to int(Inf, NaN)

(9) Compare unordered

Quiet NaN

Division by Zero
x/0, x finite <>0

+/–Inf

Overflow
Round nearest

+/–Inf.

Res/2**192 or 1536

Round to zero

+/–MAX

Res/2**192 or 1536

Round to –Inf

+MAX/–Inf

Res/2**192 or 1536

Round to +Inf

+Inf/–MAX

Res/2**192 or 1536

0/denorm

Res*2**192 or 1536

Rounded

Res

Underflow
Underflow

Inexact
Inexact

B–12 Alpha Architecture Handbook

Appendix C

Instruction Summary

This appendix summarizes all instructions and opcodes in the Alpha architecture. All values
are in hexadecimal radix.

C.1 Common Architecture Instruction Summary
This section summarizes all common Alpha instructions. Table C–1 describes the contents of
the Format and Opcode columns in Table C–2.

Table C–1: Instruction Format and Opcode Notation
Instruction
Format

Format
Symbol

Opcode
Notation

Branch
Floating- point

Bra
F-P

oo
oo.fff

Memory
Memory/ func code

Mem
Mfc

oo
oo.ffff

Memory/ branch

Mbr

oo.h

Operate

Opr

oo.ff

PALcode

Pcd

Meaning
oo is the 6-bit opcode field
oo is the 6-bit opcode field
fff is the 11-bit function code field
oo is the 6-bit opcode field
oo is the 6-bit opcode field
ffff is the 16-bit function code in the displacement field
oo is the 6-bit opcode field
h is the high-order two bits of the displacement field
oo is the 6-bit opcode field
ff is the 7-bit function code field
oo is the 6-bit opcode field; the particular
PALcode instruction is specified in the
26-bit function code field.

Table C–2 shows qualifiers for operate format instructions. Qualifiers for IEEE and VAX
floating-point instructions are shown in Sections C.2 and C.3, respectively.

C–1

Table C–2: Common Architecture Instructions
Mnemonic

Format

Opcode

Description

ADDF
ADDG
ADDL
ADDL/V
ADDQ
ADDQ/V
ADDS
ADDT
AMASK
AND
BEQ
BGE
BGT
BIC
BIS
BLBC
BLBS
BLE
BLT
BNE
BR
BSR
CALL_PAL
CMOVEQ
CMOVGE
CMOVGT
CMOVLBC
CMOVLBS
CMOVLE
CMOVLT
CMOVNE
CMPBGE
CMPEQ
CMPGEQ
CMPGLE
CMPGLT
CMPLE
CMPLT
CMPTEQ
CMPTLE
CMPTLT
CMPTUN
CMPULE
CMPULT
CPYS
CPYSE
CPYSN
CTLZ
CTPOP
CTTZ
CVTDG
CVTGD
CVTGF

F-P
F-P
Opr

15.080
15.0A0
10.00
10.40
10.20
10.60
16.080
16.0A0
11.61
11.00
39
3E
3F
11.08
11.20
38
3C
3B
3A
3D
30
34
00
11.24
11.46
11.66
11.16
11.14
11.64
11.44
11.26
10.0F
10.2D
15.0A5
15.0A7
15.0A6
10.6D
10.4D
16.0A5
16.0A7
16.0A6
16.0A4
10.3D
10.1D
17.020
17.022
17.021
1C.32
1C.30
1C.33
15.09E
15.0AD
15.0AC

Add F_floating
Add G_floating
Add longword

Opr
F-P
F-P
Opr
Opr
Bra
Bra
Bra
Opr
Opr
Bra
Bra
Bra
Bra
Bra
Bra
Mbr
Pcd
Opr
Opr
Opr
Opr
Opr
Opr
Opr
Opr
Opr
Opr
F-P
F-P
F-P
Opr
Opr
F-P
F-P
F-P
F-P
Opr
Opr
F-P
F-P
F-P
Opr
Opr
Opr
F-P
F-P
F-P

C–2 Alpha Architecture Handbook

Add quadword
Add S_floating
Add T_floating
Architecture mask
Logical product
Branch if = zero
Branch if ≥ zero
Branch if > zero
Bit clear
Logical sum
Branch if low bit clear
Branch if low bit set
Branch if ≤ zero
Branch if < zero
Branch if ≠ zero
Unconditional branch
Branch to subroutine
Trap to PALcode
CMOVE if = zero
CMOVE if ≥ zero
CMOVE if > zero
CMOVE if low bit clear
CMOVE if low bit set
CMOVE if ≤ zero
CMOVE if < zero
CMOVE if ≠ zero
Compare byte
Compare signed quadword equal
Compare G_floating equal
Compare G_floating less than or equal
Compare G_floating less than
Compare signed quadword less than or equal
Compare signed quadword less than
Compare T_floating equal
Compare T_floating less than or equal
Compare T_floating less than
Compare T_floating unordered
Compare unsigned quadword less than or equal
Compare unsigned quadword less than
Copy sign
Copy sign and exponent
Copy sign negate
Count leading zero
Count population
Count trailing zero
Convert D_floating to G_floating
Convert G_floating to D_floating
Convert G_floating to F_floating

Table C–2: Common Architecture Instructions (Continued)
Mnemonic

Format

Opcode

Description

CVTGQ
CVTLQ
CVTQF
CVTQG
CVTQL
CVTQS
CVTQT
CVTST
CVTTQ
CVTTS
DIVF
DIVG
DIVS
DIVT
ECB
EQV
EXCB
EXTBL
EXTLH
EXTLL
EXTQH
EXTQL
EXTWH
EXTWL
FBEQ
FBGE
FBGT
FBLE
FBLT
FBNE
FCMOVEQ
FCMOVGE
FCMOVGT
FCMOVLE
FCMOVLT
FCMOVNE
FETCH
FETCH_M
FTOIS
FTOIT
IMPLVER
INSBL
INSLH
INSLL
INSQH
INSQL
INSWH
INSWL
ITOFF
ITOFS
ITOFT
JMP
JSR
JSR_COROUTINE

F-P
F-P
F-P
F-P
F-P
F-P
F-P
F-P
F-P
F-P
F-P
F-P
F-P
F-P
Mfc
Opr
Mfc
Opr
Opr
Opr
Opr
Opr
Opr
Opr
Bra
Bra
Bra
Bra
Bra
Bra
F-P
F-P
F-P
F-P
F-P
F-P
Mfc
Mfc
F-P
F-P
Opr
Opr
Opr
Opr
Opr
Opr
Opr
Opr
F-P
F-P
F-P
Mbr
Mbr
Mbr

15.0AF
17.010
15.0BC
15.0BE
17.030
16.0BC
16.0BE
16.2AC
16.0AF
16.0AC
15.083
15.0A3
16.083
16.0A3
18.E800
11.48
18.0400
12.06
12.6A
12.26
12.7A
12.36
12.5A
12.16
31
36
37
33
32
35
17.02A
17.02D
17.02F
17.02E
17.02C
17.02B
18.8000
18.A000
1C.78
1C.70
11.6C
12.0B
12.67
12.2B
12.77
12.3B
12.57
12.1B
14.014
14.004
14.024
1A.0
1A.1
1A.3

Convert G_floating to quadword
Convert longword to quadword
Convert quadword to F_floating
Convert quadword to G_floating
Convert quadword to longword
Convert quadword to S_floating
Convert quadword to T_floating
Convert S_floating to T_floating
Convert T_floating to quadword
Convert T_floating to S_floating
Divide F_floating
Divide G_floating
Divide S_floating
Divide T_floating
Evict cache block
Logical equivalence
Exception barrier
Extract byte low
Extract longword high
Extract longword low
Extract quadword high
Extract quadword low
Extract word high
Extract word low
Floating branch if = zero
Floating branch if ≥ zero
Floating branch if > zero
Floating branch if ≤ zero
Floating branch if < zero
Floating branch if ≠ zero
FCMOVE if = zero
FCMOVE if ≥ zero
FCMOVE if > zero
FCMOVE if ≤ zero
FCMOVE if < zero
FCMOVE if ≠ zero
Prefetch data
Prefetch data, modify intent
Floating to integer move, S_floating
Floating to integer move, T_floating
Implementation version
Insert byte low
Insert longword high
Insert longword low
Insert quadword high
Insert quadword low
Insert word high
Insert word low
Integer to floating move, F_floating
Integer to floating move, S_floating
Integer to floating move, T_floating
Jump
Jump to subroutine
Jump to subroutine return

Instruction Summary C–3

Table C–2: Common Architecture Instructions (Continued)
Mnemonic

Format

Opcode

Description

LDA
LDAH
LDBU
LDWU
LDF
LDG
LDL
LDL_L
LDQ
LDQ_L
LDQ_U
LDS
LDT
MAXSB8
MAXSW4
MAXUB8
MAXUW4
MB
MF_FPCR
MINSB8
MINSW4
MINUB8
MINUW4
MSKBL
MSKLH
MSKLL
MSKQH
MSKQL
MSKWH
MSKWL
MT_FPCR
MULF
MULG
MULL
MULL/V
MULQ
MULQ/V
MULS
MULT
ORNOT
PERR
PKLB
PKWB
RC
RET
RPCC
RS
S4ADDL
S4ADDQ
S4SUBL
S4SUBQ
S8ADDL
S8ADDQ
S8SUBL

Mem
Mem
Mem
Mem
Mem
Mem
Mem
Mem
Mem
Mem
Mem
Mem
Mem
Opr
Opr
Opr
Opr
Mfc
F-P
Opr
Opr
Opr
Opr
Opr
Opr
Opr
Opr
Opr
Opr
Opr
F-P
F-P
F-P
Opr

08
09
0A
0C
20
21
28
2A
29
2B
0B
22
23
1C.3E
1C.3F
1C.3C
1C.3D
18.4000
17.025
1C.38
1C.39
1C.3A
1C.3B
12.02
12.62
12.22
12.72
12.32
12.52
12.12
17.024
15.082
15.0A2
13.00
13.40
13.20
13.60
16.082
16.0A2
11.28
1C.31
1C.37
1C.36
18.E000
1A.2
18.C000
18.F000
10.02
10.22
10.0B
10.2B
10.12
10.32
10.1B

Load address
Load address high
Load zero-extended byte
Load zero-extended word
Load F_floating
Load G_floating
Load sign-extended longword
Load sign-extended longword locked
Load quadword
Load quadword locked
Load unaligned quadword
Load S_floating
Load T_floating
Vector signed byte maximum
Vector signed word maximum
Vector unsigned byte maximum
Vector unsigned word maximum
Memory barrier
Move from FPCR
Vector signed byte minimum
Vector signed word minimum
Vector unsigned byte minimum
Vector unsigned word minimum
Mask byte low
Mask longword high
Mask longword low
Mask quadword high
Mask quadword low
Mask word high
Mask word low
Move to FPCR
Multiply F_floating
Multiply G_floating
Multiply longword

Opr
F-P
F-P
Opr
Opr
Opr
Opr
Mfc
Mbr
Mfc
Mfc
Opr
Opr
Opr
Opr
Opr
Opr
Opr

C–4 Alpha Architecture Handbook

Multiply quadword
Multiply S_floating
Multiply T_floating
Logical sum with complement
Pixel error
Pack longwords to bytes
Pack words to bytes
Read and clear
Return from subroutine
Read process cycle counter
Read and set
Scaled add longword by 4
Scaled add quadword by 4
Scaled subtract longword by 4
Scaled subtract quadword by 4
Scaled add longword by 8
Scaled add quadword by 8
Scaled subtract longword by 8

Table C–2: Common Architecture Instructions (Continued)
Mnemonic

Format

Opcode

Description

S8SUBQ
SEXTB
SEXTW
SLL
SQRTF
SQRTG
SQRTS
SQRTT
SRA
SRL
STB
STF
STG
STS
STL
STL_C
STQ
STQ_C
STQ_U
STT
STW
SUBF
SUBG
SUBL
SUBL/V
SUBQ
SUBQ/V
SUBS
SUBT
TRAPB
UMULH
UNPKBL
UNPKBW
WH64
WMB
XOR
ZAP
ZAPNOT

Opr
Opr
Opr
Opr
F-P
F-P
F-P
F-P
Opr
Opr
Mem
Mem
Mem
Mem
Mem
Mem
Mem
Mem
Mem
Mem
Mem
F-P
F-P
Opr

10.3B
1C.00
1C.01
12.39
14.08A
14.0AA
14.08B
14.0AB
12.3C
12.34
0E
24
25
26
2C
2E
2D
2F
0F
27
0D
15.081
15.0A1
10.09
10.49
10.29
10.69
16.081
16.0A1
18.0000
13.30
1C.35
1C.34
18.F800
18.4400
11.40
12.30
12.31

Scaled subtract quadword by 8
Sign extend byte
Sign extend word
Shift left logical
Square root F_floating
Square root G_floating
Square root S_floating
Square root T_floating
Shift right arithmetic
Shift right logical
Store byte
Store F_floating
Store G_floating
Store S_floating
Store longword
Store longword conditional
Store quadword
Store quadword conditional
Store unaligned quadword
Store T_floating
Store word
Subtract F_floating
Subtract G_floating
Subtract longword

Opr
F-P
F-P
Mfc
Opr
Opr
Opr
Mfc
Mfc
Opr
Opr
Opr

Subtract quadword
Subtract S_floating
Subtract T_floating
Trap barrier
Unsigned multiply quadword high
Unpack bytes to longwords
Unpack bytes to words
Write hint — 64 bytes
Write memory barrier
Logical difference
Zero bytes
Zero bytes not

Instruction Summary C–5

C.2 IEEE Floating-Point Instructions
Table C–3 lists the hexadecimal value of the 11-bit function code field for the IEEE floating-point instructions, with and without qualifiers. The opcode for the following instructions is
1616, except for SQRTS and SQRTT, which are opcode 1416.

Table C–3: IEEE Floating-Point Instruction Function Codes
None
ADDS
ADDT
CMPTEQ
CMPTLE
CMPTLT
CMPTUN
CVTQS
CVTQT
CVTST
CVTTQ
CVTTS
DIVS
DIVT
MULS
MULT
SQRTS
SQRTT
SUBS
SUBT

ADDS
ADDT
CMPTEQ
CMPTLE
CMPTLT
CMPTUN
CVTQS
CVTQT
CVTTS
DIVS
DIVT
MULS
MULT
SQRTS
SQRTT
SUBS
SUBT

CVTST

/UC

/UM

/UD

080
000
0A0
020
0A5
0A7
0A6
0A4
0BC
03C
0BE
03E
See below
See below
0AC
02C
083
003
0A3
023
082
002
0A2
022
08B
00B
0AB
02B
081
001
0A1
021

040
060

0C0
0E0

180
1A0

100
120

140
160

1C0
1E0

07C
07E

0FC
0FE

06C
043
063
042
062
04B
06B
041
061

0EC
0C3
0E3
0C2
0E2
0CB
0EB
0C1
0E1

1AC
183
1A3
182
1A2
18B
1AB
181
1A1

12C
103
123
102
122
10B
12B
101
121

16C
143
163
142
162
14B
16B
141
161

1EC
1C3
1E3
1C2
1E2
1CB
1EB
1C1
1E1

/SU

/SUC

/SUM

/SUD

/SUI

/SUIC

/SUIM

/SUID

580
5A0
5A5
5A7
5A6
5A4

500
520

540
560

5C0
5E0

780
7A0

700
720

740
760

7C0
7E0

5EC
5C3
5E3
5C2
5E2
5CB
5EB
5C1
5E1

7BC
7BE
7AC
783
7A3
782
7A2
78B
7AB
781
7A1

73C
73E
72C
703
723
702
722
70B
72B
701
721

77C
77E
76C
743
763
742
762
74B
76B
741
761

7FC
7FE
7EC
7C3
7E3
7C2
7E2
7CB
7EB
7C1
7E1

5AC
583
5A3
582
5A2
58B
5AB
581
5A1

52C
503
523
502
522
50B
52B
501
521

None

2AC

6AC

C–6 Alpha Architecture Handbook

56C
543
563
542
562
54B
56B
541
561

Table C–3: IEEE Floating-Point Instruction Function Codes (Continued)

CVTTQ

None

/VC

/SV

/SVC

/SVI

/SVIC

0AF

02F

1AF

12F

5AF

52F

7AF

72F

/VD

/SVD

/SVID

/VM

/SVM

/SVIM

0EF

1EF

5EF

7EF

06F

16F

56F

76F

Programming Note:
To use CMPTxx with software completion trap handling, specify the /SU IEEE trap mode,
even though an underflow trap is not possible. To use CVTQS or CVTQT with software
completion trap handling, specify the /SUI IEEE trap mode, even though an underflow trap
is not possible.

C.3 VAX Floating-Point Instructions
Table C–4 lists the hexadecimal value of the 11-bit function code field for the VAX floating-point instructions. The opcode for the following instructions is 15 16 , except for SQRTF
and SQRTG, which are opcode 14 16.

Table C–4: VAX Floating-Point Instruction Function Codes

ADDF
CVTDG
ADDG
CMPGEQ
CMPGLE
CMPGLT
CVTGD
CVTGF
CVTQF
CVTQG
CVTGQ
DIVF
DIVG
MULF
MULG
SQRTF
SQRTG
SUBF
SUBG

CVTGQ

None

/UC

/SC

/SU

/SUC

080
09E
0A0
0A5
0A7
0A6
0AD
0AC
0BC
0BE
See below
083
0A3
082
0A2
08A
0AA
081
0A1

000
01E
020

180
19E
1A0

100
11E
120

400
41E
420

580
59E
5A0

500
51E
520

02D
02C
03C
03E

1AD
1AC

12D
12C

480
49E
4A0
4A5
4A6
4A7
4AD
4AC

42D
42C

5AD
5AC

52D
52C

003
023
002
022
00A
02A
001
021

183
1A3
182
1A2
18A
1AA
181
1A1

103
123
102
122
10A
12A
101
121

483
4A3
482
4A2
48A
4AA
481
4A1

403
423
402
422
40A
42A
401
421

583
5A3
582
5A2
58A
5AA
581
5A1

503
523
502
522
50A
52A
501
521

None

/VC

/SC

/SV

/SVC

0AF

02F

1AF

12F

4AF

42F

5AF

52F

Instruction Summary C–7

C.4 Independent Floating-Point Instructions
Table C–5 lists the hexadecimal value of the 11-bit function code field for the floating-point
instructions that are not directly tied to IEEE or VAX floating point. The opcode for the following instructions is 1716.

Table C–5: Independent Floating-Point Instruction Function Codes
None
CPYS
CPYSE
CPYSN
CVTLQ
CVTQL
FCMOVEQ
FCMOVGE
FCMOVGT
FCMOVLE
FCMOVLT
MF_FPCR
MT_FPCR

020
022
021
010
030
02A
02D
02F
02E
02C
025
024

/SV

130

530

C.5 Opcode Summary
Table C–6 lists all Alpha opcodes from 00 (CALL_PAL) through 3F (BGT). In the table, the
column headings that appear over the instructions have a granularity of 816. The rows beneath
the leftmost column supply the individual hex number to resolve that granularity.
If an instruction column has a 0 (zero) in the right (low) hex digit, replace that 0 with the number to the left of the backslash in the leftmost column on the instruction’s row. If an instruction
column has an 8 in the right (low) hexadecimal digit, replace that 8 with the number to the
right of the backslash in the leftmost column.
For example, the third row (2/A) under the 10 column contains the symbol INTS*, representing all the integer shift instructions. The opcode for those instructions would then be 12 16
because the 0 in 10 is replaced by the 2 in the leftmost column. Likewise, the third row under
the 18 column contains the symbol JSR*, representing all jump instructions. The opcode for
those instructions is 1A because the 8 in the heading is replaced by the number to the right of
the backslash in the leftmost column.

C–8 Alpha Architecture Handbook

The instruction format is listed under the instruction symbol. The symbols in Table C–6 are
explained in Table C–7.

Table C–6: Opcode Summary
00

0/8

PAL*
(pal)

LDA
(mem)

INTA*
(op)

MISC*
(mem)

LDF
(mem)

LDL
(mem)

BR
(br)

BLBC
(br)

1/9

Res

LDAH
(mem)

INTL*
(op)

\PAL\

LDG
(mem)

LDQ
(mem)

FBEQ
(br)

BEQ
(br)

2/A

Res

LDBU
(mem)

INTS*
(op)

JSR*
(mem)

LDS
(mem)

LDL_L
(mem)

FBLT
(br)

BLT
(br)

3/B

Res

LDQ_U
(mem)

INTM*
(op)

\PAL\

LDT
(mem)

LDQ_L
(mem)

FBLE
(br)

BLE
(br)

4/C

Res

LDWU
(mem)

ITFP*

FPTI*

STF
(mem)

STL
(mem)

BSR
(br)

BLBS
(br)

5/D

Res

STW
(mem)

FLTV*
(op)

\PAL\

STG
(mem)

STQ
(mem)

FBNE
(br)

BNE
(br)

6/E

Res

STB
(mem)

FLTI*
(op)

\PAL\

STS
(mem)

STL_C
(mem)

FBGE
(br)

BGE
(br)

7/F

Res

STQ_U
(mem)

FLTL*
(op)

\PAL\

STT
(mem)

STQ_C
(mem)

FBGT
(br)

BGT
(br)

Table C–7: Key to Opcode Summary
Symbol

Meaning

FLTI*
FLTL*
FLTV*
FPTI*
INTA*
INTL*
INTM*
INTS*
ITFP*
JSR*
MISC*
PAL*
\PAL\

IEEE floating-point instruction opcodes
Floating-point Operate instruction opcodes
VAX floating-point instruction opcodes
Floating-point to integer register move opcodes
Integer arithmetic instruction opcodes
Integer logical instruction opcodes
Integer multiply instruction opcodes
Integer shift instruction opcodes
Integer to floating-point register move opcodes
Jump instruction opcodes
Miscellaneous instruction opcodes
PALcode instruction (CALL_PAL) opcodes
Reserved for PALcode

Res

Reserved for Compaq

Instruction Summary C–9

C.6 Common Architecture Opcodes in Numerical Order
Table C–8: Common Architecture Opcodes in Numerical Order
Opcode
00
01
02
03
04
05
06
07
08
09
0A
0B
0C
0D
0E
0F
10.00
10.02
10.09
10.0B
10.0F
10.12
10.1B
10.1D
10.20
10.22
10.29
10.2B
10.2D
10.32
10.3B
10.3D
10.40
10.49
10.4D
10.60
10.69
10.6D
11.00
11.08
11.14
11.16
11.20
11.24

Opcode
CALL_PAL
OPC01
OPC02
OPC03
OPC04
OPC05
OPC06
OPC07
LDA
LDAH
LDBU
LDQ_U
LDWU
STW
STB
STQ_U
ADDL
S4ADDL
SUBL
S4SUBL
CMPBGE
S8ADDL
S8SUBL
CMPULT
ADDQ
S4ADDQ
SUBQ
S4SUBQ
CMPEQ
S8ADDQ
S8SUBQ
CMPULE
ADDL/V
SUBL/V
CMPLT
ADDQ/V
SUBQ/V
CMPLE
AND
BIC
CMOVLBS
CMOVLBC
BIS
CMOVEQ

C–10 Alpha Architecture Handbook

11.26
11.28
11.40
11.44
11.46
11.48
11.61
11.64
11.66
11.6C
12.02
12.06
12.0B
12.12
12.16
12.1B
12.22
12.26
12.2B
12.30
12.31
12.32
12.34
12.36
12.39
12.3B
12.3C
12.52
12.57
12.5A
12.62
12.67
12.6A
12.72
12.77
12.7A
13.00
13.20
13.30
13.40
13.60
14.004
14.00A
14.00B

Opcode
CMOVNE
ORNOT
XOR
CMOVLT
CMOVGE
EQV
AMASK
CMOVLE
CMOVGT
IMPLVER
MSKBL
EXTBL
INSBL
MSKWL
EXTWL
INSWL
MSKLL
EXTLL
INSLL
ZAP
ZAPNOT
MSKQL
SRL
EXTQL
SLL
INSQL
SRA
MSKWH
INSWH
EXTWH
MSKLH
INSLH
EXTLH
MSKQH
INSQH
EXTQH
MULL
MULQ
UMULH
MULL/V
MULQ/V
ITOFS
SQRTF/C
SQRTS/C

14.014
14.024
14.02A
14.02B
14.04B
14.06B
14.08A
14.08B
14.0AA
14.0AB
14.0CB
14.0EB
14.10A
14.10B
14.12A
14.12B
14.14B
14.16B
14.18A
14.18B
14.1AA
14.1AB
14.1CB
14.1EB
14.40A
14.42A
14.48A
14.4AA
14.50A
14.50B
14.52A
14.52B
14.54B
14.56B
14.58A
14.58B
14.5AA
14.5AB
14.5CB
14.5EB
14.70B
14.72B
14.74B
14.76B

ITOFF
ITOFT
SQRTG/C
SQRTT/C
SQRTS/M
SQRTT/M
SQRTF
SQRTS
SQRTG
SQRTT
SQRTS/D
SQRTT/D
SQRTF/UC
SQRTS/UC
SQRTG/UC
SQRTT/UC
SQRTS/UM
SQRTT/UM
SQRTF/U
SQRTS/U
SQRTG/U
SQRTT/U
SQRTS/UD
SQRTT/UD
SQRTF/SC
SQRTG/SC
SQRTF/S
SQRTG/S
SQRTF/SUC
SQRTS/SUC
SQRTG/SUC
SQRTT/SUC
SQRTS/SUM
SQRTT/SUM
SQRTF/SU
SQRTS/SU
SQRTG/SU
SQRTT/SU
SQRTS/SUD
SQRTT/SUD
SQRTS/SUIC
SQRTT/SUIC
SQRTS/SUIM
SQRTT/SUIM

Table C–8: Common Architecture Opcodes in Numerical Order (Continued)
Opcode
14.78B
14.7AB
14.7CB
14.7EB
15.000
15.001
15.002
15.003
15.01E
15.020
15.021
15.022
15.023
15.02C
15.02D
15.02F
15.03C
15.03E
15.080
15.081
15.082
15.083
15.09E
15.0A0
15.0A1
15.0A2
15.0A3
15.0A5
15.0A6
15.0A7
15.0AC
15.0AD
15.0AF
15.0BC
15.0BE
15.100
15.101
15.102
15.103
15.11E
15.120
15.121
15.122
15.123
15.12C
15.12D

Opcode
SQRTS/SUI
SQRTT/SUI
SQRTS/SUID
SQRTT/SUID
ADDF/C
SUBF/C
MULF/C
DIVF/C
CVTDG/C
ADDG/C
SUBG/C
MULG/C
DIVG/C
CVTGF/C
CVTGD/C
CVTGQ/C
CVTQF/C
CVTQG/C
ADDF
SUBF
MULF
DIVF
CVTDG
ADDG
SUBG
MULG
DIVG
CMPGEQ
CMPGLT
CMPGLE
CVTGF
CVTGD
CVTGQ
CVTQF
CVTQG
ADDF/UC
SUBF/UC
MULF/UC
DIVF/UC
CVTDG/UC
ADDG/UC
SUBG/UC
MULG/UC
DIVG/UC
CVTGF/UC
CVTGD/UC

15.12F
15.180
15.181
15.182
15.183
15.19E
15.1A0
15.1A1
15.1A2
15.1A3
15.1AC
15.1AD
15.1AF
15.400
15.401
15.402
15.403
15.41E
15.420
15.421
15.422
15.423
15.42C
15.42D
15.42F
15.480
15.481
15.482
15.483
15.49E
15.4A0
15.4A1
15.4A2
15.4A3
15.4A5
15.4A6
15.4A7
15.4AC
15.4AD
15.4AF
15.500
15.501
15.502
15.503
15.51E
15.520

Opcode
CVTGQ/VC
ADDF/U
SUBF/U
MULF/U
DIVF/U
CVTDG/U
ADDG/U
SUBG/U
MULG/U
DIVG/U
CVTGF/U
CVTGD/U
CVTGQ/V
ADDF/SC
SUBF/SC
MULF/SC
DIVF/SC
CVTDG/SC
ADDG/SC
SUBG/SC
MULG/SC
DIVG/SC
CVTGF/SC
CVTGD/SC
CVTGQ/SC
ADDF/S
SUBF/S
MULF/S
DIVF/S
CVTDG/S
ADDG/S
SUBG/S
MULG/S
DIVG/S
CMPGEQ/S
CMPGLT/S
CMPGLE/S
CVTGF/S
CVTGD/S
CVTGQ/S
ADDF/SUC
SUBF/SUC
MULF/SUC
DIVF/SUC
CVTDG/SUC
ADDG/SUC

15.521
15.522
15.523
15.52C
15.52D
15.52F
15.580
15.581
15.582
15.583
15.59E
15.5A0
15.5A1
15.5A2
15.5A3
15.5AC
15.5AD
15.5AF
16.000
16.001
16.002
16.003
16.020
16.021
16.022
16.023
16.02C
16.02F
16.03C
16.03E
16.040
16.041
16.042
16.043
16.060
16.061
16.062
16.063
16.06C
16.06F
16.07C
16.07E
16.080
16.081
16.082
16.083

SUBG/SUC
MULG/SUC
DIVG/SUC
CVTGF/SUC
CVTGD/SUC
CVTGQ/SVC
ADDF/SU
SUBF/SU
MULF/SU
DIVF/SU
CVTDG/SU
ADDG/SU
SUBG/SU
MULG/SU
DIVG/SU
CVTGF/SU
CVTGD/SU
CVTGQ/SV
ADDS/C
SUBS/C
MULS/C
DIVS/C
ADDT/C
SUBT/C
MULT/C
DIVT/C
CVTTS/C
CVTTQ/C
CVTQS/C
CVTQT/C
ADDS/M
SUBS/M
MULS/M
DIVS/M
ADDT/M
SUBT/M
MULT/M
DIVT/M
CVTTS/M
CVTTQ/M
CVTQS/M
CVTQT/M
ADDS
SUBS
MULS
DIVS

Instruction Summary C–11

Table C–8: Common Architecture Opcodes in Numerical Order (Continued)
Opcode
16.0A0
16.0A1
16.0A2
16.0A3
16.0A4
16.0A5
16.0A6
16.0A7
16.0AC
16.0AF
16.0BC
16.0BE
16.0C0
16.0C1
16.0C2
16.0C3
16.0E0
16.0E1
16.0E2
16.0E3
16.0EC
16.0EF
16.0FC
16.0FE
16.100
16.101
16.102
16.103
16.120
16.121
16.122
16.123
16.12C
16.12F
16.140
16.141
16.142
16.143
16.160
16.161
16.162
16.163
16.16C
16.16F
16.180
16.181

Opcode
ADDT
SUBT
MULT
DIVT
CMPTUN
CMPTEQ
CMPTLT
CMPTLE
CVTTS
CVTTQ
CVTQS
CVTQT
ADDS/D
SUBS/D
MULS/D
DIVS/D
ADDT/D
SUBT/D
MULT/D
DIVT/D
CVTTS/D
CVTTQ/D
CVTQS/D
CVTQT/D
ADDS/UC
SUBS/UC
MULS/UC
DIVS/UC
ADDT/UC
SUBT/UC
MULT/UC
DIVT/UC
CVTTS/UC
CVTTQ/VC
ADDS/UM
SUBS/UM
MULS/UM
DIVS/UM
ADDT/UM
SUBT/UM
MULT/UM
DIVT/UM
CVTTS/UM
CVTTQ/VM
ADDS/U
SUBS/U

C–12 Alpha Architecture Handbook

16.182
16.183
16.1A0
16.1A1
16.1A2
16.1A3
16.1AC
16.1AF
16.1C0
16.1C1
16.1C2
16.1C3
16.1E0
16.1E1
16.1E2
16.1E3
16.1EC
16.1EF
16.2AC
16.500
16.501
16.502
16.503
16.520
16.521
16.522
16.523
16.52C
16.52F
16.540
16.541
16.542
16.543
16.560
16.561
16.562
16.563
16.56C
16.56F
16.580
16.581
16.582
16.583
16.5A0
16.5A1
16.5A2

Opcode
MULS/U
DIVS/U
ADDT/U
SUBT/U
MULT/U
DIVT/U
CVTTS/U
CVTTQ/V
ADDS/UD
SUBS/UD
MULS/UD
DIVS/UD
ADDT/UD
SUBT/UD
MULT/UD
DIVT/UD
CVTTS/UD
CVTTQ/VD
CVTST
ADDS/SUC
SUBS/SUC
MULS/SUC
DIVS/SUC
ADDT/SUC
SUBT/SUC
MULT/SUC
DIVT/SUC
CVTTS/SUC
CVTTQ/SVC
ADDS/SUM
SUBS/SUM
MULS/SUM
DIVS/SUM
ADDT/SUM
SUBT/SUM
MULT/SUM
DIVT/SUM
CVTTS/SUM
CVTTQ/SVM
ADDS/SU
SUBS/SU
MULS/SU
DIVS/SU
ADDT/SU
SUBT/SU
MULT/SU

16.5A3
16.5A4
16.5A5
16.5A6
16.5A7
16.5AC
16.5AF
16.5C0
16.5C1
16.5C2
16.5C3
16.5E0
16.5E1
16.5E2
16.5E3
16.5EC
16.5EF
16.6AC
16.700
16.701
16.702
16.703
16.720
16.721
16.722
16.723
16.72C
16.72F
16.73C
16.73E
16.740
16.741
16.742
16.743
16.760
16.761
16.762
16.763
16.76C
16.76F
16.77C
16.77E
16.780
16.781
16.782
16.783

DIVT/SU
CMPTUN/SU
CMPTEQ/SU
CMPTLT/SU
CMPTLE/SU
CVTTS/SU
CVTTQ/SV
ADDS/SUD
SUBS/SUD
MULS/SUD
DIVS/SUD
ADDT/SUD
SUBT/SUD
MULT/SUD
DIVT/SUD
CVTTS/SUD
CVTTQ/SVD
CVTST/S
ADDS/SUIC
SUBS/SUIC
MULS/SUIC
DIVS/SUIC
ADDT/SUIC
SUBT/SUIC
MULT/SUIC
DIVT/SUIC
CVTTS/SUIC
CVTTQ/SVIC
CVTQS/SUIC
CVTQT/SUIC
ADDS/SUIM
SUBS/SUIM
MULS/SUIM
DIVS/SUIM
ADDT/SUIM
SUBT/SUIM
MULT/SUIM
DIVT/SUIM
CVTTS/SUIM
CVTTQ/SVIM
CVTQS/SUIM
CVTQT/SUIM
ADDS/SUI
SUBS/SUI
MULS/SUI
DIVS/SUI

Table C–8: Common Architecture Opcodes in Numerical Order (Continued)
Opcode
16.7A0
16.7A1
16.7A2
16.7A3
16.7AC
16.7AF
16.7BC
16.7BE
16.7C0
16.7C1
16.7C2
16.7C3
16.7E0
16.7E1
16.7E2
16.7E3
16.7EC
16.7EF
16.7FC
16.7FE
17.010
17.020
17.021
17.022
17.024
17.025
17.02A
17.02B
17.02C
17.02D
17.02E
17.02F
17.030
17.130
17.530
18.0000
18.0400

Opcode
ADDT/SUI
SUBT/SUI
MULT/SUI
DIVT/SUI
CVTTS/SUI
CVTTQ/SVI
CVTQS/SUI
CVTQT/SUI
ADDS/SUID
SUBS/SUID
MULS/SUID
DIVS/SUID
ADDT/SUID
SUBT/SUID
MULT/SUID
DIVT/SUID
CVTTS/SUID
CVTTQ/SVID
CVTQS/SUID
CVTQT/SUID
CVTLQ
CPYS
CPYSN
CPYSE
MT_FPCR
MF_FPCR
FCMOVEQ
FCMOVNE
FCMOVLT
FCMOVGE
FCMOVLE
FCMOVGT
CVTQL
CVTQL/V
CVTQL/SV
TRAPB
EXCB

18.4000
18.4400
18.8000
18.A000
18.C000
18.E000
18.E800
18.F000
18.F800
19
1A.0
1A.1
1A.2
1A.3
1B
1C.00
1C.01
1C.30
1C.31
1C.32
1C.33
1C.34
1C.35
1C.36
1C.37
1C.38
1C.39
1C.3A
1C.3B
1C.3C
1C.3D
1C.3E
1C.3F
1C.70
1C.78
1D
1E

Opcode
MB
WMB
FETCH
FETCH_M
RPCC
RC
ECB
RS
WH64
PAL19
JMP
JSR
RET
JSR_COROUTINE
PAL1B
SEXTB
SEXTW
CTPOP
PERR
CTLZ
CTTZ
UNPKBW
UNPKBL
PKWB
PKLB
MINSB8
MINSW4
MINUB8
MINUW4
MAXUB8
MAXUW4
MAXSB8
MAXSW4
FTOIT
FTOIS
PAL1D
PAL1E

1F
20
21
22
23
24
25
26
27
28
29
2A
2B
2C
2D
2E
2F
30
31
32
33
34
35
36
37
38
39
3A
3B
3C
3D
3E
3F

PAL1F
LDF
LDG
LDS
LDT
STF
STG
STS
STT
LDL
LDQ
LDL_L
LDQ_L
STL
STQ
STL_C
STQ_C
BR
FBEQ
FBLT
FBLE
BSR
FBNE
FBGE
FBGT
BLBC
BEQ
BLT
BLE
BLBS
BNE
BGE
BGT

Instruction Summary C–13

C.7 OpenVMS Alpha PALcode Instruction Summary
Table C–9: OpenVMS Alpha Unprivileged PALcode Instructions
Mnemonic

Opcode

Description

AMOVRM
AMOVRR
BPT
BUGCHK
CHMK
CHME
CHMS
CHMU
CLRFEN
GENTRAP
IMB
INSQHIL
INSQHILR
INSQHIQ
INSQHIQR
INSQTIL
INSQTILR
INSQTIQ
INSQTIQR
INSQUEL
INSQUEL/D
INSQUEQ
INSQUEQ/D
PROBER
PROBEW
RD_PS
READ_UNQ
REI
REMQHIL
REMQHILR
REMQHIQ
REMQHIQR
REMQTIL
REMQTILR
REMQTIQ
REMQTIQR
REMQUEL
REMQUEL/D
REMQUEQ
REMQUEQ/D
RSCC
SWASTEN
WRITE_UNQ
WR_PS_SW

00.00A1
00.00A0
00.0080
00.0081
00.0083
00.0082
00.0084
00.0085
00.00AE
00.00AA
00.0086
00.0087
00.00A2
00.0089
00.00A4
00.0088
00.00A3
00.008A
00.00A5
00.008B
00.008D
00.008C
00.008E
00.008F
00.0090
00.0091
00.009E
00.0092
00.0093
00.00A6
00.0095
00.00A8
00.0094
00.00A7
00.0096
00.00A9
00.0097
00.0099
00.0098
00.009A
00.009D
00.009B
00.009F
00.009C

Atomic move from register to memory
Atomic move from register to register
Breakpoint
Bugcheck
Change mode to kernel
Change mode to executive
Change mode to supervisor
Change mode to user
Clear floating-point enable
Generate software trap
I-stream memory barrier
Insert into longword queue at head interlocked
Insert into longword queue at head interlocked resident
Insert into quadword queue at head interlocked
Insert into quadword queue at head interlocked resident
Insert into longword queue at tail interlocked
Insert into longword queue at tail interlocked resident
Insert into quadword queue at tail interlocked
Insert into quadword queue at tail interlockedresident
Insert entry into longword queue
Insert entry into longword queue deferred
Insert entry into quadword queue
Insert entry into quadword queue deferred
Probe for read access
Probe for write access
Move processor status
Read unique context
Return from exception or interrupt
Remove from longword queue at head interlocked
Remove from longword queue at head interlocked resident
Remove from quadword queue at head interlocked
Remove from quadword queue at head interlocked resident
Remove from longword queue at tail interlocked
Remove from longword queue at tail interlocked resident
Remove from quadword queue at tail interlocked
Remove from quadword queue at tail interlocked resident
Remove entry from longword queue
Remove entry from longword queue deferred
Remove entry from quadword queue
Remove entry from quadword queue deferred
Read system cycle counter
Swap AST enable for current mode
Write unique context
Write processor status software field

C–14 Alpha Architecture Handbook

Table C–10: OpenVMS Alpha Privileged PALcode Instructions
Mnemonic

Opcode

Description

CFLUSH
CSERVE
DRAINA
HALT
LDQP
MFPR_ASN
MFPR_ESP
MFPR_FEN
MFPR_IPL
MFPR_MCES
MFPR_PCBB
MFPR_PRBR
MFPR_PTBR
MFPR_SCBB
MFPR_SISR
MFPR_SSP
MFPR_TBCHK
MFPR_USP
MFPR_VPTB
MFPR_WHAMI
MTPR_ASTEN
MTPR_ASTSR
MTPR_DATFX
MTPR_ESP
MTPR_FEN
MTPR_IPIR
MTPR_IPL
MTPR_MCES
MTPR_PERFMON
MTPR_PRBR
MTPR_SCBB
MTPR_SIRR
MTPR_SSP
MTPR_TBIA
MTPR_TBIAP
MTPR_TBIS
MTPR_TBISD
MTPR_TBISI
MTPR_USP
MTPR_VPTB
STQP
SWPCTX
SWPPAL
WTINT

00.0001
00.0009
00.0002
00.0000
00.0003
00.0006
00.001E
00.000B
00.000E
00.0010
00.0012
00.0013
00.0015
00.0016
00.0019
00.0020
00.001A
00.0022
00.0029
00.003F
00.0026
00.0027
00.002E
00.001F
00.000B
00.000D
00.000E
00.0011
00.002B
00.0014
00.0017
00.0018
00.0021
00.001B
00.001C
00.001D
00.0024
00.0025
00.0023
00.002A
00.0004
00.0005
00.000A
00.003E

Cache flush
Console service
Drain aborts
Halt processor
Load quadword physical
Move from processor register ASN
Move from processor register ESP
Move from processor register FEN
Move from processor register IPL
Move from processor register MCES
Move from processor register PCBB
Move from processor register PRBR
Move from processor register PTBR
Move from processor register SCBB
Move from processor register SISR
Move from processor register SSP
Move from processor register TBCHK
Move from processor register USP
Move from processor register VPTB
Move from processor register WHAMI
Move to processor register ASTEN
Move to processor register ASTSR
Move to processor register DATFX
Move to processor register ESP
Move to processor register FEN
Move to processor register IPRI
Move to processor register IPL
Move to processor register MCES
Move to processor register PERFMON
Move to processor register PRBR
Move to processor register SCBB
Move to processor register SIRR
Move to processor register SSP
Move to processor register TBIA
Move to processor register TBIAP
Move to processor register TBIS
Move to processor register TBISD
Move to processor register TBISI
Move to processor register USP
Move to processor register VPTB
Store quadword physical
Swap privileged context
Swap PALcode image
Wait for interrupt

Instruction Summary C–15

C.8 DIGITAL UNIX PALcode Instruction Summary
Table C–11: DIGITAL UNIX Unprivileged PALcode Instructions
Mnemonic

Opcode

Description

bpt
bugchk
callsys
clrfen
gentrap
imb
rdunique
urti
wrunique

00.0080
00.0081
00.0083
00.00AE
00.00AA
00.0086
00.009E
00.0092
00.009F

Breakpoint trap
Bugcheck
System call
Clear floating-point enable
Generate software trap
I-stream memory barrier
Read unique value
Return from user mode trap
Write unique value

Table C–12: DIGITAL UNIX Privileged PALcode Instructions
Mnemonic

Opcode

Description

cflush
cserve
draina
halt
rdmces
rdps
rdusp
rdval
retsys
rti
swpctx
swpipl
swppal
tbi
whami
wrent
wrfen
wripir
wrkgp
wrmces
wrperfmon
wrusp
wrval
wrvptptr
wtint

00.0001
00.0009
00.0002
00.0000
00.0010
00.0036
00.003A
00.0032
00.003D
00.003F
00.0030
00.0035
00.000A
00.0033
00.003C
00.0034
00.002B
00.000D
00.0037
00.0011
00.0039
00.0038
00.0031
00.002D
00.003E

Cache flush
Console service
Drain aborts
Halt the processor
Read machine check error summary register
Read processor status
Read user stack pointer
Read system value
Return from system call
Return from trap or interrupt
Swap privileged context
Swap interrupt priority level
Swap PALcode image
Translation buffer invalidate
Who am I
Write system entry address
Write floating-point enable
Write interprocessor interrupt request
Write kernel global pointer
Write machine check error summary register
Performance monitoring function
Write user stack pointer
Write system value
Write virtual page table pointer
Wait for interrupt

C–16 Alpha Architecture Handbook

C.9 Windows NT Alpha Instruction Summary
Table C–13: Windows NT Alpha Unprivileged PALcode Instructions
Mnemonic

Opcode

Description

bpt
callkd
callsys
gentrap
imb
kbpt
rdteb

00.0080
00.00AD
00.0083
00.00AA
00.0086
00.00AC
00.00AB

Breakpoint trap
Call kernel debugger
Call system service
Generate trap
Instruction memory barrier
Kernel breakpoint trap
Read TEB internal processor register

Table C–14: Windows NT Alpha Privileged PALcode instructions
Mnemonic

Opcode

Description

csir
dalnfix
di
draina
dtbis
ealnfix
ei
halt
initpal
initpcr
rdcounters
rdirql
rdksp
rdmces
rdpcr
rdpsr
rdstate
rdthread
reboot
restart
retsys
rfe
swpirql
swpksp
swppal
swpprocess
swpctx
ssir
tbia
tbim
tbimasn
tbis
tbisasn
wrentry
wrmces
wrperfmon

00.000D
00.0025
00.0008
00.0002
00.0016
00.0024
00.0009
00.0000
00.0004
00.0038
00.0030
00.0007
00.0018
00.0012
00.001C
00.001A
00.0031
00.001E
00.0002
00.0001
00.000F
00.000E
00.0006
00.0019
00.000A
00.0011
00.0010
00.000C
00.0014
00.0020
00.0021
00.0015
00.0017
00.0005
00.0013
00.0032

Clear software interrupt request
Disable alignment fixups
Disable interrupts
Drain aborts
Data translation buffer invalidate single
Enable alignment fixups
Enable interrupts
Trap to illegal instruction
Initialize the PALcode
Initialize processor control region data
Read PALcode event counters
Read current IRQL
Read initial kernel stack
Read machine check error summary
Read PCR (processor control registers)
Read processor status register
Read internal processor state
Read the current thread value
Transfer to console firmware
Restart the processor
Return from system service call
Return from exception
Swap IRQL
Swap initial kernel stack
Swap PALcode
Swap privileged process context
Swap privileged thread context
Set software interrupt request
Translation buffer invalidate all
Translation buffer invalidate multiple
Translation buffer invalidate multiple ASN
Translation buffer invalidate single
Translation buffer invalidate single ASN
Write system entry
Write machine check error summary
Write performance monitoring values

Instruction Summary C–17

C.10 PALcode Opcodes in Numerical Order
Opcodes 00.003816 through 00.003F16 are reserved for processor implementation-specific
PALcode instructions. All other opcodes are reserved for use by Compaq.

Table C–15: PALcode Opcodes in Numerical Order
Opcode16

Opcode10

OpenVMS Alpha

DIGITAL
UNIX

Windows NT
Alpha

00.0000
00.0001
00.0002
00.0003
00.0004
00.0005
00.0006
00.0007
00.0008
00.0009
00.000A
00.000B
00.000C
00.000D
00.000E
00.000F
00.0010
00.0011
00.0012
00.0013
00.0014
00.0015
00.0016
00.0017
00.0018
00.0019
00.001A
00.001B
00.001C
00.001D
00.001E
00.001F
00.0020
00.0021
00.0022
00.0023
00.0024
00.0025
00.0026
00.0027
00.0029
00.002A
00.002B
00.002D
00.002E
00.0030
00.0031

00.0000
00.0001
00.0002
00.0003
00.0004
00.0005
00.0006
00.0007
00.0008
00.0009
00.0010
00.0011
00.0012
00.0013
00.0014
00.0015
00.0016
00.0017
00.0018
00.0019
00.0020
00.0021
00.0022
00.0023
00.0024
00.0025
00.0026
00.0027
00.0028
00.0029
00.0030
00.0031
00.0032
00.0033
00.0034
00.0035
00.0036
00.0037
00.0038
00.0039
00.0041
00.0042
00.0043
00.0045
00.0046
00.0048
00.0049

HALT
CFLUSH
DRAINA
LDQP
STQP
SWPCTX
MFPR_ASN
MTPR_ASTEN
MTPR_ASTSR
CSERVE
SWPPAL
MFPR_FEN
MTPR_FEN
MTPR_IPIR
MFPR_IPL
MTPR_IPL
MFPR_MCES
MTPR_MCES
MFPR_PCBB
MFPR_PRBR
MTPR_PRBR
MFPR_PTBR
MFPR_SCBB
MTPR_SCBB
MTPR_SIRR
MFPR_SISR
MFPR_TBCHK
MTPR_TBIA
MTPR_TBIAP
MTPR_TBIS
MFPR_ESP
MTPR_ESP
MFPR_SSP
MTPR_SSP
MFPR_USP
MTPR_USP
MTPR_TBISD
MTPR_TBISI
MFPR_ASTEN
MFPR_ASTSR
MFPR_VPTB
MTPR_VPTB
MTPR_PERFMON
—
MTPR_DATFX
—
—

halt
cflush
draina
—
—
—
—
—
—
cserve
swppal
—
—
wripir
—
—
rdmces
wrmces
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
wrfen
wrvptptr
—
swpctx
wrval

halt
restart
draina
reboot
initpal
wrentry
swpirql
rdirql
di
ei
swppal
—
ssir
csir
rfe
retsys
swpctx
swpprocess
rdmes
wrmces
tbia
tbis
dtbis
tbisasn
rdksp
swpksp
rdpsr
—
rdpcr
—
rdthread
—
tbim
tbimasn
—
—
ealnfix
dalnfix
—
—
—
—
—
—
—
rdcounters
rdstate

C–18 Alpha Architecture Handbook

Table C–15: PALcode Opcodes in Numerical Order (Continued)
Opcode16

Opcode10

OpenVMS Alpha

00.0032
00.0033
00.0034
00.0035
00.0036
00.0037
00.0038
00.0039
00.003A
00.003C
00.003D
00.003E
00.003F
00.0080
00.0081
00.0082
00.0083
00.0084
00.0085
00.0086
00.0087
00.0088
00.0089
00.008A
00.008B
00.008C
00.008D
00.008E
00.008F
00.0090
00.0091
00.0092
00.0093
00.0094
00.0095
00.0096
00.0097
00.0098
00.0099
00.009A
00.009B
00.009C
00.009D
00.009E
00.009F
00.00A0
00.00A1
00.00A2
00.00A3
00.00A4
00.00A5
00.00A6
00.00A7

00.0050
00.0051
00.0052
00.0053
00.0054
00.0055
00.0056
00.0057
00.0058
00.0060
00.0061
00.0062
00.0063
00.0128
00.0129
00.0130
00.0131
00.0132
00.0133
00.0134
00.0135
00.0136
00.0137
00.0138
00.0139
00.0140
00.0141
00.0142
00.0143
00.0144
00.0145
00.0146
00.0147
00.0148
00.0149
00.0150
00.0151
00.0152
00.0153
00.0154
00.0155
00.0156
00.0157
00.0158
00.0159
00.0160
00.0161
00.0162
00.0163
00.0164
00.0165
00.0166
00.0167

—
—
—
—
—
—
—
—
—
—
—
WTINT
MFPR_WHAMI
BPT
BUGCHK
CHME
CHMK
CHMS
CHMU
IMB
INSQHIL
INSQTIL
INSQHIQ
INSQTIQ
INSQUEL
INSQUEQ
INSQUEL/D
INSQUEQ/D
PROBER
PROBEW
RD_PS
REI
REMQHIL
REMQTIL
REMQHIQ
REMQTIQ
REMQUEL
REMQUEQ
REMQUEL/D
REMQUEQ/D
SWASTEN
WR_PS_SW
RSCC
READ_UNQ
WRITE_UNQ
AMOVRR
AMOVRM
INSQHILR
INSQTILR
INSQHIQR
INSQTIQR
REMQHILR
REMQTILR

DIGITAL
UNIX

Windows NT
Alpha

rdval
tbi
wrent
swpipl
rdps
wrkgp
wrusp
wrperfmon
rdusp
whami
retsys
wtint
rti
bpt
bugchk
—
callsys
—
—
imb
—
—
—
—
—
—
—
—
—
—
—
urti
—
—
—
—
—
—

wrperfmon
—
—
—
—
initpcr
—
—
—
—
—
—
—
bpt
—
—
callsys
—
—
imb
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—

—
—
—
—
rdunique
wrunique
—
—
—
—
—
—
—
—

Instruction Summary C–19

Table C–15: PALcode Opcodes in Numerical Order (Continued)
Opcode16

Opcode10

OpenVMS Alpha

DIGITAL
UNIX

Windows NT
Alpha

00.00A8
00.00A9
00.00AA
00.00AB
00.00AC
00.00AD
00.00AE

00.0168
00.0169
00.0170
00.0171
00.0172
00.0173
00.0174

REMQHIQR
REMQTIQR
GENTRAP
—
—
—
CLRFEN

—
—
gentrap
—
—
—
clrfen

—
—
gentrap
rdteb
kbpt
callkd

C.11 Required PALcode Opcodes
The opcodes listed in Table C–16 are required for all Alpha implementations. The notation
used is oo.ffff, where oo is the hexadecimal 6-bit opcode and ffff is the hexadecimal 26-bit
function code.

Table C–16: Required PALcode Opcodes
Mnemonic

Type

Opcode

DRAINA
HALT
IMB

Privileged
Privileged
Unprivileged

00.0002
00.0000
00.0086

C.12 Opcodes Reserved to PALcode
The opcodes listed in Table C–17 are reserved for use in implementing PALcode.

Table C–17: Opcodes Reserved for PALcode
Mnemonic
PAL19
PAL1E

Mnemonic
19
1E

C–20 Alpha Architecture Handbook

PAL1B
PAL1F

Mnemonic
1B
1F

PAL1D

C.13 Opcodes Reserved to Compaq
The opcodes listed in Table C–18 are reserved to Compaq.

Table C–18: Opcodes Reserved for Compaq
Mnemonic
OPC01
OPC04
OPC07

Mnemonic
01
04
07

OPC02
OPC05

Mnemonic
02
05

OPC03
OPC06

03
06

Programming Note:
The code points 18.4800 and 18.4C00 are reserved for adding weaker memory barrier
instructions. Those code points must operate as a Memory Barrier instruction (MB
18.4000) for implementations that precede their definition as weaker memory barrier
instructions. Software must use the 18.4000 code point for MB.

C.14 Unused Function Code Behavior
Unused function codes for all opcodes assigned (not reserved) in the Version 5 Alpha architecture specification (May 1992) produce UNPREDICTABLE but not UNDEFINED results; they
are not security holes.
Unused function codes for opcodes defined as reserved in the Version 5 Alpha architecture
specification produce an illegal instruction trap. Those opcodes are 01, 02, 03, 04, 05, 06, 07,
0A, 0C, 0D, 0E, 14, 19, 1B, 1C, 1D, 1E, and 1F. Unused function codes for those opcodes
reserved to PALcode produce an illegal instruction trap only if not used in the PALcode
environment.

Instruction Summary C–21

C.15 ASCII Character Set
Table C–19 shows the 7-bit ASCII character set and the corresponding hexadecimal value for
each character.

Table C–19: ASCII Character Set
Char

Hex
Code

Char

Hex
Code

Char

Hex
Code

Char

Hex
Code

NUL
SQH
STX
ETX
EOT
ENQ
ACK
BEL
BS
HT
LF
VT
FF
CR
SO
SI
DLE
DC1
DC2
DC3
DC4
NAK
SYN
ETB
CAN
EM
SUB
ESC
FS
GS
RS
US

0
1
2
3
4
5
6
7
8
9
A
B
C
D
E
F
10
11
12
13
14
15
16
17
18
19
1A
1B
1C
1D
1E
1F

SP
!
"
#
$
%
&
'
(
)
*
+
,
.
/
0
1
2
3
4
5
6
7
8
9
:
;
<
=
>
?

20
21
22
23
24
25
26
27
28
29
2A
2B
2C
2D
2E
2F
30
31
32
33
34
35
36
37
38
39
3A
3B
3C
3D
3E
3F

@
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
Q
R
S
T
U
V
W
X
Y
Z
[
\
]
^
_

40
41
42
43
44
45
46
47
48
49
4A
4B
4C
4D
4E
4F
50
51
52
53
54
55
56
57
58
59
5A
5B
5C
5D
5E
5F

‘
a
b
c
d
e
f
g
h
i
j
k
l
m
n
o
p
q
r
s
t
u
v
w
x
y
z
{
|
}
~
DEL

60
61
62
63
64
65
66
67
68
69
6A
6B
6C
6D
6E
6F
70
71
72
73
74
75
76
77
78
79
7A
7B
7C
7D
7E
7F

C–22 Alpha Architecture Handbook

Appendix D

Registered System and Processor Identifiers

This appendix contains a table of the processor type assignments, PALcode implementation
information, and the architecture mask (AMASK) and implementation value (IMPLVER)
assignments.

D.1 Processor Type Assignments
The following processor types are defined.

Table D–1: Processor Type Assignments
Major Type
1=

EV3

EV4 (21064)

Simulation

LCA Family:
LCA4s (21066)
LCA4s embedded (21068)
LCA45 (21066A, 21068A)

Minor Type

Pass 2 or 2.1

Pass 3 (also EV4s)

Reserved

Pass 1 or 1.1 (21066)

Pass 2 (21066)

Pass 1 or 1.1 (21068)

Pass 2 (21068)

Pass 1 (21066A)

Pass 1 (21068A)

D–1

Table D–1: Processor Type Assignments (Continued)
Major Type
5=

Minor Type

EV5 (21164)

EV45 (21064A)

EV56 (21164A)

EV6 (21264)

PCA56 (21164PC)

Reserved (Pass 1)

Pass 2, 2.2 (rev BA, CA)

Pass 2.3 (rev DA, EA)

Pass 3

Pass 3.2

Pass 4

Reserved

Pass 1

Pass 1.1

Pass 2

Reserved

Pass 1

Pass 2

Reserved

Pass 1

Pass 2, 2.1

Pass 2.2

Pass 2.3

Pass 3

Reserved

Pass 1

For OpenVMS Alpha and DIGITAL UNIX, the processor types are stored in the Per-CPU Slot
Table (SLOT[176]), pointed to by HWRPB[160].

D.2 PALcode Variation Assignments
The PALcode variation assignments are as follows:

Table D–2: PALcode Variation Assignments
Token

PALcode Type

Summary Table

Console

N/A

OpenVMS Alpha

Console Interface (III), Chapter 3, in the
Alpha Architecture Reference Manual.

D–2 Alpha Architecture Handbook

Table D–2: PALcode Variation Assignments
Token

PALcode Type

Summary Table

DIGITAL UNIX

Console Interface (III), Chapter 3 in the
Alpha Architecture Reference Manual

3–127

Reserved to Compaq

128–255

Reserved to non-Compaq

D.3 Architecture Mask and Implementation Values
The following bits are defined for the AMASK instruction.

Table D–3: AMASK Bit Assignments
Bit

Meaning

Support for the byte/word extension (BWX)
The instructions that comprise the BWX extension are LDBU, LDWU, SEXTB,
SEXTW, STB, and STW.

Support for the square-root and floating-point convert extension (FIX)
The instructions that comprise the FIX extension are FTOIS, FTOIT, ITOFF,
ITOFS, ITOFT, SQRTF, SQRTG, SQRTS, and SQRTT.

Support for the count extension (CIX)
The instructions that comprise the CIX extension are CTLZ, CTPOP, and CTTZ.

Support for the multimedia extension (MVI)
The instructions that comprise the MVI extension are MAXSB8, MAXSW4,
MAXUB8, MAXUW4, MINSB8, MINSW4, MINUB8, MINUW4, PERR, PKLB,
PKWB, UNPKBL, and UNPKBW.

Support for precise arithmetic trap reporting in hardware. The trap PC is the same
as the instruction PC after the trapping instruction is executed.

The following values are defined for the IMPLVER instruction.

Table D–4: IMPLVER Value Assignments
Value

Meaning

21064 (EV4)
21064A (EV45)
21066A/21068A (LCA45)

21164 (EV5)
21164A (EV56)
21164PC (PCA56)

21264 (EV6)

Registered System and Processor Identifiers D–3

Appendix E

Waivers and Implementation-Dependent
Functionality

This appendix describes waivers to the Alpha architecture and functionality that is specific to
particular hardware implementations.

E.1 Waivers
The following waivers have been passed for the Alpha architecture.

E.1.1 DECchip 21064, DECchip 21066, and DECchip 21068 IEEE Divide
Instruction Violation
The DECchip 21064, DECchip 21066, and DECchip 21068 CPUs violate the architected handling of IEEE divide instructions DIVS and DIVT with respect to reporting Inexact Result
exceptions.

Note:
The DECchip 21064A, DECchip 21066A, and DECchip 21068A CPUs are compliant and
require no waiver. The DECchip 21164 is also compliant.
As specified by the architecture, floating-point exceptions generated by the CPU are recorded
in two places for all IEEE floating-point instructions:
1. If an exception is detected and the corresponding trap is enabled (such as ADD/U for
underflow), the CPU initiates a trap and records the exception in the exception summary register (EXC_SUM).
2. The exceptions are also recorded as flags that can be tested in the floating-point control
register (FPCR). The FPCR can only be accessed with MTPR/MFPR instructions and
an explicit MT_FPCR is required to clear the FPCR. The FPCR is updated irrespective
of whether the trap is enabled or not.

E–1

The DECchip 21064, DECchip 21066, and DECchip 21068 implementations differ from the
above specification in handling the Inexact condition for the IEEE DIVS and DIVT instructions in two ways:
1. The DIVS and DIVT instructions with the /Inexact modifier trap unconditionally and
report the INE exception in the EXC_SUM register (except for NaN, infinity, and
denormal inputs that result in INVs). This allows for a software calculation to determine the correct INE status.
2. The FPCR <INE> bit is never set by DIVS or DIVT. This is because the DECchip
21064, DECchip 21066, and DECchip 21068 do not include hardware to determine that
particular exactness.

E.1.2 DECchip 21064, DECchip 21066, and DECchip 21068 Write Buffer
Violation
The DECchip 21064, DECchip 21066, and DECchip 21068 CPUs can be made to violate the
architecture by, under one contrived case, indefinitely delaying a buffered off-chip write.

Note:
The DECchip 21064A, DECchip 21066A, and DECchip 21068A CPUs are compliant and
require no waiver. The DECchip 21164 is also compliant.
The CPUs in violation can send a buffered write off-chip when one of the following conditions is met:
1. The write buffer contains at least two valid entries.
2. The write buffer contains one valid entry and 256 cycles have elapsed since the execution of the last write.
3. The write buffer contains an MB or STx_C instruction.
4. A load miss hits an entry in the write buffer.
The write can be delayed indefinitely under condition 2 above, when there is an indefinite
stream of writes to addresses within the same aligned 32-byte write buffer block.

E.1.3 DECchip 21264 LDx_L/STx_C with WH64 Violation
The DECchip 21264 violates the architected relationship between the LDx_L and STx_C
instructions when an intervening WH64 instruction is executed.
As specified in Section 4.2.4:
If any other memory access (ECB, LDx, LDQ_U, STx, STQ_U, WH64) is executed on the
given processor between the LDx_L and the STx_C, the sequence above may always fail
on some implementations; hence, no useful program should do this.

E–2 Alpha Architecture Handbook

The DECchip 21264 varies from that description, with regard to the WH64 instruction, as
follows:
If any other memory access (ECB, LDx, LDQ_U, STx, STQ_U) is executed on the given
processor between the LDx_L and the STx_C, the sequence above may always fail on
some implementations; hence, no useful program should do this.
If a WH64 memory access is executed on any given 21264 processor between the LDx_L
and STx_C, and:
–

The WH64 access is to the same aligned 64-byte block that STx_C is accessing,
and

–

No CALL_PAL REI, rei, or rfe instruction has been executed since the most-recent
LDx_L (ensuring that the sequence cannot occur as the result of unfortunate coincidences with interrupts)

then, the load-locked/store-conditional sequence may sometimes fail when it would
otherwise succeed and sometimes succeed when it otherwise would fail; hence no useful
program should do this.

E.2 Implementation-Specific Functionality
The following functionality, although a documentated part of the Alpha architecture, is implemented in a manner that is specific to the particular hardware implementation.

E.2.1 DECchip 21064/21066/21068 Performance Monitoring
Note:
All functions, arguments, and descriptions in this section apply to the DECchip
21064/21064A, 21066/21066A, and 21068/21068A.
PALcode instructions control the DECchip 21064/21066/21068 on-chip performance counters.
For OpenVMS Alpha, the instruction is MTPR_PERFMON; for DIGITAL UNIX and Windows NT Alpha, the instruction is wrperfmon.
The instruction arguments and results are described in the following sections. The scratch register usage is operating system specific.
Two on-chip counters count events. The bit width of the counters (8, 12, or 16 bits) can be
selected and the event that they count can be switched among a number of available events.
One possible event is an "external" event. For example, the processor board can supply an
event that causes the counter to increment. In this manner, off-chip events can be counted.
The two counters can be switched independently. There is no hardware support for reading,
writing, or resetting the counters. The only way to monitor the counters is to enable them to
cause an interrupt on overflow.

Waivers and Implementation-Dependent Functionality E–3

The performance monitor functions, described in Section E.2.1.2, can provide the following,
depending on implementation:

•

Enable the performance counters to interrupt and trap into the performance monitoring
vector in the operating system.

•

Disable the performance counter from interrupting. This does not necessarily mean that
the counters will stop counting.

•

Select which events will be monitored and set the width of the two counters.

•

In the case of OpenVMS Alpha and DIGITAL UNIX, implementations can choose to
monitor selected processes. If that option is selected, the PME bit in the PCB controls
the enabling of the counters. Since the counters cannot be read/written/reset, if more
than one process is being monitored, the rounding error may become significant.

E.2.1.1 DECchip 21064/21066/21068 Performance Monitor Interrupt Mechanism
The performance monitoring interrupt mechanism varies according to the particular operating
system.
For the OpenVMS Alpha Operating System

When a counter overflows and interrupt enabling conditions are correct, the counter causes an
interrupt to PALcode. The PALcode builds an appropriate stack frame. The PALcode then dispatches in the form of an exception (not in the form of an interrupt) to the operating system by
vectoring to the SCB performance monitor entry point through SC BB+650
(HWSCB$Q_PERF_MONITOR), at IPL 29, in kernel mode.
Two interrupts are generated if both counters overflow. For each interrupt, the status of each
counter overflow is indicated by register R4:
R4 = 0 if performance counter 0 caused the interrupt
R4 = 1 if performance counter 1 caused the interrupt
When the interrupt is taken, the PC is saved on the stack frame as the old PC.
For the DIGITAL UNIX Operating System

When a counter overflows and interrupt enabling conditions are correct, the counter causes an
interrupt to PALcode. The PALcode builds an appropriate stack frame and dispatches to the
operating system by vectoring to the interrupt entry point entINT, at IPL 6, in kernel mode.
Two interrupts are generated if both counters overflow. For each interrupt, registers a0..a2 are
as follows:
a0 = osfint$c_perf (4)
a1 = scb$v_perfmon (650)
a2 = 0 if performance counter 0 caused the interrupt
a2 = 1 if performance counter 1 caused the interrupt
When the interrupt is taken, the PC is saved on the stack frame as the old PC.
For the Windows NT Alpha Operating System

When a counter overflows and interrupt enabling conditions are correct, the counter causes an
interrupt to PALcode. The PALcode builds a frame on the kernel stack and dispatches to the
kernel at the interrupt entry point.
E–4 Alpha Architecture Handbook

E.2.1.2 Functions and Arguments for the DECchip 21064/21066/21068
The functions execute on a single (the current running) processor only and are described in
Table E–1.

•
•
•

The OpenVMS Alpha MTPR_PERFMON instruction is called with a function code in
R16, a function-specific argument in R17, and status is returned in R0.
The DIGITAL UNIX wrperfmon instruction is called with a function code in a0, a function specific argument in a1, and status is returned in v0.
The Windows NT Alpha wrperfmon instruction is called with input parameters a0
through a3, as shown in Table E–1.

Table E–1: DECchip 21064/21066/21068 Performance Monitoring Functions

Function

Comments

Enable performance monitoring
DIGITAL UNIX
Input:
Output:

a0 = 1
a1 = 0
v0 = 1
v0 = 0

Function code
Argument
Success
Failure (not generated)

OpenVMS Alpha
Input:
R16 = 1
R17 = 0
Output: R0 = 1
R0 = 0
Windows NT Alpha
Input:
a0 = 0
a0 = 1
a1 = 1

Function code
Argument
Success
Failure (not generated)
Select counter 0
Select counter 1
Enable selected counter

Disable performance monitoring
DIGITAL UNIX
Input:

Enable takes effect at the next IPL change

Disable takes effect at the next IPL change

a0 = 0
a1 = 0
v0 = 1
v0 = 0

Function code
Argument
Success
Failure (not generated)

OpenVMS Alpha
Input:
R16 = 0
R17 = 0
Output: R0 = 1
R0 = 0

Function code
Argument
Success
Failure (not generated)

Output:

Waivers and Implementation-Dependent Functionality E–5

Table E–1: DECchip 21064/21066/21068 Performance Monitoring Functions
(Continued)
Function

Windows NT Alpha
Input:
a0 = 0
a0 = 1
a1 = 0

Comments
Select counter 0
Select counter 1
Disable selected counter

Select desired events (mux_ctl)
DIGITAL UNIX
Input:

Output:

a0 = 2
a1 = mux_ctl

v0 = 1
v0 = 0

OpenVMS Alpha
Input:
R16 = 2
R17 = mux_ctl

Output:

R0 = 1
R0 = 0
Windows NT Alpha
Input:
a2 = PCMUX0
a2 = PCMUX1
a3 = PC0
a3 = PC1

Function code
mux_ctl is the exact contents of those fields
from the ICCSR register, in write format,
described in Table E–2.
Success
Failure (not generated)
Function code
mux_ctl is the exact contents of those fields
from the ICCSR register, in write format,
described in Table E–2.
Success
Failure (not generated)
For ICCSR<PCMUX0> field when a0 = 0
For ICCSR<PCMUX1> field when a0 = 1
For ICCSR<PC0> field when a0 = 0
For ICCSR<PC1> field when a0 = 1

Select performance monitoring options
DIGITAL UNIX
Input:

Output:

a0 = 3
a1 = opt

v0 = 1
v0 = 0

E–6 Alpha Architecture Handbook

Function code
Function argument opt is:
<0> = log all processes if set
<1> = log only selected if set
Success
Failure (not generated)

Table E–1: DECchip 21064/21066/21068 Performance Monitoring Functions
(Continued)
Function

Comments

OpenVMS Alpha
Input:
R16 = 3
R17 = opt

Output:

Function code
Function argument opt is:
<0> = log all processes if set
<1> = log only selected if set
Success
Failure (not generated)

R0 = 1
R0 = 0

Table E–2: DECchip 21064/21066/21068 MUX Control Fields in ICCSR Register

Bits

Option

Description

34:32

PCMUX1

Event selection, counter 1:

Value

Description

0
1
2
3
4
5
6
7

Total D-cache misses
Total I-cache misses
Cycles of dual issue
Branch mispredicts (conditional, JSR, HW_REI)
FP operate instructions (not BR, LOAD, STORE)
Integer operates (including LDA, LDAH into R0–R30)
Total store instructions
External events supplied by pin

Waivers and Implementation-Dependent Functionality E–7

Table E–2: DECchip 21064/21066/21068 MUX Control Fields in ICCSR Register
(Continued)
Bits

Option

Description

11:8

PCMUX0

Event selection, counter 0:

PC0

PC1

Value

Description

0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15

Total issues divided by 2
Unused
Nothing issued, no valid I-stream data
Unused
All load instructions
Unused
Nothing issued, resource conflict
Unused
All branches (conditional, unconditional, JSR, HW_REI)
Unused
Total cycles
Cycles while in PALcode environment
Total nonissues divided by 2
Unused
External event supplied by pin.
Unused

Frequency setting, counter 0:
Value

Description

2**16 (65536) events per interrupt

2**12 (4096) events per interrupt

Frequency setting, counter 1:
Value

Description

0
1

2**12 (4096) events per interrupt
2**8 (256) events per interrupt

E–8 Alpha Architecture Handbook

E.2.2 DECchip 21164/21164PC Performance Monitoring
Unless otherwise stated, the term "21164" in this section means implementations of the 21164
at all frequencies.
PALcode instructions control the DECchip 21164/21164PC on-chip performance counters. For
OpenVMS Alpha, the instruction is MTPR_PERFMON; for DIGITAL UNIX and Windows
NT Alpha, the instruction is wrperfmon.
The instruction arguments and results are described in the following sections. The scratch register usage is operating system specific.
Three on-chip counters count events. Counters 0 and 1 are 16-bit counters; counter 2 is a 14-bit
counter. Each counter can be individually programmed. Counters can be read and written and
are not required to interrupt. The counters can be collectively restricted according to the processor mode.
Processes can be selectively monitored with the PME bit.

E.2.2.1 Performance Monitor Interrupt Mechanism
The performance monitoring interrupt mechanism varies according to the particular operating
system.
For the OpenVMS Alpha Operating System

When a counter overflows and interrupt enabling conditions are correct, the counter causes an
interrupt to PALcode. The PALcode builds an appropriate stack frame. The PALcode then dispatches in the form of an exception (not in the form of an interrupt) to the operating system by
vectoring to the SCB performance monitor entry point through SC BB+650
(HWSCB$Q_PERF_MONITOR), at IPL 29, in kernel mode.
An interrupt is generated for each counter overflow. For each interrupt, the status of each
counter overflow is indicated by register R4:
R4 = 0 if performance counter 0 caused the interrupt
R4 = 1 if performance counter 1 caused the interrupt
R4 = 2 if performance counter 2 caused the interrupt
When the interrupt is taken, the PC is saved on the stack frame as the old PC.
For the DIGITAL UNIX Operating System

When a counter overflows and interrupt enabling conditions are correct, the counter causes an
interrupt to PALcode. The PALcode builds an appropriate stack frame and dispatches to the
operating system by vectoring to the interrupt entry point entINT, at IPL 6, in kernel mode.
An interrupt is generated for each counter overflow. For each interrupt, registers a0..a2 are as
follows:
a0 = osfint$c_perf (4)
a1 = scb$v_perfmon (650)
a2 = 0 if performance counter 0 caused the interrupt
a2 = 1 if performance counter 1 caused the interrupt

Waivers and Implementation-Dependent Functionality E–9

For the Windows NT Alpha Operating System

E.2.2.2 Windows NT Alpha Functions and Argument
The functions for Windows NT Alpha execute on only a single (the current running) processor.
The wrperfmon instruction is called with the following input registers:
Input
Register

Contents
(Bits)

63–0

The register in Table E–3, which contains the value to be written
to the hardware PMCTR register.

When a1 = 0, write a0 to the hardware PMCTR register.

Meaning

When a1 = 1, read the hardware PMCTR register. The returned
PMCTR register is written to register v0.
a2

2–0

Has meaning when PCSEL1 in Table E–3 has the value 0xF. Contents are determined by processor type:
Processor

Contents

Reference

21164
21164PC

CBOX1
PM0_MUX

Table E–15
Table E–17

Has meaning when PCSEL2 in Table E–3 has the value 0xF. Contents are determined by processor type:
Processor

Contents

Reference

21164
21164PC

CBOX2
PM1_MUX

Table E–16
Table E–18

E–10 Alpha Architecture Handbook

Table E–3: Bit Summary of PMCTR Register for Windows NT Alpha

Bits

Name

Meaning

63–48

CTR0

Counter 0 value

47–32

CTR1

Counter 1 value

PCSEL0

Counter 0 selection:

Value

Meaning

0
1

Cycles
Issues

Must be set to one1

29–16

CTR2

Counter 2 value

15–14

CTL0

Counter 0 control:

13–12

11–10

CTL1

CTL2

Value

Meaning

0
1
2
3

Counter disable, interrupt disable
Counter enable, interrupt disable
Counter enable, interrupt at count 65536
Counter enable, interrupt at count 256

Counter 1 control:
Value

Meaning

0
1
2
3

Counter disable, interrupt disable
Counter enable, interrupt disable
Counter enable, interrupt at count 65536
Counter enable, interrupt at count 256

Counter 2 control:
Value

Meaning

0
1
2
3

Counter disable, interrupt disable
Counter enable, interrupt disable
Counter enable, interrupt at count 16384
Counter enable, interrupt at count 256

Waivers and Implementation-Dependent Functionality E–11

Table E–3: Bit Summary of PMCTR Register for Windows NT Alpha
(Continued)
Bits

Name

Meaning

9–8

MODE_SELECT 1

Select modes in which to count:
Value

Meaning

0
1
2
3

Count all modes
Count PALmode only
Count all modes except PALmode
Count only user mode

7–4

PCSEL1

Counter 1 selection. See Table E–13

3–0

PCSEL2

Counter 2 selection. See Table E–14

Windows NT Alpha uses bits 30 and 9–8 differently than as documented in the 21164 Hardware Reference Manual; it uses the processor executive mode to run user (nonprivileged)
code. Therefore, bit 30 is always set to one and bits 9–8 are used to select the mode.

E.2.2.3 OpenVMS Alpha and DIGITAL UNIX Functions and Arguments
The functions execute only on a single (the current running) processor and are described in
Table E–4.
The OpenVMS Alpha MTPR_PERFMON instruction is called with a function code in R16, a
function-specific argument in R17, and status is returned in R0.
The DIGITAL UNIX wrperfmon instruction is called with a function code in a0, a function
specific argument in a1, and status is returned in v0.

Table E–4: OpenVMS Alpha and DIGITAL UNIX Performance Monitoring
Functions
Function

Comments

Enable performance monitoring; do not reset counters
DIGITAL UNIX
Input:
Output:
OpenVMS Alpha
Input:
Output:

a0 = 1
a1 = arg
v0 = 1
v0 = 0

Function code value
Argument from Table E–5
Success
Failure (not generated)

R16 = 1
R17 = arg
R0 = 1
R0 = 0

Function code value
Argument from Table E–5
Success
Failure (not generated)

E–12 Alpha Architecture Handbook

Table E–4: OpenVMS Alpha and DIGITAL UNIX Performance Monitoring
Functions (Continued)
Function

Comments

Enable performance monitoring; start the counters from zero
DIGITAL UNIX
Input:
Output:
OpenVMS Alpha
Input:
Output:

a0 = 7
a1 = arg
v0 = 1
v0 = 0

Function code value
Argument from Table E–5
Success
Failure (not generated)

R16 = 7
R17 = arg
R0 = 1
R0 = 0

Function code value
Argument from Table E–5
Success
Failure (not generated)

Disable performance monitoring; do not reset counters
DIGITAL UNIX
Input:
Output:
OpenVMS Alpha
Input:
Output:

a0 = 0
a1 = arg
v0 = 1
v0 = 0

Function code value
Argument from Table E–6
Success
Failure (not generated)

R16 = 0
R17 = arg
R0 = 1
R0 = 0

Function code value
Argument from Table E–6
Success
Failure (not generated)

Select desired events (MUX_SELECT)
DIGITAL UNIX
Input:
Output:
OpenVMS Alpha
Input:
Output:

a0 = 2
a1 = arg
v0 = 1
v0 = 0

Function code value
Argument from Table E–7 or E–8
Success
Failure (not generated)

R16 = 2
R17 = arg
R0 = 1
R0 = 0

Function code value
Argument from Table E–7 or E–8
Success
Failure (not generated)

Waivers and Implementation-Dependent Functionality E–13

Table E–4: OpenVMS Alpha and DIGITAL UNIX Performance Monitoring
Functions (Continued)
Function

Comments

Select Processor Mode options
DIGITAL UNIX
Input:
Output:
OpenVMS Alpha
Input:
Output:

a0 = 3
a1 = arg
v0 = 1
v0 = 0

Function code value
Argument from Table E–9
Success
Failure (not generated)

R16 = 3
R17 = arg
R0 = 1
R0 = 0

Function code value
Argument from Table E–9
Success
Failure (not generated)

Select interrupt frequencies
DIGITAL UNIX
Input:
Output:
OpenVMS Alpha
Input:
Output:

a0 = 4
a1 = arg
v0 = 1
v0 = 0

Function code value
Argument from Table E–10
Success
Failure (not generated)

R16 = 4
R17 = arg
R0 = 1
R0 = 0

Function code value
Argument from Table E–10
Success
Failure (not generated)

a0 = 5
a1 = arg
v0 = val

Function code value
Argument from Table E–11
Return value from Table E–11

R16 = 5
R17 = arg
R0 = val

Function code value
Argument from Table E–11
Return value from Table E–11

Read the counters
DIGITAL UNIX
Input:
Output:
OpenVMS Alpha
Input:
Output:

E–14 Alpha Architecture Handbook

Table E–4: OpenVMS Alpha and DIGITAL UNIX Performance Monitoring
Functions (Continued)
Function

Comments

Write the counters
DIGITAL UNIX
Input:
Output:
OpenVMS Alpha
Input:
Output:

a0 = 6
a1 = arg
v0 = 1
v0 = 0

Function code value
Argument from Table E–12
Success
Failure (not generated)

R16 = 6
R17 = arg
R0 = 1
R0 = 0

Function code value
Argument from Table E–12
Success
Failure (not generated)

Table E–5: 21164/21164PC Enable Counters for OpenVMS Alpha and DIGITAL
UNIX
Bits

Meaning When Set

Operate on counter 2

Operate on counter 1

Operate on counter 0

Table E–6: 21164/21164PC Disable Counters for OpenVMS Alpha and DIGITAL
UNIX
Bits

Meaning When Set

Operate on counter 2

Operate on counter 1

Operate on counter 0

Waivers and Implementation-Dependent Functionality E–15

Table E–7: 21164 Select Desired Events for OpenVMS Alpha and DIGITAL
UNIX
Bits

Name

63:32
31

Meaning
MBZ

PCSEL0

30:25

Counter 0 selection:
Value

Meaning

0
1

Cycles
Issues

MBZ

24:22

CBOX2

CBOX2 event selection (only has meaning when event selection field
PCSEL2 is value <15>; otherwise MBZ). CBOX2 described in Table E–
16.

21:19

CBOX1

CBOX1 event selection (only has meaning when event selection field
PCSEL1 is value <15>; otherwise MBZ). CBOX1 described in Table E–
15.

18:8

MBZ

7:4

PCSEL1

Counter 1 event selection. PCSEL1 described in Table E–13.

3:0

PCSEL2

Counter 2 event selection. PCSEL2 described in Table E–14.

Table E–8: 21164PC Select Desired Events for OpenVMS Alpha and DIGITAL
UNIX
Bits

Name

63:32
31

Meaning
MBZ

PCSEL0

30:14

Counter 0 selection:
Value

Meaning

0
1

Cycles
Issues

MBZ

13:11

PM1_MUX

PM1_MUX event selection (only has meaning when event selection field PCSEL2 is value <15>; otherwise MBZ). PM1_MUX is
described in Table E–18.

10:8

PM0_MUX

PM0_MUX event selection (only has meaning when event selection field PCSEL1 is value <15>; otherwise MBZ). PM0_MUX is
described in Table E–17.

E–16 Alpha Architecture Handbook

Table E–8: 21164PC Select Desired Events for OpenVMS Alpha and DIGITAL
UNIX (Continued)
Bits

Name

Meaning

7:4

PCSEL1

Counter 1 event selection. PCSEL1 described in Table E–13.

3:0

PCSEL2

Counter 2 event selection. PCSEL2 described in Table E–14.

Table E–9: 21164/21164PC Select Special Options for OpenVMS Alpha and
DIGITAL UNIX
Bits

Meaning

63:31

MBZ

Stop count in user mode

29:10

MBZ

Stop count in PALmode

Stop count in kernel mode

7:1

MBZ

Monitor selected processes (when clear monitor all processes)

Setting any of the "NOT" bits causes the counters to not count when the processor is running in
the specified mode. Under OpenVMS Alpha, "NOT_KERNEL" also stops the count in executive and supervisor mode, except as noted below:

NOT_BITS

Counters Operate Under These Modes When Bits Set:

K U

K E S U P

K E S U

K E S

U P

P
E S

(here "NOT_KERNEL" stops kernel counter only)

Note:
DIGITAL UNIX counts user mode by using the executive counter; that is, the count for
executive mode is returned as the user mode count.
Waivers and Implementation-Dependent Functionality E–17

Table E–10: 21164/21164PC Select Desired Frequencies for OpenVMS Alpha and
DIGITAL UNIX
Table E–10 contains the selection definitions for each of the three counters. All frequency
fields are two-bit fields with the following values defined:

Bits

Meaning When Set

63:10

MBZ

9:8

Counter 0 frequency:

7:6

5:4

3:0

Value

Meaning

0
1
2
3

Do not interrupt
Unused
Low frequency (2**16 (65536) events per interrupt)
High frequency (2**8 (256) events per interrupt)

Counter 1 frequency:
Value

Meaning

0
1
2
3

Do not interrupt
Unused
Low frequency (2**16 (65536) events per interrupt)
High frequency (2**8 (256) events per interrupt)

Counter 2 frequency:
Value

Meaning

0
1
2
3

Do not interrupt
Unused
Low frequency (2**14 (16384) events per interrupt)
High frequency (2**8 (256) events per interrupt)

MBZ

E–18 Alpha Architecture Handbook

Table E–11: 21164/21164PC Read Counters for OpenVMS Alpha and DIGITAL
UNIX
Bits

Meaning When Returned

63:48

Counter 0 returned value

47:32

Counter 1 returned value

31:30

MBZ

29:16

Counter 2 returned value

15:1

MBZ

Set means success; clear means failure

Table E–12: 21164/21164PC Write Counters for OpenVMS Alpha and DIGITAL
UNIX
Bits

Meaning

63:48

Counter 0 written value

47:32

Counter 1 written value

31:30

MBZ

29:16

Counter 2 written value

15:0

MBZ

Table E–13: 21164/21164PC Counter 1 (PCSEL1) Event Selection
The following values choose the counter 1 (PCSEL1) event selection:
Value

Meaning

Nothing issued, pipeline frozen

Some but not all issuable instructions issued

Nothing issued, pipeline dry

Replay traps (ldu, wb/maf, litmus test)

Single issue cycles

Dual issue cycles

Triple issue cycles

Quad issue cycles

Flow change (all branches, jsr-ret, hw_rei), where:
If PCSEL2 has value 3, flow change is a conditional branch
If PCSEL2 has value 2, flow change is a JSR-RET

Waivers and Implementation-Dependent Functionality E–19

Table E–13: 21164/21164PC Counter 1 (PCSEL1) Event Selection (Continued)
The following values choose the counter 1 (PCSEL1) event selection:
Value

Meaning

Integer operate instructions

Floating point operate instructions

Load instructions

Store instructions

Instruction cache access

Data cache access

For the 21164, use CBOX1 event selection in Table E–15.
For the 21164PC, use PM0_MUX event selection in Table E–17.

Table E–14: 21164/21164PC Counter 2 (PCSEL2) Event Selection
The following values choose the counter 2 (PCSEL2) event selection:
Value

Meaning

Long stalls (> 15 cycles)

Unused value

PC mispredicts

Branch mispredicts

I-cache misses

ITB misses

D-cache misses

DTB misses

Loads merged in MAF

LDU replays

WB/MAF full replays

Event from external pin

Cycles

Memory barrier instructions

LDx/L instructions

For the 21164, use CBOX2 event selection in Table E–16.
For the 21164PC, use PM1_MUX event selection in Table E–18.

E–20 Alpha Architecture Handbook

Table E–15: 21164 CBOX1 Event Selection
The following values choose the CBOX1 event selection.

Value

Meaning

S-cache access

S-cache read

S-cache write

S-cache victim

Unused value

B-cache hit

B-cache victim

System request

Table E–16: 21164 CBOX2 Event Selection
The following values choose the CBOX2 event selection.
Value

Meaning

S-cache misses

S-cache read misses

S-cache write misses

S-cache shared writes

S-cache writes

B-cache misses

System invalidates

System read requests

Waivers and Implementation-Dependent Functionality E–21

Table E–17: 21164PC PM0_MUX Event Selection
The following values choose the PM0_MUX event selection and perform the chosen operation
in Counter 0.
Value

Meaning

B-cache read operations

B-cache D read hits

B-cache D read fills

B-cache write operations

Undefined

B-cache clean write hits

B-cache victims

Read miss 2 launched

Table E–18: 21164PC PM1_MUX Event Selection
The following values choose the PM1_MUX event selection and perform the chosen operation
in Counter 1.
Value

Meaning

B-cache D read operations

B-cache read hits

B-cache read fills

B-cache write hits

B-cache write fills

System read/flush B-cache hits

System read/flush B-cache misses

Read miss 3 launched

E–22 Alpha Architecture Handbook

E.2.3 21264 Performance Monitoring
PALcode instructions control the 21264 on-chip performance counters. For OpenVMS Alpha,
the instruction is MTPR_PERFMON; for DIGITAL UNIX and Windows NT Alpha, the
instruction is wrperfmon.
The instruction arguments and results are described in the following sections. The scratch register usage is operating system specific.
Two 20-bit on chip counters count events. Counters can be individually programmed, read, and
written.
Processes can be selectively monitored with the PME bit.
Profile monitoring for the 21264 is called aggregate mode profile monitoring because it provides an aggregate count.

E.2.3.1 Performance Monitor Interrupt Mechanism
The performance monitoring interrupt mechanism varies according to the particular operating
system.
For the OpenVMS Alpha Operating System

When a counter overflows and interrupt enabling conditions are correct, the counter causes an
interrupt to PALcode. The PALcode builds an appropriate stack frame. The PALcode then dispatches in the form of an exception (not in the form of an interrupt) to the operating system by
vectoring to the SCB performance monitor entry point through SC BB+650
(HWSCB$Q_PERF_MONITOR), at IPL 29, in kernel mode.
An interrupt is generated for each counter overflow. For each interrupt, the status of each
counter overflow is indicated by register R4:
R4 = 0 if performance counter 0 caused the interrupt
R4 = 1 if performance counter 1 caused the interrupt
When the interrupt is taken, the PC is saved on the stack frame as the old PC.
For the DIGITAL UNIX Operating System

When a counter overflows and interrupt enabling conditions are correct, the counter causes an
interrupt to PALcode. The PALcode builds an appropriate stack frame and dispatches to the
operating system by vectoring to the interrupt entry point entINT, at IPL 6, in kernel mode.
An interrupt is generated for each counter overflow. For each interrupt, registers a0..a2 are as
follows:
a0 = osfint$c_perf (4)
a1 = scb$v_perfmon (650)
a2 = 0 if performance counter 0 caused the interrupt
a2 = 1 if performance counter 1 caused the interrupt

Waivers and Implementation-Dependent Functionality E–23

For the Windows NT Alpha Operating System

E.2.3.2 Windows NT Alpha Functions and Argument
The functions for Windows NT Alpha execute on only a single (the current running) processor.
The wrperfmon instruction is called with the following input registers:
Input
Register

Contents
(Bits)

63–0

The register in Table E–19, which contains the value to be written
to the hardware PCTR_CTL register.

When a1 = 0, write a0 to the hardware PCTR_CTL register.

Meaning

When a1 = 1, read the hardware PCTR_CTL register. The
returned PCTR_CTL register is written to register v0.

Table E–19: Bit Summary of PCTR_CTL Register for Windows NT Alpha
Bits

Name

63–48

SEXT[PCTR0_CTL[47]

47–28

PCTR0

27–26

Reserved

25–6

PCTR1

Reserved

SL0

Meaning

Counter 0 value.
Enabled by setting I_CTL[PCT0_EN] and either
I_CTL[SPCE] or PCTX[PPCE]. On overflow, an
interrupt is triggered at ISUM[PC0], if enabled by
IER_CM[PCEN0].
Mode is determined by SL0 and operation is
described in SL1.

Counter 1 value.
Enabled by setting I_CTL[PCT1_EN] and either
I_CTL[SPCE] or PCTX[PPCE]. On overflow, an
interrupt is triggered at ISUM[PC1], if enabled by
IER_CM[PCEN1].
Operation is described in SL1.

PCTR0 input selecter:
Value Meaning
0
1

E–24 Alpha Architecture Handbook

Aggregate counting mode
Reserved

Table E–19: Bit Summary of PCTR_CTL Register for Windows NT Alpha
Bits

Name

Meaning

3–2

SL1

PCTR1 input selector. If SL0 value is 0:
Bit value

Meaning

0000
0001

Counter 1 counts cycles.
Counter 1 counts retired conditional
branches.
Counter 1 counts retired branch mispredicts.
Counter 1 counts retired DTB single
misses * 2.
Counter 1 counts retired DTB double
double misses.
Counter 1 counts retired ITB misses.
Counter 1 counts retired unaligned traps.
Counter 1 counts replay traps.

0010
0011
0100
0101
0110
0111
1–0

Reserved

E.2.3.3 OpenVMS Alpha and DIGITAL UNIX Functions and Arguments
The functions execute only on a single (the current running) processor and are described in
Table E–20.
The OpenVMS Alpha MTPR_PERFMON instruction is called with a function code in R16, a
function-specific argument in R17, and any output is returned in R0.
The DIGITAL UNIX wrperfmon instruction is called with a function code in a0, a functionspecific argument in a1, and any output is returned in v0.

Table E–20: OpenVMS Alpha and DIGITAL UNIX Performance Monitoring
Functions
Function

Comments

Enable performance monitoring
DIGITAL UNIX
Input:
OpenVMS Alpha
Input:

a0 = 1
a1 = arg

Function code value
Argument from Table E–21

R16 = 1
R17 = arg

Function code value
Argument from Table E–21

Waivers and Implementation-Dependent Functionality E–25

Table E–20: OpenVMS Alpha and DIGITAL UNIX Performance Monitoring
Functions
Function

Comments

Disable performance monitoring
DIGITAL UNIX
Input:
OpenVMS Alpha
Input:

a0 = 0
a1 = arg

Function code value
Argument from Table E–22

R16 = 0
R17 = arg

Function code value
Argument from Table E–22

Select desired events (MUX_SELECT)
DIGITAL UNIX
Input:
OpenVMS Alpha
Input:

a0 = 2
a1 = arg

Function code value
Argument from Table E–23

R16 = 2
R17 = arg

Function code value
Argument from Table E–23

a0 = 3
a1[0] = 1
a1[0] = 0

Function code value
Log all processes
Log only selected processes

R16 = 3
R17[0] = 1
R17[0] = 0

Function code value
Log all processes
Log only selected processes

Select logging options
DIGITAL UNIX
Input:

OpenVMS Alpha
Input:

Read the counters
DIGITAL UNIX
Input:
Output:
OpenVMS Alpha
Input:
Output:

E–26 Alpha Architecture Handbook

a0 = 5
Function code value
v0 = contents of the counters; see Table E–24
R16 = 5
Function code value
R0 = contents of the counters; see Table E–24

Table E–20: OpenVMS Alpha and DIGITAL UNIX Performance Monitoring
Functions
Function

Comments

Write the counters
DIGITAL UNIX
Input:
OpenVMS Alpha
Input:

a0 = 6
a1 = arg

Function code value
Argument from Table E–25

R16 = 6
R17 = arg

Function code value
Argument from Table E–25

Enable and write selected counters
DIGITAL UNIX
Input:
OpenVMS Alpha
Input:

a0 = 7
a1 = arg

Function code value
Argument from Table E–26

R16 = 7
R17 = arg

Function code value
Argument from Table E–26

Table E–21: 21264 Enable Counters for OpenVMS Alpha and DIGITAL UNIX
R17/a1 Bits

Meaning When Set

Set I_CTL[PCT1_EN], which enables counter 1

Set I_CTL[PCT0_EN], which enables counter 0

Table E–22: 21264 Disable Counters for OpenVMS Alpha and DIGITAL UNIX
R17/a1 Bits

Meaning When Set

Clear I_CTL[PCT1_EN], which disables counter 1

Clear I_CTL[PCT0_EN], which disables counter 0

Waivers and Implementation-Dependent Functionality E–27

Table E–23: 21264 Select Desired Events for OpenVMS Alpha and DIGITAL
UNIX
R17/a1 Bits

Meaning

Bit value Meaning
1
0

3–2

Counter 0 counts retired instructions.
Counter 0 counts cycles.

Bit value Meaning
0000
0001
0010
0011
0100
0101
0110
0111

Counter 1 counts cycles.
Counter 1 counts retired conditional branches.
Counter 1 counts retired branch mispredicts.
Counter 1 counts retired DTB single misses * 2.
Counter 1 counts retired DTB double double misses.
Counter 1 counts retired ITB misses.
Counter 1 counts retired unaligned traps.
Counter 1 counts replay traps.

Table E–24: 21264 Read Counters for OpenVMS Alpha and DIGITAL UNIX
R0/v0 Bits

Meaning When Returned

63–48

Reserved

47–28

Counter 0 returned value

27–26

Reserved

25–6

Counter 1 returned value

5–0

Reserved

Table E–25: 21264 Write Counters for OpenVMS Alpha and DIGITAL UNIX
R17/a1 Bits

Meaning

63–48

Reserved

47–28

Counter 0 value to write

27–26

Reserved

25–6

Counter 1 value to write

E–28 Alpha Architecture Handbook

Table E–25: 21264 Write Counters for OpenVMS Alpha and DIGITAL UNIX
R17/a1 Bits

Meaning

5–2

Reserved

When set, write to Counter 1

When set, write to Counter 0

Table E–26: 21264 Enable and Write Counters for OpenVMS Alpha and
DIGITAL UNIX
R17/a1 Bits

Meaning

63–48

Reserved

47–28

Counter 0 value to write; writing zeroes clears the counter

27–26

Reserved

25–6

Counter 1 value to write; writing zeroes clears the counter

5–2

Reserved

When set, enable and write to Counter 1

When set, enable and write to Counter 0

Waivers and Implementation-Dependent Functionality E–29

Index
A
Aborts, forcing, 6–6
ACCESS(x,y) operator, 3–7
Add instructions
add longword, 4–25
add quadword, 4–27
add scaled longword, 4–26
add scaled quadword, 4–28
See also Floating-point operate
ADDF instruction, 4–110

Alpha architecture
addressing, 2–1
overview, 1–1
porting operating systems to, 1–1
programming implications, 5–1
registers, 3–1
security, 1–7
See also Conventions
Alpha privileged architecture library. See PALcode
AMASK (Architecture mask) instruction, 4–133
AMASK bit assignments, D–3

ADDG instruction, 4–110

AND instruction, 4–42

ADDL instruction, 4–25

AND operator, 3–7

ADDQ instruction, 4–27

Architecture extensions,AMASK with, 4–133

Address space match (ASM)
virtual cache coherency, 5–4
Address space number (ASN) register
virtual cache coherency, 5–4
ADDS instruction, 4–111

ARITH_RIGHT_SHIFT(x,y) operator, 3–7

ADDT instruction, 4–111
AFTER, defined for memory access , 5–12
Aligned byte/word memory accesses, A–9
ALIGNED data objects, 1–8
Alignment
atomic byte, 5–3
atomic longword, 5–2
atomic quadword, 5–2
D_floating, 2–6
data considerations, A–4
double-width data paths , A–1
F_floating , 2–4
G_floating, 2–5
instruction, A–2
longword, 2–2
longword integer, 2–12
memory accesses, A–9
quadword , 2–3
quadword integer, 2–12
S_floating , 2–8
T_floating, 2–9
X_floating, 2–10

Arithmetic instructions, 4–24
See also specific arithmetic instructions
Arithmetic left shift instruction, 4–41
Arithmetic traps
denormal operand exception disabling, 4–81
denormal operand exception enabled for, B–5
denormal operand status of, B–5
disabling, 4–78
division by zero, 4–77, 4–81
division by zero, disabling, 4–81
division by zero, enabling, B–6
division by zero, status of, B–5
dynamic rounding mode, 4–80
enabling, B–5
inexact result, 4–78, 4–81
inexact result, disabling, 4–80
inexact result, enabling, B–6
inexact result, status of, B–5
integer overflow, 4–78, 4–81
integer overflow, disabling, B–5
integer overflow, enabling, B–5
invalid operation, 4–76, 4–81
invalid operation, disabling, 4–81
invalid operation, enabling, B–6
invalid operation, status of, B–5
overflow, 4–77, 4–81
overflow, disabling, 4–81
overflow, enabling, B–6
overflow, status of, B–5

Index–1

programming implications for, 5–30
TRAPB instruction with, 4–144
underflow, 4–78, 4–81
underflow to zero, disabling, 4–80
underflow, disabling, 4–80
underflow, enabling, B–6
underflow, status of, B–5
ASCII character set, C–22
Atomic access, 5–3
Atomic operations
accessing longword datum, 5–2
accessing quadword datum , 5–2
updating shared data structures, 5–7
using load locked and store conditional, 5–7
Atomic sequences, A–16

B
BEFORE, defined for memory access, 5–12
BEQ instruction, 4–20
BGE instruction, 4–20
BGT instruction, 4–20
BIC instruction, 4–42
Big-endian addressing, 2–13
byte operation examples, 4–54
byte swapping for, A–11
extract byte with, 4–51
insert byte with, 4–55
load F_floating with, 4–91
load long/quad locked with, 4–9
load S_floating with, 4–93
mask byte with, 4–57
store byte/word with, 4–15
store F_floating with, 4–95
store long/quad conditional with, 4–12
store long/quad with, 4–15
store S_floating with, 4–97
Big-endian data types, X_floating, 2–10
BIS instruction, 4–42
BLBC instruction, 4–20
BLBS instruction, 4–20
BLE instruction, 4–20
BLT instruction, 4–20
BNE instruction, 4–20
Boolean instructions, 4–41
logical functions , 4–42
Boolean stylized code forms, A–13
BPT (PALcode) instruction
required recognition of, 6–4
bpt (PALcode) instruction
required recognition of, 6–4

Index–2

BR instruction, 4–21
Branch instructions, 4–18
backward conditional, 4–20
conditional branch, 4–20
floating-point, summarized, 4–99
format of, 3–12
forward conditional, 4–20
opcodes and format summarized, C–1
unconditional branch, 4–21
See also Control instructions
Branch prediction model, 4–18
Branch prediction stack,with BSR instruction, 4–21
BSR instruction, 4–21
BUGCHK (PALcode) instruction
required recognition of, 6–4
bugchk (PALcode) instruction
required recognition of, 6–4
Byte data type, 2–1
atomic access of, 5–3
Byte manipulation, 1–2
Byte manipulation instructions, 4–47
Byte swapping, A–11
BYTE_ZAP(x,y) operator, 3–7

C
/C opcode qualifier
IEEE floating-point, 4–67
VAX floating-point, 4–67
C opcode qualifier, 4–67
Cache coherency
barrier instructions for, 5–25
defined, 5–2
in multiprocessor environment, 5–6
Caches
design considerations, A–1
I-stream considerations, A–4
MB and IMB instructions with, 5–25
requirements for, 5–5
translation buffer conflicts, A–6
with powerfail/recovery, 5–5
CALL_PAL (call privileged architecture library)
instruction, 4–135
CASE operator, 3–8
Causal loops, 5–15
CFLUSH (PALcode) instruction
ECB compared with, 4–138

Changed datum, 5–6

CMOVLBC instruction, 4–43

notation, 3–10
numbering, 1–7
ranges, 1–8
Count instructions
Count leading zero, 4–31
Count population, 4–32
Count trailing zero, 4–33
CPYS instruction, 4–105

Clear a register, A–12
CMOVEQ instruction, 4–43
CMOVGE instruction, 4–43
CMOVGT instruction, 4–43
CMOVLE instruction, 4–43

CPYSE instruction, 4–105

CMOVLT instruction, 4–43

CPYSN instruction, 4–105

CMOVNE instruction, 4–43

CSERVE (PALcode) instruction
required recognition of, 6–4
cserve (PALcode) instruction
required recognition of, 6–4
CTLZ instruction, 4–31

CMPBGE instruction, 4–49
CMPEQ instruction, 4–29
CMPGLE instruction, 4–112
CMPGLT instruction, 4–112
CMPLE instruction, 4–29
CMPLT instruction, 4–29
CMPTEQ instruction, 4–113
CMPTLE instruction, 4–113
CMPTLT instruction, 4–113
CMPTUN instruction, 4–113
CMPULE instruction, 4–30
CMPULT instruction, 4–30
Code forms, stylized, A–11
Boolean, A–13
load literal, A–12
negate, A–13
NOP, A–11
NOT, A–13
register, clear, A–12
register-to-register move, A–13
Code scheduling
IMPLVER instruction with, 4–141
Code sequences, A–9

CTPOP instruction, 4–32
CTTZ instruction, 4–33
CVTDG instruction, 4–116
CVTGD instruction, 4–116
CVTGF instruction, 4–116
CVTGQ instruction, 4–114
CVTLQ instruction, 4–106
CVTQF instruction, 4–115
CVTQG instruction, 4–115
CVTQL instruction, 4–106
FP_C quadword with, B–5
CVTQS instruction, 4–118
CVTQT instruction, 4–118
CVTST instruction, 4–120
CVTTQ instruction, 4–117
FP_C quadword with, B–5
CVTTS instruction, 4–119

CODEC, 4–151

Coherency
cache, 5–2
memory, 5–1
Compare instructions
compare integer signed, 4–29
compare integer unsigned, 4–30
See also Floating-point operate
Conditional move instructions, 4–43
See also Floating-point operate
Console overview, 7–1

/D opcode qualifier
FPCR (floating-point control register), 4–79
IEEE floating-point, 4–67
D_floating data type, 2–5
alignment of, 2–6
mapping, 2–6
restricted, 2–6
Data alignment, A–4

Control instructions, 4–18
Conventions
code examples, 1–9
extents, 1–8
figures, 1–9
instruction format, 3–10

Data caches
ECB instruction with, 4–136
WH64 instruction with, 4–145
Data format, overview, 1–3
Data sharing (multiprocessor), A–5
synchonization requirement, 5–6

Index–3

Data stream considerations, A–4
Data structures, shared, 5–6
Data types
byte, 2–1
IEEE floating-point, 2–6
longword, 2–2
longword integer, 2–11
quadword , 2–2
quadword integer, 2–12
unsupported in hardware, 2–12
VAX floating-point, 2–3
word, 2–1
Denormal, 4–64
Denormal operand exception disable, 4–81
Denormal operand exception enable (DNOE)
FP_C quadword bit, B–5
Denormal operand status (DNOS)
FP_C quadword bit , B–5
Denormal operands to zero, 4–81

DZED bit. See Trap disable bits, division by zero

E
ECB (Evict data cache block) instruction, 4–136
CFLUSH (PALcode) instruction with, 4–138
EQV instruction, 4–42
EXCB (exception barrier) instruction, 4–138
with FPCR, 4–84
Exception handlers, B–3
TRAPB instruction with, 4–144
Exceptions
F31 with, 3–2
R31 with, 3–1
EXTBL instruction, 4–51
EXTLH instruction, 4–51
EXTLL instruction, 4–51
EXTQH instruction, 4–51

Depends order (DP), 5–15

EXTQL instruction, 4–51

DIGITAL UNIX PALcode, instruction summary,
C–16

Extract byte instructions, 4–51

Dirty zero, 4–64

EXTWL instruction, 4–51

DIV operator, 3–8
DIVF instruction, 4–121
DIVG instruction, 4–121
Division
integer, A–10
performance impact of, A–10
Division by zero enable (DZEE)
FP_C quadword bit, B–6
Division by zero status (DZES)
FP_C quadword bit, B–5
DIVS instruction, 4–122

EXTWH instruction, 4–51

F
F_floating data type, 2–3
alignment of, 2–4
compared to IEEE S_floating, 2–8
MAX/MIN, 4–65
FBEQ instruction, 4–100
FBGE instruction, 4–100
FBGT instruction, 4–100
FBLE instruction, 4–100

DIVT instruction, 4–122

FBLT instruction, 4–100

DNOD bit. See Denormal operand exception disable

FBNE instruction, 4–100

DNZ. See Denormal operands to zero

FCMOVEQ instruction, 4–107

DP. See Depends order

FCMOVGE instruction, 4–107

DRAINA (PALcode) instruction
required, 6–5
draina (PALcode) instruction
required, 6–5
DYN bit. See Arithmetic traps, dynamic rounding
mode

FCMOVGT instruction, 4–107

DZE bit
See also Arithmetic traps, division by zero

FCMOVLE instruction, 4–107
FCMOVLT instruction, 4–107
FCMOVNE instruction, 4–107
FETCH (prefetch data) instruction, 4–139
FETCH_M (prefetch data, modify intent) instruction,
4–139
Finite number, Alpha, contrasted with VAX, 4–63
Floating-point branch instructions, 4–99
Floating-point control register (FPCR)
accessing, 4–82

Index–4

at processor initialization, 4–83
bit descriptions, 4–80
instructions to read/write, 4–109
operate instructions that use, 4–102
saving and restoring, 4–83
trap disable bits in , 4–78
Floating-point convert instructions, 3–14
Fa field requirements, 3–14
Floating-point division, performance impact of,
A–10
Floating-point format, number representation
(encodings), 4–65
Floating-point instructions
branch, 4–99
faults, 4–62
function field format, 4–84
introduced, 4–62
memory format, 4–90
opcodes and format summarized, C–1
operate, 4–102
rounding modes, 4–66
terminology, 4–63
trapping modes, 4–69
traps, 4–62
Floating-point load instructions, 4–90
load F_floating, 4–91
load G_floating, 4–92
load S_floating, 4–93
load T_floating, 4–94
with non-finite values, 4–90
Floating-point operate instructions, 4–102
add (IEEE), 4–111
add (VAX), 4–110
compare (IEEE), 4–113
compare (VAX), 4–112
conditional move, 4–107
convert IEEE floating to integer, 4–117
convert integer to IEEE floating, 4–118
convert integer to integer, 4–106
convert integer to VAX floating, 4–115
convert S_floating to T_floating, 4–119
convert T_floating to S_floating, 4–120
convert VAX floating to integer, 4–114
convert VAX floating to VAX floating, 4–116
copy sign, 4–105
divide (IEEE), 4–122
divide (VAX), 4–121
format of, 3–13
from integer moves, 4–124
move from/to FPCR, 4–109
multiply (IEEE), 4–127
multiply (VAX), 4–126
subtract (IEEE), 4–131
subtract (VAX), 4–130
to integer moves, 4–123
unused function codes with, 3–14

Floating-point registers, 3–2
Floating-point single-precision operations, 4–62
Floating-point store instructions, 4–90
store F_floating, 4–95
store G_floating, 4–96
store S_floating, 4–97
store T_floating, 4–98
with non-finite values, 4–90
Floating-point support
floating-point control (FP_C) quadword, B–4
IEEE, 2–6
IEEE standard 754-1985, 4–88
instruction overview, 4–62
longword integer, 2–11
operate instructions, 4–102
optional, 4–2
quadword integer, 2–12
rounding modes, 4–66
single-precision operations, 4–62
trap modes, 4–69
VAX, 2–3
Floating-point to integer move, 4–123
Floating-point to integer move instructions, 3–14
Floating-point trapping modes, 4–69
See also Arithmetic traps
FNOP code form, A–11
FP_C quadword, B–4
FPCR. See Floating-point control register
FTOIS instruction, 4–123
FTOIT instruction, 4–123
Function codes
IEEE floating-point, C–6
in numerical order, C–10
independent floating-point, C–8
VAX floating-point, C–7
See also Opcodes

G
G_floating data type, 2–4
alignment of, 2–5
mapping, 2–5
MAX/MIN, 4–65
GENTRAP (PALcode) instruction
required recognition of, 6–4
gentrap (PALcode) instruction
required recognition of, 6–4

H
HALT (PALcode) instruction
required, 6–7
halt (PALcode) instruction
required, 6–7

Index–5

I
I/O devices, DMA
MB and WMB with, 5–22
reliably communicating with processor, 5–27
shared memory locations with, 5–11
I/O interface overview, 8–1
IEEE floating-point
exception handlers, B–3
floating-point control (FP_C) quadword, B–4
format, 2–6
FPCR (floating-point control register), 4–79
function field format, 4–85
hardware support, B–2
NaN , 2–6
options, B–1
S_floating , 2–7
standard charts, B–12
standard, mapping to, B–6
T_floating, 2–8
trap handling, B–6
X_floating, 2–9
See also Floating-point instructions
IEEE floating-point control word, B–4
IEEE floating-point instructions
add instructions, 4–111
compare instructions, 4–113
convert from integer instructions, 4–118
convert S_floating to T_floating, 4–119
convert T_floating to S_floating, 4–120
convert to integer instructions, 4–117
divide instructions, 4–122
from integer moves, 4–124
function codes for, C–6
multiply instructions, 4–127
operate instructions, 4–102
square root instructions, 4–129
subtract instructions, 4–131
to register moves, 4–123
IEEE standard, 4–88
conformance to, B–1
mapping to, B–6
IGN (ignore) , 1–9
IMB (PALcode) instruction, 5–23
required, 6–8
virtual I-cache coherency, 5–5
imb (PALcode) instruction
required, 6–8
IMP (implementation dependent), 1–9
IMPLVER (Implementation version) instruction,
4–141
IMPLVER value assignments, D–3
Independent floating-point function codes , C–8
INE bit
See also Arithmetic traps, inexact result

Index–6

INED bit. See Trap disable bits, inexact result trap
Inexact result enable (INEE)
FP_C quadword bit, B–6
Inexact result status (INES)
FP_C quadword bit, B–5
Infinity, 4–64
conversion to integer, 4–88
INSBL instruction, 4–55
Insert byte instructions, 4–55
INSLH instruction, 4–55
INSLL instruction, 4–55
INSQH instruction, 4–55
INSQL instruction, 4–55
Instruction encodings
common architecture, C–1
numerical order, C–10
opcodes and format summarized, C–1
Instruction fetches (memory), 5–11
Instruction formats
branch, 3–12
conventions, 3–10
floating-point convert, 3–14
floating-point operate, 3–13
floating-point to integer move, 3–14
memory, 3–11
memory jump, 3–12
operand values, 3–10
operators, 3–6
overview, 1–4
PALcode, 3–14
registers, 3–1
Instruction set
access type field, 3–5
Boolean, 4–41
branch, 4–18
byte manipulate, 4–47
conditional move (integer), 4–43
data type field, 3–6
floating-point subsetting, 4–2
integer arithmetic, 4–24
introduced, 1–6
jump, 4–18
load memory integer, 4–4
miscellaneous, 4–132
multimedia, 4–151
name field, 3–5
opcode qualifiers, 4–3
operand notation, 3–5
overview, 4–1
shift, arithmetic, 4–46
software emulation rules, 4–3
store memory integer, 4–4
VAX compatibility, 4–149
See also Floating-point instructions

Instruction stream. See I-stream
Instructions, overview, 1–4
INSWH instruction, 4–55
INSWL instruction, 4–55
Integer division, A–10
Integer registers
defined, 3–1
R31 restrictions, 3–1
INV bit
See also Arithmetic traps, invalid operation
Invalid operation enable (INVE)
FP_C quadword bit , B–6
Invalid operation status (INVS)
FP_C quadword bit , B–5
INVD bit. See Trap disable bits, invalid operation
IOV bit
See also Arithmetic traps, integer overflow
I-stream
coherency of, 6–8
design considerations, A–2
modifying physical, 5–5
modifying virtual, 5–5
PALcode with, 6–2
with caches, 5–5
ITOFF instruction, 4–124

with STx_C instruction, 4–9
LDQ instruction, 4–6
LDQ_L instruction, 4–9
restrictions, 4–10
with processor lock register/flag, 4–10
with STx_C instruction, 4–10
LDQ_U instruction, 4–8
LDS instruction, 4–93
with FPCR, 4–84
LDT instruction, 4–94
LDWU instruction, 4–6
LEFT_SHIFT(x,y) operator, 3–8
lg operator, 3–8
Literals, operand notation, 3–5
Litmus tests, shared data veracity, 5–17

Load instructions
emulation of, 4–3
FETCH instruction, 4–139
Load address, 4–5
Load address high, 4–5
load byte, 4–6
load longword, 4–6
load quadword, 4–6
load quadword locked, 4–10
load sign-extended longword locked, 4–9
load unaligned quadword, 4–8
load word, 4–6
multiprocessor environment, 5–6
serialization, 4–142
See also Floating-point load instructions
Load literal, A–12

JMP instruction, 4–22

Load memory integer instructions, 4–4

ITOFS instruction, 4–124
ITOFT instruction , 4–124

JSR instruction, 4–22

LOAD_LOCKED operator, 3–8

JSR_COROUTINE instruction, 4–22

Load-locked, defined, 5–16

Jump instructions, 4–18, 4–22
branch prediction logic, 4–22
coroutine linkage, 4–23
return from subroutine, 4–22
unconditional long jump , 4–23
See also Control instructions

L
LDA instruction, 4–5
LDAH instruction, 4–5
LDBU instruction, 4–6
LDF instruction, 4–91
LDG instruction, 4–92
LDL instruction, 4–6

Location, 5–11
Location access constraints, 5–14
Lock flag, per-processor
defined, 3–2
when cleared, 4–10
with load locked instructions, 4–10
Lock registers, per-processor
defined, 3–2
with load locked instructions, 4–10
Lock variables, with WMB instruction, 4–148
Logical instructions. See Boolean instructions
Longword data type, 2–2
alignment of, 2–12
atomic access of, 5–2
LSB (least significant bit), defined for floating-point,

LDL_L instruction, 4–9
restrictions, 4–10
with processor lock register/flag, 4–10

Index–7

4–64

Memory-like behavior, 5–3
MF_FPCR instruction, 4–109
MIN, defined for floating-point, 4–65

/M opcode qualifier, IEEE floating-point, 4–67

MINS(x,y) operator, 3–8

MAP_F function, 2–4

MINSB8 instruction, 4–152

MAP_S function, 2–7

MINSW4 instruction, 4–152

MAP_x operator, 3–8

MINU(x,y) operator, 3–8

Mask byte instructions, 4–57

MINUB8 instruction, 4–152

MAX, defined for floating-point, 4–65

MINUW4 instruction, 4–152

MAXS(x,y) operator, 3–8

Miscellaneous instructions, 4–132

MAXSB8 instruction, 4–152

Move instructions (conditional). See Conditional
move instructions

MAXSW4 instruction, 4–152
MAXU(x,y) operator, 3–8
MAXUB8 instruction, 4–152
MAXUW4 instruction, 4–152
MB (Memory barrier) instruction, 4–142
compared with WMB, 4–148
multiprocessors only, 4–142
with DMA I/O, 5–22
with LDx_L/STx_C, 4–14
with multiprocessor D-stream, 5–22
with shared data structures, 5–9
See also IMB, WMB
MBZ (must be zero), 1–9
Memory access
aligned byte/word, A–9
coherency of, 5–1
granularity of, 5–2
width of, 5–3
with WMB instruction, 4–147
Memory alignment, requirement for, 5–2
Memory barrier instructions. See MB, IMB
(PALcode), and WMB instructions
Memory barriers, 5–22
Memory format instructions
opcodes and format summarized, C–1
Memory instruction format, 3–11
Memory jump instruction format, 3–12
Memory management
support in PALcode, 6–2
Memory prefetch registers
defined, 3–3

Index–8

Move, register-to-register, A–13
MSKBL instruction, 4–57
MSKLH instruction, 4–57
MSKLL instruction, 4–57
MSKQL instruction, 4–57
MSKWH instruction, 4–57
MSKWL instruction, 4–57
MT_FPCR instruction, 4–109
synchronization requirement, 4–82
MULF instruction, 4–126
MULG instruction, 4–126
MULL instruction, 4–34
with MULQ, 4–34
MULQ instruction, 4–35
with MULL, 4–34
with UMULH, 4–35
MULS instruction, 4–127
MULT instruction, 4–127
Multimedia instructions, 4–151
Multiply instructions
multiply longword, 4–34
multiply quadword, 4–35
multiply unsigned quadward high, 4–36
See also Floating-point operate
Multiprocessor environment
cache coherency in, 5–6
context switching, 5–24
I-stream reliability, 5–23
MB and WMB with, 5–22
no implied barriers, 5–22
read/write ordering, 5–10
serialization requirements in, 4–142
shared data, 5–6, A–5

N
NaN (Not-a-Number)
conversion to integer, 4–88
copying, generating, propograting, 4–89
defined, 2–6
quiet, 4–64
signaling, 4–64
NATURALLY ALIGNED data objects, 1–8
Negate stylized code form, A–13
Non-finite number, 4–64
Nonmemory-like behavior, 5–3
NOP, universal (UNOP), A–11
NOT instruction, ORNOT with zero, 4–42
NOT operator, 3–9
NOT stylized code form, A–13

O
Opcode qualifiers
default values, 4–3
notation, 4–3
See also specific qualifiers
Opcodes
common architecture, C–1
DIGITAL UNIX PALcode, C–16
in numerical order, C–10
OpenVMS Alpha PALcode, C–14
PALcode in numerical order, C–18
reserved, C–21
summary, C–8
unused function codes for, C–21
Windows NT Alpha PALcode, C–17
See also Function codes
OpenVMS Alpha PALcode, instruction summary,
C–14
Operand expressions, 3–4
Operand notation
defined, 3–4
Operand values, 3–4
Operate instruction format
unused function codes with, 3–13
Operate instructions
opcodes and format summarized, C–1
Operate instructions, convert with integer overflow ,
4–78
Operators, instruction format, 3–6
Optimization. See Performance optimizations
OR operator, 3–9
ORNOT instruction, 4–42
Overflow enable (OVFE)
FP_C quadword bit, B–6

Overflow status (OVFS)
FP_C quadword bit, B–5
Overlap
with location access constraints, 5–14
with processor issue constraints, 5–13
with visibility, 5–14
OVF bit
See also Arithmetic traps, overflow
OVFD bit. See Trap disable bits, overflow disable

P
Pack to bytes instructions, 4–155
PALcode
barriers with, 5–22
CALL_PAL instruction, 4–135
compared to hardware instructions, 6–1
implementation-specific, 6–2
instead of microcode, 6–1
instruction format, 3–14
overview, 6–1
recognized instructions, 6–4
replacing, 6–3
required, 6–2
required instructions, 6–5
running environment, 6–2
special functions function support, 6–2
PALcode instructions
opcodes and format summarized, C–1
required, C–20
reserved, function codes for, C–20
PALcode instructions, required privileged, 6–5
PALcode instructions, required unprivileged, 6–5
PALcode opcodes in numerical order, C–18
PALcode variation assignments, D–2
PCC_CNT, 3–3, 4–143
PCC_OFF, 3–3, 4–143
Performance monitoring, E–3, E–9, E–23
Performance optimizations
branch prediction, A–2
code sequences, A–9
data stream, A–4
for I-streams, A–2
instruction alignment, A–2
instruction scheduling, A–4
I-stream density, A–4
shared data, A–5
Performance tuning
IMPLVER instruction with, 4–141
PERR (Pixel error) instruction, 4–154
Physical address space
described, 5–1
PHYSICAL_ADDRESS operator, 3–9
Pipelined implementations, using EXCB instruction

Index–9

with, 4–138

Pixel error instruction, 4–154

Relational Operators, 3–9

PKLB (Pack longwords to bytes) instruction, 4–155

Representative result, 4–64

PKWB (Pack words to bytes) instruction, 4–155

Reserved instructions, opcodes for, C–21

Prefetch data (FETCH instruction), 4–139

Result latency, A–4

PRIORITY_ENCODE operator, 3–9

RET instruction, 4–22

Privileged Architecture Library. See PALcode

RIGHT_SHIFT(x,y) operator, 3–9

Processor communication, 5–15

Rounding modes. See Floating-point rounding modes

Processor cycle counter (PCC) register, 3–3
RPCC instruction with, 4–143
Processor issue constraints, 5–12

RPCC (read processor cycle counter) instruction,
4–143
RS (read and set) instruction, 4–150

Processor issue sequence, 5–12
Processor type assignments, D–1
Program counter (PC) register, 3–1
with EXCB instruction, 4–138
Pseudo-ops, A–14

Q
Quadword data type, 2–2
alignment of, 2–3, 2–12
atomic access of, 5–2
integer floating-point format, 2–12
T_floating with, 2–12

R
R31
restrictions, 3–1
RAZ (read as zero), 1–9
RC (read and clear) instruction, 4–150
RDUNIQUE (PALcode) instruction
required recognition of, 6–4
Read/write ordering (multiprocessor), 5–10
determining requirements, 5–10
hardware implications for, 5–29
memory location defined, 5–11
Read/write, sequential, A–8
Regions in physical address space, 5–1
Registers, 3–1
floating-point, 3–2
integer, 3–1
lock, 3–2
memory prefetch, 3–3
optional, 3–3
processor cycle counter, 3–3
program counter (PC), 3–1
value when unused, 3–10
VAX compatibility, 3–3
See also specific registers

Index–10

S
S_floating data type
alignment of, 2–8
compared to F_floating, 2–8
exceptions, 2–8
mapping, 2–7
MAX/MIN, 4–65
NaN with T_floating convert, 4–88
operations, 4–62
S4ADDL instruction, 4–26
S4ADDQ instruction, 4–28
S4SUBL instruction, 4–38
S4SUBQ instruction, 4–40
S8ADDL instruction, 4–26
S8ADDQ instruction, 4–28
S8SUBL instruction, 4–38
S8SUBQ instruction, 4–40
SBZ (should be zero), 1–9
Security holes, 1–7
with UNPREDICTABLE results, 1–8
Sequential read/write, A–8
Serialization, MB instruction with, 4–142
SEXT(x) operator, 3–9
Shared data (multiprocessor), A–5
changed vs. updated datum , 5–6
Shared data structures
atomic update, 5–7
ordering considerations, 5–9
using memory barrier (MB) instruction, 5–9
Shared memory
accessing, 5–11
defined, 5–10

Shift arithmetic instructions, 4–46

STT instruction, 4–98

Sign extend instructions, 4–60

STW instruction, 4–15

Single-precision floating-point, 4–62

SUBF instruction, 4–130

SLL instruction, 4–45

SUBG instruction, 4–130

Software considerations, A–1
See also Performance optimizations
SQRTF instruction, 4–128

SUBL instruction, 4–37

SQRTG instruction, 4–128
SQRTS instruction, 4–129
SQRTT instruction, 4–129
Square root instructions
IEEE, 4–129
VAX, 4–128
SRA instruction, 4–46
SRL instruction, 4–45
STB instruction, 4–15
STF instruction, 4–95
STG instruction, 4–96
STL instruction, 4–15
STL_C instruction, 4–12
when guaranteed ordering with LDL_L, 4–14
with LDx_L instruction, 4–12
with processor lock register/flag, 4–12
Storage, defined, 5–14
Store instructions
emulation of, 4–3
FETCH instruction, 4–139
multiprocessor environment, 5–6
serialization, 4–142
Store byte, 4–15
store longword, 4–15
store longword conditional, 4–12
store quadword, 4–15
store quadword conditional, 4–12
Store word, 4–15
STQ_U , 4–17
See also Floating-point store instructions
Store memory integer instructions, 4–4
STORE_CONDITIONAL operator, 3–9
Store-conditional, defined, 5–16
STQ instruction, 4–15
STQ_C instruction, 4–12
when guaranteed ordering with LDQ_L, 4–14
with LDx_L instruction, 4–12
with processor lock register/flag, 4–12
STQ_U instruction, 4–17
STS instruction, 4–97
with FPCR, 4–84

SUBQ instruction, 4–39
SUBS instruction, 4–131
SUBT instruction, 4–131
Subtract instructions
subtract longword, 4–37
subtract quadword, 4–39
subtract scaled longword, 4–38
subtract scaled quadword, 4–40
See also Floating-point operate
SUM bit. See Summary bit
Summary bit, in FPCR, 4–80
SWPPAL (PALcode) instruction
required recognition of, 6–4
swppal (PALcode) instruction
required recognition of, 6–4

T
T_floating data type
alignment of, 2–9
exceptions, 2–9
format, 2–9
MAX/MIN, 4–65
NaN with S_floating convert, 4–88
TEST(x,cond) operator, 3–10
Timeliness of location access, 5–17
Timing considerations, atomic sequences, A–16
Trap disable bits, 4–78
denormal operand exception, 4–81
division by zero, 4–81
DZED with DZE arithmetic trap, 4–77
DZED with INV arithmetic trap, 4–76
IEEE compliance and, B–4
inexact result, 4–80
invalid operation, 4–81
overflow disable, 4–81
underflow, 4–80
underflow to zero, 4–80
when unimplemented, 4–78
Trap enable bits, B–5
Trap handler, with non-finite arithmetic operands,
4–74
Trap handling, IEEE floating-point, B–6
Trap modes
floating-point, 4–69
Trap shadow
defined for floating-point, 4–64
programming implications for, 5–30
Index–11

TRAPB (trap barrier) instruction
described, 4–144
with FPCR, 4–84
True result, 4–64
True zero, 4–65

U
UMULH instruction, 4–36
with MULQ, 4–35
UNALIGNED data objects, 1–8
Unconditional long jump, 4–23
UNDEFINED operations, 1–7
Underflow enable (UNFE)
FP_C quadword bit, B–6
Underflow status (UNFS)
FP_C quadword bit, B–5
UNDZ bit. See Trap disable bits, underflow to zero
UNF bit
See also Arithmetic traps, underflow
UNFD bit. See Trap disable bits, underflow
UNOP code form, A–11
UNORDERED memory references, 5–10
Unpack to bytes instructions, 4–156
UNPKBL (Unpack bytes to longwords) instruction,
4–156
UNPKBW (Unpack bytes to words) instruction,
4–156
UNPREDICTABLE results, 1–7

function field format, 4–87
multiply instructions, 4–126
operate instructions, 4–102
square root instructions, 4–128
subtract instructions, 4–130
VAX rounding modes, 4–66
Vector instructions
byte and word maximum, 4–152
byte and word minimum, 4–152
Virtual D-cache, 5–4
Virtual I-cache, 5–4
maintaining coherency of, 5–5
Visibility, defined, 5–14

W
Waivers, E–1
WH64 (Write hint) instruction, 4–145
WH64 instruction
lock_flag with, 4–10
Windows NT Alpha PALcode, instruction summary,
C–17
WMB (Write memory barrier) instruction, 4–147
atomic operations with, 5–8
compared with MB, 4–148
with shared data structures, 5–9
Word data type, 2–1
atomic access of, 5–3
Write buffers, requirements for, 5–5
Write-back caches, requirements for, 5–5

Updated datum, 5–6

wrunique (PALcode) instruction
required recognition of, 6–4

VAX compatibility instructions, restrictions for,
4–149

x MOD y operator, 3–8

VAX compatibility register, 3–3
VAX floating-point
D_floating, 2–5
F_floating , 2–3
G_floating, 2–4
See also Floating-point instructions
VAX floating-point instructions
add instructions, 4–110
compare instructionsCMPGEQ instruction,
4–112
convert from integer instructions, 4–115
convert to integer instructions, 4–114
convert VAX floating format instructions,
4–116
divide instructions, 4–121
from integer move, 4–124
function codes for, C–7
Index–12

X_floating data type, 2–9
alignment of, 2–10
big-endian format, 2–10
MAX/MIN, 4–65

XOR instruction, 4–42
XOR operator, 3–10

Y
YUV coordinates, interleaved, 4–151

Z
ZAP instruction, 4–61
ZAPNOT instruction, 4–61
Zero byte instructions, 4–61
ZEXT(x)operator, 3–10

Index–13