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MISC-684EC462

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Document:	PDP11 Handbook
Order Number:	MISC-684EC462
Revision:
Pages:	112
Original Filename:	PDP%2011%20Handbook%201969.pdf

OCR Text

alMml
digihl

equipmentcorpomtion

Digital

PDP is a registered
of Digital Equipment,

trademark
Corporation

The material
in this handbook
is for information
purposes only and is subject to change without
notice.

TABLE OF CONTENTS
CHAPIER

PDP-11

SYSTEMS

UNIBUS

lNTRODUCTlOR

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .:.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . .

. . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . , . . . . . . . . .. . . . . . . . . . . . . . . .

KAl 1 PROCESSOR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .?.
Priority Interrupts
,...............,.....
. . . . . . . . . . . . . . . . .. . . . . . . .. . . . . . . . . . . . . . . .
Reentrant Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
General Registers . . . .. . . . . .. . . . . . . . . .:... . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .
Instruction
Set . . . . . . . . . .._........................................................
Addressing
. ..~...............................................~........,...,.........
Operation
...... ........... ........ ............ ...... ......... L
Asynchronous
PACKAGJNG . . . . . . . . . . . . . .
...
.
.. . . . . . . . . . . . . . . . . . . ..I . . . . . . . . . . . . . .
SOmARE
.: ........................................................
........................
CHAPTER 2 SYSTEM INTRODUCTION
SYSTEM DEFINITION
..................................................................
SYSTEM COMPONENTS
...............................................................
UNIBUS
...................................................
. ..................................
Single Bus .................................................................
. ..........
Bidirectional
Lines ............ ..........................................
..* ....
Relation
Master-Slave
...........................
..............................
................................................
Interlocked .Communication
Dynamic Master-Slave
Relation ...........................................
KAll CENTRAL PROCESSOR ......................................................
General Registers ................................................................
Central Processor Status Register ........................................
CORE MEMORY .............................................................................
PERIPHERAL DEVICES ................................................................
SYSTEM INTERACTION ................................................................
TRANSFER OF BUS MASTER ......................................................
PRIORITY STRUCTURE ............ ....................................................
NPR Requests .......................................
.I ...........................
Interrupt
Requests ..............................................................
CHAPTER

ADDRESSING

INSTRUCTlOil

2
z
2
2
3

5
5
5
5
5
5

:
6
6
6
6
7
5
7
:

MODES

INTRODUCTION
. .....................
.............................
.; .....................
ADDRESS FIELDS ........................................................
. ...............
General Register Addressing .......................
.:. ......................
*Indexed Addressing ........................
. .....................................
Autoincrement
Mode Addressing
........................................
Autodecrement
Addressing
....................
. .............................
STACK PROCESSING ..................................................................
USE OF THE PC AS A GENERAL REGISTER ..........................
_...
Immediate
Addressing
.........................................................
Absolute Addressing
.:.
.........................................................
Relative Addressing ..............................................................
Deferred Relative Addressing
..........................................
..:.
USE OF THE SP AS A GENERAL REGISTER ................................
DOUBLE OPERAND ADDRESSING ..............................................
&iAPTER

1
1

11
11
11
12
12
12
13
13
13
13
14
14
14
14

TIMING ................................................................
INSTRUCTION
NOTATION ....................................................................................
Ill

17
17

.
DOUBLE OPERAND-INSTRUCTIONS .......................... i.. ...............

Arithmetic
Operations
.................. 1........................................
Boolean Instructions
.............................................................
BRANCHES ..........................................................................
.:. .....
Unconditional
Branch ..........................................................
I
Simple Conditional
Branches ..............................................
Signed Conditional
Branches ..............................................
Unsigned Conditional
Branches ..........................................
JUMP ....................................................................................
SUBROUTINES
.............................................................................
............................................................................
Examples
............................................
SINGLE OPERAND INSTRUCTIONS
Multiple Precision Operations ..............................................
Rotates ................................................................................
..................................................................................
Shifts
Examples
............................................................................
BYTE OPERATIONS ......................................................................
Double Operand Byte Instructions
......................................
Example
.......... . ...................................................................
Single Operand Instructions
................................................
CONDITION
CODE OPERATORS ................................................
MISCELLANEOUS
CONTROL INSTRUCTIONS
..............................
,
PROCESSOR TRAPS ....................................................................
Trap Instructions
............................
. ...................................
Stack Overflow Trap ............................................................
Bus Error Traps ..................................................................
Trace Traps ..........................................................................
CHAPTER 5 I ADDRESS ALLOCATION
ADDRESS MAP ............................................................................
Interrupt and Trap Vector ....................................................
Processor Stack and General Storage ..................................
Peripheral
Registers ............................................................
CORE MEMORY ............................................................................
Read-Write Core Memory ..................................
, .................
Read-Only Core Memory ......................................................
Wordlet Memory ..................................................................

18
20
21

CHAPTER 6 PROGRAMMING OF PERIPHERALS
DEViCE REGISTERS ......................................................
..... . .........
CONTROL & STATUS REGISTERS ...............................................
Device Function Bits .......................................
:.. ................
. ......
Memory Extension
.......................................................
Done Enable and Interrupt
Enable ......................................
..i ....
Condition
Bits .............................................................
Unit Bits ..............................................................................
Error Bits .............. I ..:. .........................................................
DATA BUFFER REGISTERS ..................................................
.: ......
PROGRAMMING
EXAMPLES--NON
INTERRUPT ........................
INTERRUPT STRUCTURE ............................................................
PROGRAMMING
EXAMPLE ..........................................................
.
CHAPTER 7 PERIPHERAL BULLETINS
TELETYPE (MODEL LT33-DC/DD)
..............................................
Size ......................................................................................
Power Requirement
............................................................
IV

;:
23
9:
27
28
zi

33
34

36
36
36
37
38
i!
41
41
ii
43

45
46
z
46
46
g

;;
$
ii
g
48
48
50
51
;;
53

TELETYPE CONTROL (MODEL KLll)
.........................................
Teletype Control ..........................................................
r .......
Keyboard/Reader
Operation
................................................
Registers (TKS, TKB) ..................................................
Teleprinter/Punch
................................................................
Registers (TPS, TPB) ....................................................
Programming
Example ........................................................
Peripheral Address Assignments
..........................................
Mounting
......................................
.......................................
HIGH-SPEED PERFORATED TAPE READER (MODEL PCll) ......
Tape Reader ........................................................................
Registers (PRS, PRB) ...................................................
Programming
Example ................................................
Peripheral
Address Assignments
................................
Tape Punch ..........................................................................
Registers (PPS, PPB) ..................
., ................................
Programming
Example
................................................
Peripheral Address Assignments
..................................
Mounting
...............................................................................
Environmental
.......................................................................
Line Frequency Clock (Model KWll-L)
............ I,....................
Register .....................................................................
.:. .... . ....
Peripheral Address Assignments
..........................................
Mounting
...............................................................................
Vector Address ....................................................................
Priority Level ........................................................................
DESCRIPTION OF THE UNIBUS
(
GENERAL CONCEPTS OF THE UNIBUS ......................................
Single Bus ............................................................................
Bidirectional
Bus ................................................................
Master-Slave
Relation ..........................................................
. Interlocked Communication
..................................................
Dynamic Master-Slave
Relation .............................................
UNIBUS SIGNALS ........................................................................
NON-INTERRUPT
SIGNALS ..........................................................
Data Lines ..................................
..r.~. ...................................
Address Lines ......................................................................
Control Lines .:. .....................................................................
Master Sync &‘Slave Sync ..................................................
Parity Available & Parity Bit ................................................
Initialization
........................................................................
Spare 1 & Spare 2 ..............................................................
INTERRUPT SIGNALS .......................................
*. .......................
Bus Request Lines ...........................................................
9..
I Bus Grant Lines ................. r.. ..............................................
Non-Processor
Request ........................................................
Non-Processor
Grant ............................................................
Selection
Acknowledge
........................................................
Interrupt
(and) Bus Busy ....................................................
UNIBUS DATA TRANSFER OPERATIONS ....................................
DATO and DATOB ................................................................
DATI and DATIP ..................................................................
Examples of Data Transfers
......... . .................. ........ . ...........
Signal Description
of Data Transfers
......... . ........................

53
53
53
54
54
.55
55
55
55
55
55
56
56
56
56
57
E
57
58
5588
58
58
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58

CHAPTER 8

59
59
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59

60
60
60
:?I
E
::
61
,61
61
z:
61
61
6":
61
:z
62
63

UNIBUS CONTROL ............. ’.........................................................
Priority Arbitration
.................................................................
..............................................
Selection of Next m m
Interrupt Sequence ...................................
. ..........................
Example of Interrupt,
etc. ..................................................
Example of NPR Operation ..................................................
CHAPTER 9 WI’TERF~MC
REGISTERS ..................................................................................
BUS DRIVERS AND RECEIVERS ..................................................
ADDRESS SELECTOR ..................................................................
.............................
INTERRUPT CONTRbL ...................................
DEVICE CONTROL LOGIC ............................................................

69
69
76

CHAPTER 10 CONFlGUMTICM
AND MSTALLATIOW PlANNlm;
MODULAR CONSTRUCTION
........................................................
MOUNTING BOXES AND CABINETS ............................................
PDP-11 Tabletop Box for 11/20, Etc. ..................................
PDP-11 Basic Mounting Box ................................................
PDP-11 Tabletop Extension Mounting Box ..........................
PDP-1 l-Freestanding
Base Cabinet .............................
. ......
Freestanding
Programmer’s
Table .........................................
SYSTEM UNITS AND CABLES ....................................................
Peripheral Mounting
Unit ....................................................
Blank System Unit ...........................................................
.: ...
Unibus Module ....................................................................
Unibus Cable ...... :. ................................................................
CABLE REQUIREMENTS
..............................................................
PDP-11/20
POWER REQUIREMENTS
..........................................
TELETYPE REQUIREMENTS
........................................................

ENVIRONMENTAL

INSTALLATION

REQUIREMENTS

PROCEDURE

............................................

.....................

................................

CHAPTER 11 PAPER TAPE SOFlWARE SYSTEM
PTS FEATURES ............................................................................
PAL-11A Assembler

..............................................................

ED11 Editor ..........................................................................
ODT On-Line Debugging ......................................................
IOX Input/Output,
etc. ........................................................
Math Package .......................................................................
Loaders ................................................................................
Core Dump Routines .....................................................
CHAPTER 12 THE OPERATOR’S CONSOLE
CQNSOLE ELEMENTS ..................................................................
Indicator Lights .....................................................................
Register Displays ..................................................................
Switch Register ....................................................................
Control Switches ..........................................................
CONTROL SWITCH OPERATION ..................................................

i’. .....

:;
x;
a7
89

..................

APPENDIX B-ADDRESSING
SUMMARY ....................
..- .............
ADDRESSING
MODES .........................................
. ......................
General Register Addressing ................................................
PC Register Addressing
............. \. ........... . ...........................

APPENDIX

. ......

A-PDP-11

INSTRUCTION

REPERTOIRE

E
95

INSTRUCTION

FORMATS

._.___...........___........................................

APPENDIX

C-ADDRESS

MAP . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .

APPENDIX

D-UNIBUS

OPERATIONS

.......... ........ ............ ........ ..

DATA-TRANSFERS .__............._._.....................................................
DATI and DATIP . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
DATO and DATOB . . . .._..........................................................
/
F’TR-PRIORITY TRANSFER . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . ..__......................
INTR-INTerRupt
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . .
GENERAL NOTES ON THE BUS OPERATIONS _.__._,...._...__.,.,,......

VII

99
lzo”

101
102
102

The PDP-11”is available
in two versions-PDP-ll/lO
and PDP11/20.
The basic PDP-ll/lO
contains
1,024 words of read only
memory in conjunction
with 128 words of read/write
memory and
the basic PDP-ll/PO
includes 4,096,words
of read/write
memory.

VIII

CHAPTER 1
INTRODUCTION
This publication
is a handbook
for Digital Equipment
Corporation’s
PDP-11.
It provides a comprehensive
overview of the system structure,
the instruction
repertoire,
input/output
programming,
peripherals,
general interfacing,
software, and console operation.
PDP-11 is Digital’s answer to the demand for a modular system for real-time
data acquisition,
analysis and control.
PDP-11 systems can handle a wide
variety
of real-time
control
applications-each
system
being individually
tailored
from a comprehensive
array of modular
building
blocks. Digital is
unique among manufacturers
of small-scale
computers-in
its ability to provide not only fast and efficient processing
units, but also a large family of its
own compatible
I/O devices including
A/D and D/A converters,
magnetic
tape, disk storage,
paper tape, and displays, as well as a wide range of
general-purpose
modules. This capability offers the user a hew, more efficient
approach to real-time systems.
The following
paragraphs
introduce
the new PDP-11 by way of highlighting
several of the important
design features that set it apart from other machines
in its class. Subsequent
chapters of this manual place these features in their
proper context and provide detailed descriptions
of each.

PDP-11 SYSTEMS
The PDP-11 is available in two versions designated
as PDP-ll/ 10 and PDP11/20.
The PDP-ll/ 10 contains
a KAll
processor,
1,024 words of 16-bit
read-only
memory,
and 128 16-bit words of read-write
memory.
The basic
PDP-ll/PO
contains a KAll
processor
and 4,096
words of 16.bit
read-write
core memory,
a programmer’s
console, and an ASR-33 Teletype.
Both versions can be similarly expanded
with either read-write
or read-only memory
and peripheral devices.

UNIBUS

Unibus is the name given to the single bus structure
of the PDP-11. The
processor,
memory
and all peripheral
devices share the same high-speed
bus. The Unibus enables the processor to view peripheral
devices-as
active
memory locations which perform special functions.
Peripherals
can thus be
addressed
as memory.
In other words, memory
reference
insfructions
can
operate
directly on’control,
status, or data registers in peripheral
devices.
Data transfers
from input to output devices can bypass the processor completely.

KAll PROCESSOR
The KAll processor incorporates
a unique
not previously available in ldbit computers.

combination

of powerful

features

Priority

Interrupts-A
four-level
automatic
priority interrupt
system permits
the processor to respond automatically
to conditions
outside the system, or
in the processor itself. Any number of separate devices can be attached to
each level.
Each perkpheral device in a PDP-11 system has a hardware pointer to its own
unique pair of memory locations which, in turn, point to the device’s service
routine. This unique identification
eliminates
the need for polling of devices

to identify

an interrupt,

since
the appropriate

begins executing

the interrupt
servicing
service routine.

The device’s interrupt
priority and service
This allows dynamic adjustment
of system
conditions.

hardware

selects and

routine priority are independent.
behavior in response to real-time

The interrupt
system allows the processor continually
to compare
its own
priority levels with the levels of any interrupting
devices and to acknowledge
the device with the highest level above the processor’s
priority level. Servicing an interrupt
for a device can be interrupted
for servicing a higher priority
device. Service to the lower priority device can be resumed
aUtOmaticallY
upon completion
of the higher level servicing. Such a process, called nested
interrupt servicing, can be carried out to any level.

Rentrant
Code-Both
the interrupt
handling
hardware
and the subroutine
call hardware
are designed
to facilitate
writing
reentrant
code for the
PDP-11. This type of code allows use of a single copy of a given subroutine
or program to be shared by more than one process or task. This reduces,the
amount
of core needed for multi-task
applications
such as the concurrent
servicing of many peripheral devices.
General

Register-The
PDP-11 is equipped
with eight general registers. All
are program-accessible
and can be used as accumulators,
as pointers
to
memory locations,
or as full-word index registers. Six registers are used for
general-purpose
access while the seventh and eighth registers are used as
a stack pointer and program counter respectively.

Instruction
Set-An
important
feature of the PDP-11 instruction
set is the
availability
of double operand instructions.
These instructions
allow memoryto-memory
processing and eliminate
the need to use registers for Storage of
intermediate
results. By using double operand
instructions,
every memory
location can be treated
as an accumulator.
This significantly
reduces the
length of programs
by eliminating
load and store operations
associated with
single operand machines.
Addressing-Much
of the power of the PDP-11 is derived from its wide range
of addressing
capabilities.
PDP-11 addressing
modes include list sequential
addressing,
full address
indexing,
full 16qbit word addressing,
8-bit byte
addressing,
stack addressing,
and direct addressing
to 32K words.
Variable length instruction
formatting
allows a minimum
number of bits to
be used for each addressing
mode. This results in efficient use of program
storage space.
Asynchronous
Operation-The
PDP-11’s memory
and processor
are asynchronous.
As a result, I/O devices transferring
directly
memory may steal memory cycles during instruction
operation.

PACKAGING

operations
to or from

’

The PDP-11 has adopted a modular approach to allow custom configuring
Of
systems, easy expansion,
and easy servicing. Systems are composed
of basic
building blocks, called System Units, which are completely
independent
subsystems
connected
only by pluggable
Unibus
and power connections.
There is no fixed wiring between them. An example of this type of subsystem
is a 4,096-word
memory module.
System Units can be mounted
in many combinations
within the PDP-11
hardware,
since there are no fixed positions for memory or l/O device controllers. Additional
units can be mounted
easily and connected
to the system

in the field. In case maintenance
is required,
replaced with spares and operation resumed

-E

defective System Units can be
within a few minutes.

’

A corn-e
package of user-oriented
software includes:
Absolute assembler providing object and source listings
Stritig-oriented
editor
Debugging
routines capable of operating
in a priority
mint
Input/output
handlers for standard peripherals
Relocatable integer and floating point math library

interrupt

environ-

All PDP-11 processors,
memories and peripherals
are electrically
and mechanically
modular subsystems
.supported in System Units.
which are simply plugged together to form a computer tailored to
user needs.
‘4

I
CHAPTER i
SYSTEM
SYSTEM

INTRODUCTION

DEFINITION

Digital Equipment
Corporation’s
PDP-11 is a 16.bit, general-purpose,
parallellogic computer
using two’s complement
arithmetic.
The PDP-11 is a variable
word length
processor
which .directly
addresses
32,768
16.bit words or
65,536 8-bit bytes. All communication
between system components
is done
on a single- high-speed
bus called a Unibus. Standard features of the system
include eight general-purpose
registers which can be used as accumulators.
index registers, or address pointers,
and a multi’level
automatic
priority interrupt system.

SYSTEM

COMPONENTS

UNIBW-There
are five concepts that are very important
both the hardware and software implications
of the Unibus.

for understanding

Single Bus--The
Unibus is a single, common
path that connects the central
processor memory, and all peripherals.
Addresses, data, and control information are sent along the 56 lines of the bus.
The form of communication
is the same for every device on the Unibus. The
processor uses the same set of signals to communicate
with memory as with
peripheral devices. Peripheral devices also use this set of signals when communicating
with the processor, memory, or other peripheral devices.
Peripheral device registers may be manipulated
as flexibly as core memory
by the central processor. All the instructions
that can be applied to data in
core memory can be applied equally well to data in peripheral
device regis-,
.ters. This is an especially powerful feature, considering
the special capability
of PDP-11 instructions
to process data in any memory location as though it
were an accumulator.
Bidirectiona)
Lines--Unibus
lines are bidirectional,
so that the same signals
which are received as input can be driven as output.
This means that a
peripheral
device register can be either read or set by the central processor
or other peripheral
devices; thus, the same register can be used. for both
input and output functions.
Master-Slave
Relation-Communication
between two devices on the bus is
in the form of a master-slave
relationship.
At any point in time, there is one
device that has control of the bus. This controlling
device is termed
the
“bus master.” The master device controls the bus when communicating
with
another
device on the bus, termed
the “slave.”
A typical example of this
relationship
is the processor,
as master, fetching an instruction
from memory (which is always a slave). Another example is the disk, as master, transferring data to memory, as slave.
interlocked
Communication--Communication
on tliq Unibus is interlocked
so that for each control signal issued by the master device, there must be-a
response from the slave in order to complete
the transfer.
Therefore,
communication
is independent
of the physical bus length and the response time
of the master and slave devices. The maximum
transfer
rate on the Unibus
is one 16-bit word every 750 nanoseconds,
or 1.3 million 16-bit words- per
second.

’

Dvnamlc
Master-tive
#?e!&eMastar-slave
,processor, for example,
could then communicate

relationships
are dynamic. The
may p&s bus control to a disk. The disk, as master,
with a slave memory bank.

Since the Unibus is used by the processor and all I/O devices, there is a
priority structure to determine
which device gets control of the bus. There
fore, every device on the Unibus which is capable of becoming bus master
has a ‘Priority assigned to it. When two devices which are kapable of becorning a bus master
request use of the bus simultaneously,
the device with the
higher priority will receive control first. Details of what conditions
must be
satisfied before a device will get control of the bus are given in the section
on System Interaction.
,+(A11 CENTRAL PROCESSOR-There
are four major features which are of
particular
interest to the programmer:
l), the General Registers:
2), the
Processor Status Word; (3), the Addressing
Modes; and 4), the Instruction
Set. The addressing
modes and the instruction
set of the PDP-11 processor
will be discussed in detail in Chapters 3 and 4.

KAll processor contains eight 16.bit general regis‘Ganeral Registers-The
Ms. These eight general registers (referred to as RO, Rl, . . . . . R7) may
be used as accumulators,
as index registers, or as stack pointers. One of
these registers,
R7, is reserved as. a program counter (PC). Generally, the
PC holds the address of the next instruction,
but it may point to data or
to an address of data. The register R6 has the special function of processor
stack pointer.
Central Processor Status Register-The
Central Processor Status Register
(PS) contains information
on the current priority of the processor, the result
of previous operations,
and an indicator for detecting
the execution of an
instruction
to be trapped
during program
debugging.
The priority of the
central processor can be set under program control to any one of eight.
levels. This information
is held in bits 5, 6, and 7 of the PS.
Four bits of the PS are assigned to monitoring
different results of previous
instructions.
These bits are set as follows:
Z-if the result was zero
.
N-if
the result was negative
C-if
the operation
resulted in a carry from the most significant
bit
V-if
the operation resulted in an arithmetic
overflow
The T bit is used in program debugging and can be set or cleared under program control. If this bit is set, when an instruction
is fetched from memory
a processor
trap will’ be caused by the completion
of the instruction’s
execution.

Central Processor Status Register (PS)
CORE MEI;(ORy-The
PDP-11 allows both 16.bit word and 8-bit byte addressing. The address space may be filled by core memory and peripheral
device registers. The top 4,096 words generally are reserved for peripheral
device registers. The remainder of address space can be used for read-write
core memory or read-only core memory.
Read-write core memory is currently available in 4,096 1Qbit word segments.
This memory has a cycle time of 1.2. microseconds
and an access time of
500 nanoseconds.
It is a standard part of a PDP-ll/PO
system.
6

Read-only core memory
(ROM) is available in 1,024 16 bit-word segments.
The access time of the ROM is 500 nanoseconds.
Memory is also available in
128 16-bit word segments with a 2.0 microsecond
cycle time. Both 1,024
words of read-only memory
and 128 words of read-write
memory mount in
a single System Unit and are a standard part of the PDP-ll/lO
system.
PERIPHERAL
DEVICES-The
ASR-33 Teletype
with low-speed
paper tape
reader and punch is provided in the basic PDP-11/20
system. Options for the
.PDP-11 include a paper tape reader capable of reading 300 characters
per
second, a paper tape punch with an output capacity of 50 characters
per
second, and additional
Teletype
units. Provision is made for the addition
of numerous
peripheral
devices. These include standard
DEC peripherals
as
well as other devices which will be unique to the PDP-11.

SYSTEM INTERACTION
At any point in time only one device can be in control of the bus, or be bus
master. The master communicates
with another device on the bus which is
called the slave. Usually, the established
master will communicate
with the
slave in the form of data transfers.
Full 16-bit words or 8-bit bytes of information
can be transferred
on the bus
between the master and the slave. The information
can be instructions,
addresses, or data. This type of ‘operation
occurs when the processor,
as
master, is fetching
instructions,
operands,
and data from memory,
and restoring the results into memory
after execution
of instructions.
Pure data
transfers
occur between a disk control and memory.
TRANSFER OF BUS MASTER-When
a device (other than the central processor) is capable of becoming
bus master and requests use of the bus, it is
1) to make a non-processor
transfer
of
generally for one of two purposes:
data directly to or from memory,
or 2) to interrupt
program execution
and
force the processor
to branch to a specific address where an interrupt
service routine is located.
PRIORITY STRUCTURE-When
a device capable
of becoming.
bus master
requests use of the bus, the handling of that request depends on the location of that device in the priority structure.
These factors must be considered
to determine the priority of the request;
1. The processor’s
priority can be set under program control to one of
eight levels using bits 7, 6, and 5 in the processor status register.
These three bits set a priority level that inhibits granting
of bus requests on lower levels.
2. Bus requests from external devices can be made on one of five request lines. A non-processor
request (NPR) has the highest priority,
and its request is honored by the processor
between bus cycles of
an instruction
execution.
Bus request 7 (BR7) is the next highest
--priority, and BR4 is the lowest. The four lower level priority requests
are honored
by the processor
between instructions.
When the processor’s priority is set to a level, for example 6, all bus requests on
BR6 and below are ignored.
3. When more than one device is connected
to the same bus request
(BR) line, a device nearer the central processor has a higher priority
than a device farther away. Any number of devices can be connected
to a given BR or NPR line.
Once’s device other than the processor has control of the bus, it is for one
of two types of requests: 1) NPR Request, 2) ‘Interrupt
Request.

NPR Requeata-NPR

data transfers

can be made between

any two peripheral

devices without
the supervision
of the processor.
Normally,
NPR transfers
are between
a mass storage
device, such as a disk, and core memory.
The structure
of the bus also permits
device-to-device
trat’ISfer%
allowing.
customer-designed
peripheral
controllers
to access other devices such as
disks directly.
An NPR device has very fast access to the bus and can transfer at high data
rates once it has control. The processor state is not affected by the transfer:
therefore
the processor
can relinquish
control
while an instruction
is in
Progress. This can occur at the end of any bus cycle except in between a
read-modify-write-sequence.
(See Chapter 8 for details).
In the PDP-11, an
NPR device can gain bus control in 3.5 microseconds
or less. An NPR device
in COritrOl of the bus may transfer
ldbit
words from memory
at memory
speed or every 1.2 microseconds
in the PDP-ll/EO
or every 1.0 microseconds
in the PDP-ll/lO.
IIIterrUpt
Requests-Devices
that request interrupts
on the bus request lines
(BR7, BR6, BR5, BR4) can take advantage
of the power and flexibility
of
the processor.
The entire instruction
set is available for manipulating
data
and status registers. When a device servicing program must be run, the task
currently
under way in the central processor
is interrupted
and the device
service routine is initiated.
Once the device request has been satisfied, the
processor returns to the interrupted
task.
In the PDP-11, the return address for the interrupted
routine and the processor status word are held in a “stack.”
A stack is a dvnamic
seauential
list of data with special provision for access from one end. A stack-is also
called a “push
down”
or “LIFO”
(Last-In
First-Out)
list. Storaee and retrieval from stacks is called “pushing”
and “popping”
respecti&ly.
These
operations
are illustrated
in Figure 2-1.
In the PDP-11, a stack is automatically
maintained
by the hardware for interrupt processing.
Thus, higher level requests can interrupt
the processing
of
lower level interrupt
service, and automatically
return control to the lower
level interrupt
service routines when the higher level servicing is completed.
Here is an example
of this procedure.
requests use of the bus at one of the.BR

operations

undertaken

to “service”

A peripheral
levels (BR7,

requires
service
BR6, BR5, BR4).

and
The

the device are as follows:
,E0

l.AN EMPTY
STACK

3.PlJStlING
ANOTHER
~tiW&CNTO
THE

El
El

El
5 POP

4. ANOTHER
PUSH

Fig 2-1

Illustration

PUSH

of Push and Pop Operations

Priorities
permitting,
the processor
relinquishes
the bus to that
device.
When the device has control of the bus, it sends the processor an
interrupt
command
with the address of the words in memory containing
the address
and status of the appropriate
device service
routine.
The processor
then “pushes”first, the current
central processor
status (PS) and then, the current
program
counter
(PC) onto the
processor stack.
The new. PC and PS (the “interrupt
vector”)
are taken from the location specified
by the.device
and the next location. and the device

Figure

2-2

Nested

device

Servicing

5.
6.

service routine is begun. Note that those operations
all occur automatically and that no device-polling
is required to determine
which
service routine to execute.
7.2 microseconds
is the time interval between the central Processor’s
receiving the interrupt command
and the fetching of the first inStruCtion. This assumes there were no NPR transfers
during this time.
The device service routine can resume the interrupted
process by
executing the RTI (Return from interrupt)
instrudion
which ‘ipops”
the processor
stack back into the PC and PS. This requires 4.5
microseconds
if there are no intervening
NPR’s.
A device service routine can be interrupted
in turn by a sufficiently
high priority bus request any time after completion
of its first instruction.
lf such an interrupt
occurs, the PC and PS of the device service
routine are aUtOITX3tiCally
pushed into the stack and the new device
routine initiated as above. This “nesting”
of priority interrupts
can
go on to any level, limited only by the core available for the stack.
More commonly,
this process will nest only four levels deep since
there are four levels of BR signals. An example of nested device
servicing is shown in Figure 2-2. A rough core map is given for each
step of the process. The SP points to the top word of the stack as
shown.

CHAPTER 3
ADDRESSING jdODES‘
Most data in a program is structured
in some way-in
a table, in a stack, in
a table of addresses, op perhap$ in a small set of frequently-used
variables‘
local to a limited region of a program. The PDP-11 handles these common
data structures -with addressing
modes specifically
designed for each kind
of access. In addition,
addressing
for unstructured
data is general enotfgh
to permit direct random ac%ess to all of core. Memory is not brokeri up into
pages and fields (often awkward and wasteful of core storage).

*’

Addressing
in the PDP-11 is dohe through the general registers. PrOWems
requiring
several stacks can use the general registers for stack pointers.
Those requiring many local variables can use general registers as accumulators. The general registers can be used interchangeably
as index ,registen
or as sequential list pointers to access tabularrdata.
Address arithmetic
may
be done directly in the general registers.
,

ADDRESS FIELDS
PDP-11 instruction
words contain a 6-bit address field divided
fields selecting the general register and the mode generating
address.

The register subfield specifies
used in the address calculation.
is to be used in determining
in the following paragraphs.

into two subthe effective

which of the eight general registers is to be
The mode subfield indicates how this register
the operand. These modes will be described

GENERAL REGISTER ADDRESSING-The
general registers will be used .as
simple accumulators
for operating
on frequently-accessed
variables.
In this
mode, the operand is held directly in the general register. The general registers are in iast memory, (280-nanosecond
cycle time) resulting in a speed
improvement
for operations on these variables.
PAL-11, the PDP-11 assembler,

interprets

instructions

of the form

OPR R
as general register operations.
R has been defineb as a register name and
OPR is used to represent a general instruction
mnemonic.
ihe address field
for general register operations is

Operands that are pointed to -by addresses (indirect
or deferred)
noted to PAL-11 by the @ symbol. Thus, instructions
of the form
specify

deferred

OPR @R
addressing
and have the following

address

are de*.
field.

Deferred
register
OPR (R).

addressing

INDEXED AD~RE&G-T~IZ
jr&en to per&t
random
tions of the form
specify indexed
and the contents

may also

access

in PAL-11

by the

form

general fegisters
may be used as index reg
of items in tables or stacks of data. InStrUC-

mode addressing.
of the specified

The index word containing

be selected

OPR X(R)
The effective
general- register

X follows

address
R.

the instructi&

is the

sum

of X

word.

Index mode addressing
can be deferred to permit access of data elements
through
tables or stacks of their addresses.
The address field for index deferred mode is

It is specified

by instructions

of the form
OPR @X(R)

AUTOINCREMENT
ADDRESSlNCiAutoincrement
addressing
provides
for
automatic
stepping
of a pointer
through
sequential
elements
of a table
of operands.
In this mode, the address of the operand
is taken from the
general register and then the contents
of the register are stepped
(incremented
by one or two) to address/the
next word or byte depending
upon
whether the instruction
operates
on byte or word data. Instructions
of the
*
form
specify autoincrement
dressing is

This mode

addressing.

may also be deferred.

specify deferred
address field.

autoincrement

01% (RI+
The address

field

for autoincrement

Instructions

of the form

OPR @(Wk
addressing

and assemble

AUTODECREMENT
ADDRESSlN*Autode&ement
cified general register to the next lower byte

with

ad-

the followihg

addressing
steps the spe(decrement
by one) or word

(decrement
by two) address
ister as the operand address.
specify autodecrement
dressing is

and-uses
the new contents
Instructions
of the form

-0PR -(RI
addressing.
The address

field

of the general

for autodecrement

This mode also may be deferred
and specified
by instructions
OPR @ -(R).
When deferred the address field is

STACK

reg-

ad- ’

of the form

PROCESSING

The combination
of autoincrement
addressing
in which the general register is
stepped forward after the operand address is determined
and autodecrement
addressing
in which the general
register
is stepped
backward
before the
operand address is determined
is the basic requirement
for convenient
low
overhead stack operations.
The PDP-11 has extensive
stack processing
capabilities.
The stack pointer
(SP), R6, maintains
a stack for the nested handling of interrupts.
All of the
general registers can maintain
stacks under program
control.
Elements
in
the middle of stacks may be accessed through
indexed addressing.
This
provides for convenient
access of dynamically
assigned temporary
storage,
especially useful in nested procedures.

USE OF THE PC AS A GENERAL REGISTER
There are special implications
in the use of the addressing
modes already
described when applied to the PC (R7). The use of the PC with the addressing modes described
above generates
immediate,
absolute,
relative,
and
deferred relative addressing.
IMMEDIATE
ADDRESSING-Immediate
addressing
provides time and space
improvement
for access of constant
operands
by including
the constant
in
the instruction.
The instruction
word referencing
an immediate
operand
specifies autoincrement
addressing
through
the program
counter.
The address field would be

The program counter points to the word after the instruction
word. The contents of this word are therefore
used as the operand and the program counter
is stepped to the next word. PAL-11 recognizes address expressions
of the
form “#n”
as immediate
operands
and codes them with the address field
shown above followed by a word of data (n).
A full word is assembled
so that instruction
words
ABSOLUTE

for immediate
operands
even in byte instructions
are always fetched from even locations.

ADDRESSING-The

contents

of the location

following

the instruc

tion word may be taken as the address of an operand by specifying
in immediate
mode addressing.
That is, instructions
of the form
refer to the operand at, address
this form into an address field

followed

by a word containing

A. PAL-11 assembles

address

deferral

cxprwssions

the o erand address.
P

MUTmE
ADDRESSIF&-Relative
addressing
specifies the operand address
relative to the instruction
location. This is accomplished
by using
the pc as
an index regkter.
The PC is considered
as a base address. The of&&,
the
distance
betwe’en the. location of the operand
and the PC, is held in the
index word of the instruction.
PAL-11 assembles instructions
of the form
OPR A
(where A has not been assigned as a name
instruction
word with the address field

followed

of a general

as an

by an index word of the form
k-f

OFTM*IIIID.2

DEFERRED RELATIVE ADDRESSING-Deferral
of relative addressing
permits
access to data through
memory
locations
holding operand
addresses.
The
“@I” character specifies deferred addressing:
i.e., OPR @A. The address field
for deferred relative addressing
is

USE OF THE SP AS A GENERAL REGISTER
The processor stack pointer will in most cases be the general register used
in PDP-11 stack operations.
Note that the content of SP, (SP), refers to the
top element
of the stack, that -(SP)
will push data onto the stack, that
(SP)+ will pop data off the stack, and that X(SP) will permit random access
of items on the stack. Since the SP is used by the processor for interrupt
handling,
it has a special attribute:
autoincrements
and autodecrements
are
always done in steps of two. Byte operations
using the SP in this way will
simply leave odd addresses unmodified.

DOUBLE OPFRAND ADDRESSING
Operations
which imply two operands
such as add, subtract .and compare
are presented in the PDP11 by instructions
which specify two addresses. The
instruction
word for 6uch operations
is of the form

Instruction

Word-Double

Operand

Instructions

and is followed by index words and immediate
operands
for the source and
destination
address fields as appropriate.
Source address calculations
are
performed
before destination
address calculations.
Since each operand may
be anywhere in core storage or in the general registers, each memory location
is thus effectively provided with the arithmetic
capabilities
of an accumulator.
Further, since peripheral device registers and memory location are addressed
in the same way, the contents
of peripheral
data buffers can be stored or
loaded directly to and from memory without use of any general register. This
means that interrupt
routines can be executed without saving and restoring
any of the general registers.

CHAPTER 4
INSTRUCTION SET
This chapter Presents the order code for the PDP-11. Each PDP-11 instruction is described
in terms of five parameters:
operation,
effect on condition
Special
codes, base timing, assembler
mnemonics,
and octal representation.
comments
are included where appropriate.

NOTATION
The following
(XXX)
src
dst
A
V
tf

notations will be used in this section:
: The contents of XXX
: The Source Address
: The Destination
Address
: Boolean “AND” Function
: Boolean “OR” Function
: Boolean “Exclusive OR” Function
: Boolean ‘NOT” Function (Complement)
: “becomes”
: “is popped from the stack”
: “is pushed onto the stack”

INSTRUCTION

TIMING

The PDP-11 is an asynchronous
processor in which, in many cases, memory
and processor operations
are overlapped.
The execution time for an instruction is the sum of a basic instruction
time and the time to determine
and
fetch the source and/or destination
operands.
The following table shows the
addressing
times required for the various modes of addressing
source and
destination
operands.
The instruction
time for each operation
is given
(throughout
this chapter)
for the 11/20 configuration.
All times stated are
subject to +20%
variation.
ADDRESSING FORM
(src or dst)
R
(RI or @R

TIMING
src bs)t
0
1.5
1.5

dst Wt
0
1.4*
1.4’
1.4*
2.6*
2.6*

L”c’Rf
@(W +
G?-(R)
BASE(R)
@BASE(R)
l

dst

time

none
of
t referencing

,’

::7”
2:;
3.9

or @(R)

.4 ws. less than
listed
time
if instruction
CoMPare.
CoMPare
Byte
Bit Test,
Bit Test
Byte
TeST,
or TeST
Byte
which
ever
modify
the destination
word.
bytes
at odd addresses
adds
0.6~s
to

2.6*
3.8’

was

and

dst

times.

DOUBLE OPERAND INSTRUCTIONS-Double
Operand Instructions
are represented in assembly language as:
OPR src, dst
where src and dst are the addresses of the source and destination
operands
respectively.
The execution
time for these operations
is comprised
of the
source time, the destination
time; and the instruction
time. The source and
destination
times depend on addressing
modes and are described in the pre.
ceding table.
17

Arithmetic

Operations-

Operation:

(src) + (dst)

Condition Codes:
Z: set if (src) = 0; cleared
N: set if (src) < 0; cleared
C: not affected
V: cleared

otherwise
otherwise

Description:
Moves the source operand to the destination
vious contents
of the destination
are lost. The contents
not affected.

location. The preof the source are

The MOV instruction
is a generalization
of ‘load,”
“pop,’ and interregister
transfer operations.

“store,”

“setup,”

General registers may be loaded
instructions
of the form:

with the contents

of memory

Registers
structions:

a counter,

‘push,”

addresses

with

MOV src, R
may

be loaded

iwhich
Operands

may be pushed

and may be popped
Interregister

with

MOV #n.
loads the number

and

values

with

MOV in-

MOV instruction

in the

R
n into register

onto a stack by:
MOV src, -(R)

off a stack by:
MOV (R)+,

transfers

pointer

dst

are simply:
MOV RA, RB
(RA and RB are general registers)

Memory-to-memory
general form:

transfers

may be done

with

the

MOV src, dst

2.3~

ADD WC. dsi

ADD
,

I
12

Operation:

(src) + (dst) + (dst)

Condition

Codes:

I
6

dst

I
0

Z: set if result = 0; cleared otherwise
N: set if result < 0; cleared otherwise
C: set if there was a carry from the most significant
bit
of the result; cleared otherwise
V: set if there was arithmetic
overflow as a result of the
operation,
that is, if both dperands were of the same
sign and the result was of the opposite sign; cleared
otherwise

Description:
Adds the source operand to the destination
operand and stores
the re&lt at the destination
address. The original contents-of
the destination
are lost. Ttr, ktmcI
of the sowe
are not a#ectod. Two’s cwpt
addition is perfoti.
The ADD instruction
inckdes as special cases the “add-to-register,”
memory,”
and ‘Md-reister-to-rwister”
functions:
Add-to&gister
ADD WC, R ,
ADD R, +t
A&l-to*mory
Add Register-to-Register
ADD RA. RB
Arithmetic
instruction

may also

ADD
.

src,

be done

“add-t@

directly

in memory

by the general

considerable

loadindand

storing

form

ADD

dst

Use of this

form

saves

Two special
interpreters,

cases of the ADD instruction
are particularly
and other stack arithmeti
processes:

of accumulators.
useful

in coppilers,

ADD (R +, (RI
(where R is th II stack pointer)
which replaces the top two elements ‘pf the stack with their sum; and ADD
src. (R), which increases the top eletient
of the stack by the contents
of
the source address.
The “Add Immediate”
ized .ADD iristruction:

operation

is y&t another
ADD

Immediate
that:

operations

are useful

#n,

special

in dealing

with

All these special cases of the ADD instruction
double operand instruetions
that follow.
suBtract
1

(dst) Codes:

4
Description:
leaves the
destination

I
12

Operation:
Condition

SUB
t

constant

operinds.

Note

well to the other
2.3 YI

WC, drt
WC

general-

dst *

ADD #n. R
register R (which may be an index
the need for a special “add-to-index

steps the
eliminating

case of this

drt
I

II
6

t
5

I
0

(src) + (dst) [in detail, (dst) + - (src) + 1 + (dst)]
2: set if result = 0; cleared otherwise
N: set if result < 0; cleared otherwise
C: cleared if there was a carry from the most significant
bit of the result: set othen@se
V: set if there was arithmetic
overflow as a result of the
operation,
that is, if-the operands
were of opposite
signs and the sign of source was the Same as the
sign of the result; cleared otherwise.

Subtract;
the source operand from the destination
operand
and
result at the destination
address. The original contents
of the
are lost. The contents
of the source are not affected.

I
12

.Operation:
Condition

1
6

Description:
Arithmetically
Affects
neither
operand.
appropriately.

compares
The only

Boolean
Instructions-These
double operand
arithmetic
bit level.

t
0

BIS
5

the source and destination
operands.
action
is to set the condition
codes

instructions
group. They

Bll set
,

dst

(dst) [in detail, (src) + - (dst) + 11
Z: set if result = 0; cleared otherwise
N: set if ‘result < 0; cleared ptherwise
C: cleared if there was a carry from the most significant
bit of the result; set otherwise
V: set if there was arithmetic
overflow; that is, operands
were of opposite signs and the sign of the destination
was the same as the sign of the result; cleared
otherwise.

(src) Codes:

2.3ur’

CMP rrc.dst

COMParo

have the same
permit operations

format- as the
on data at the
2.31~~

src.dst

It~IJ*I~~I’~l
12

Operation:

(src) V (dst) + (dst)

Condition

Codes:

,Z: set if resu’lt = 0; cleared otherwise
N: set if high-order
bit of result set; cleared
C: not affected
V: cleared

otherwise

Description:
Performs “Inclusive
OR” transfer
between the source and destination
operands
and leaves the result at the destination
address; that is,
corresponding
bits set in the source are set in the destination.
The original
contents
of the destination
are lost. The source is not affected.
Bit

BIC

Clear
0

Operation:

Conditions

L
12

(src)

Codes:

src,

src

2 9us

dst

t
6

dst
t

I
0

A (dst) + (dst)

Z:
N:
C:
V:

set if result = 0; cleared otherwise
set if high-order
bit of result set; cleared
not affected
cleared .

otherwise

Description:
The BIC instructi,on
clears each bit in the destination
that carresponds to a set bit in the source. The original contents
of the destination
are lost. The contents of the sources are unaffected.
*There is no read/modify/write
cycle in the CMP, BIT. and TST operations.
0.4 ws in all destination
address modes except register mode.

’

This.iaves

Bll

Test

BIT

WC
I

.2.9***

rrc,dst

I
12

Operation:

(src) A (dst)

Condition

Codes:

Z:
N:
C:
V:

dd
I

set if result = 0; cleared otherwise
set if high-order
bit of result set; cleared
not affected
cleared

otherwise

Description:
Performs logical “and” comparison
of the source and destination
operands and modifies condition
codes accordingly.
Neither the source nor
destination
operands are affected.
The BIT instruction
may be used to test whether any of the corresponding
bits that are set in the destination
are also set in the source or whether
all corresponding
bits set in the destination
are clear in the source.
Note.that

the operations

of BIS, BIC, and BIT are parallel

in that the same

mask may be used to set, clear and test the state of particular
BRANCHES-Branches

have the instruction

Opwotion
I

operation

exx IOC
code

Instruction

Time

offs*1

bits in a word.

format

I,1
6

II’1

The offset is treated as a signed two’s complement
displacement
to be multiplied by 2 and applied to the program counter. The program counter points
to the next word in sequence. The effect is to cause the next instruction
to
be taken from an address, “lot”,
located up to 127. words back (-254
bytes) or 128 wordsahead
(+ 256 bytes) of the branch instruction.
PAL-11
gives an error indication
in the instruction
if “lot” is outside this range.
The PDP-11 assembler
handles address arithmetic
for the user and computes and assembles the proper offsets field for branch instructions
in the
form
Bxx
lot
where lot
instructions

is the address to which the branch
have no effect on condition codes.

Unconditional

Branch-

BRbnch

Wncondilionol)

lOI

I Ol

I4
6

I
0

lot + (PC)

Description:

Provides

range

a one word

struction

The branch

2.6~s

IOC

Operation:

is to be made.

with

a way of transferring
instruction.

program

The execution

time (2.6~s) for the operation.
21

control
time

within

is equal

a limited
to the

in-

gbnpk corldttbnrit
Bran&es-Conditioned
branches combine
tion a conditional sMp, unconditional branch sequence.

in one instruc-

Timing for the conditional
dition is not met, followed
a program branch occurs).

branches is shown as execution time if the con-.
by the execution time if the condition is met (end

Branch on Eauol(z.ro)
0 ,

BEQIOC
I I I

I 4 I

Operation:

~.SILS,~,~.~ILS

offrrt

lot + (PC) if Z = 1

Description:
Tests the state of the Z-bit and causes a. branch if Z is set. It
is used to test equality following a CMP operation,
to test that no bits set
in the destination
were also set in the source fdllowing a BIT operation,
and
generally, to test that the result of the previouq operation was zero.
Thus the sequence
CMP
BEQ

A,B
C

; compare A and B
; branch if they are equal

will branch to C if A = B
and the sequence

(A -

ADD
BEQ

A.B
C

B A 0)
; addAtoB
; branch if the result = 0

will branch to C if A + B = 0.
Branch
0

on Not
1

EqualGk~)

BNE

1.5113.2.6

IOC

I0 I

Operation:

offset

III

ILS

I
0

lot + (PC) if Z = 0

Description:
Tests the state of the Z-bit and causes a branch if the Z-bit is
clear. BNE is the complementary
-operation
to BEQ. It is. used to test inequality following a CMP, to test that some bits set in the destination
were
also set in the source, following a BIT end, generally, to test that the result
of the previous operation was not zero.
Branch

on Minus

BMI

IQC

I 0 III

Operation:

1.5u.s,

Ill

2.6~

offsri

lot --, (PC) if N = 1

Description:
Tests the state of the’N-bit
and causes a branch if N is set. It
is used to test the sign (most significant
bit) of the result of the previous
operation.
Branch
1
15

6PL

on PLUS
I

0
-.

IOC

t.5 AU,

II 0
B

I
7

0ffS.t

2.61~

1
0

Operation:

lot + (PC) if N = 0.

Description:
Tests the state of the N-bit and causes
BPL is the complementary
operation to BMI.
Branch

on Carry

Set

BCS

lot

if N is,clear.

1.5~s

Operation:

a branch

,Z.~ALS
I

lot + (PC) if C = 1

Description:
Tests the state of the C-bit and causes
is used to test for a carry in the result of a previous
Bmnch

on Carry
,

Clear

BCC
I

lot + (PC) if C’=

IOffset III

on overflow

I
0

Description:
Tests the state of the C-bit and causes
BCC is the complementary
operation to BCS.
Branch

if C is set. It

t.5 U.S. 2.61~5

IOC

Operation:

a branch
operation.

a branch

1.5us,2.6us

BVS IOC

set

if C is clear.

offset
1 ,
15

Operation:

I 2

I
0

lot + (PC) if V = 1

Description:
Tests the state of the V-bit (overflow)
the V-bit is set. BVS is used to detect arithmetic
operation.
eranch
1

on
I

Overflow
0

clear

121

and causes a branch. if
overflow in the previous

1.5us.2.61~~

IOC

oftset

1 I
0

Operation:

lot + (PC) if V = 0

Description:
Tests the state of the V-bit and causes
clear. BVC is the complementary
operation to BVS.

a branch

if the V-bit is

Signed Condiiional
Branches--Particular
combinations
of the condition
code
bits are tested with the signed conditioned
branches. These instructions
are
used to test the results of instructions
in which the operands were considered as signed (two’s complement)
values.
Note that the sense of signed comparisons
differs from that of unsigned
comparisons
in that in signed
16-bit, two’s complement
arithmetic
the
sequence of values is as follows:

largest

.. ......... ....

. . . . . . . . . . . . 077777
077776
.
.
.

positive

000001
000000
177777
177776
negative
lQo00l
smallest
whereas

in unsigned

100000

16-bit arithmetic
highest

the sequence

is considered

to be .

. . . . ~. . . . . . . . . . . . . . . . . . . . . . . . . 177777
.

000002

00000 1
_. .._. . . . _.. ._ ,. .___, 000000

lowest
Branch

on Less

BLT

ThadZero)
0

l.Sir.

lot
II

offset
I
I

2.6~

lot + (PC) if N V V = 1

Operation:

Description:
Causes a branch if the “Exclusive OR” of the N- and V-bits are’
1. Thus BLT will always branch following an operation
that added two neg
ative numbers, even if overflow occurred.
In particular,
BLT will always cause a branch if it follows a CMP instruction
operating
on a negative source and a positive destination
(even if overfloti
occurred).
Further,
BLT will never cause a branch when it follows a CMP
instruction
operating
on a positive
source and negative
destination.
BLT
will not cause a branch if the result .of the previous operation
was zero
(without overflow).
Branch
0

on Greater
1

than
I

1.5 rrs.2.6~~

lot
offset

Operation:

BGE

or Equal (Zero)

I
0

lot + (PC) if N V V = 0

Description:
Causes a branch if N and V are either both clear or both set.
BGE is .the complementary
operation
to BLT. Thus BGE will always cause
a branch when it follows an operation
that caused addition
to two positive
numbers.
BGE will also cause a branch on a zero result.

Branch

on Less

than

01 to,

Equal(Z*ro)

BLE

1.5~~.

lot

I ‘31 14

offset
t
I

2.6~
I

I
0

Operation:

lot + (PC) if Z v (N V V) = 1

Description:
Operation
of BLE is similar to that of BLT but in addition
cause a branch if the result of the previous operation
was zero.
Branch

on Greater

Than

0,
15

(Zero)

BGT

I 3 I

Operation:

1.51u,2.6u~

lot

II 0

will

offset
1

’

”

lot -+ (PC) if Z v (N tt V) = 0

Description:
Operation
of BGT is’ similar
cause a branch on a zero result.

Unsigned Conditional
Branches-The
vide a means of testing the result
operands are considered as unsigned
Branch
1

on Hlgher
,

that

BGT will not

Unsigned
Conditional
of comparison
operations
values.

Branches
proin which the

l.Sur,2.6ur

BHI

1 I

offset

I ‘0 11.1

‘t 5

Gperation:

to BGE, except

I
0

lot + (PC) if both C and Z = 0

Description:
Causes a branch if the previous operation
caused neither a carry
nor a zero result. This will happen in comparison
(CMP) operations
as long
as the source has a higher unsigned value than the destination.
Branchon Lower of same
I,0

BLOS

I'
8

Operation:

1.5~.

lot

offset

2.61~s

1 1
0

lot + (PC) if C v Z = 1

Description:
Causes a branch if the previous operation
caused either a carry
or a zero result. BLOS is the complementary
operation
to BHI. The branch
will occur in comparison
operations
as long as the source is equal to, or has
a lower unsigned value than, the destination.
Comparison
of unsigned
values with the CMP instruction
can be tested for
“higher or same” and “higher”
by a simple test of the C-bit. For convenience,
the mnemonics
BHIS (Branch on Higher. or Same) and BLOS (Branch on
Lower Or Same) have been defined such that BHIS = BCC and BLO = BCS.
Bmnch
t

on Higher
1

Q Same

Operation:
Description:

BHIS

IQC

1.5~.

-8

2.6~

I
0

lot + (PC) if C = 0
BHIS is the same instruction

offset

as BCC

’

Branch

BLO

on LOwn

t.5 YS, 2.6~s

IOC
Offset

t 1
95

Operation:
Description:

1 4
e

1.1

lot + (PC) if C = 1
BLO is the same instruction

as BCS

q The following
example
illustrates
the use of some of the instructions
and
addressing
modes described
thus far. Two new instructions
are used: INC
(INCrement)
and ASL (Arithmetic
Shift Left) which respectively,
add 1 (INC)
and multiply an operand by 2 (ASL). Their operation
is fully described later
in this chapter.
This example demonstrates
the generation
of a table (histogram)
that shows
the frequency
of occurrence
of each value in another table (within a range
of values l-100).
Histogram
generation
(including
initialization)
requires
22 words. Values outside the range l-1OB are ignored.
HIST:

;set up to clear output table
;lOO entries in output table
;clear next entry
;check if done
;if not, continue clearing
;set up input pointer
;length of table
;max input value
;get next input value
;ignore if less than or equal zero
;check against max value
;ignore if greater
;2 bytes per table entry
;increment
proper element
;input done?
;if not, continue scanning
;histogram
complete

MOV #OTABLE, RO
MOV #-loo.,
Rl
‘yg p+

CLOOP:

HLOOP:

NOCOUNT:

BNE CLOOP
MOV #ITABLE, RO
MOV # -lOOO., Rl
MOV #lOO., R2
MOV (RO)+, R4
BLE NOCOUNT
CMP R4, R2
BGT NOCOUNT
ASL R4
INC OTABLE (R4)
INC- Rl
BNE HLOOP
HALT

The JUMP Instruction-JMP
(JUMP) provides more flexible program branching then is provided with the branch instructions.
Control may be transferred
to any location in memory
(no range limitation)
and can be accomplished
with the full flexibility of the PDP-11 addressing
modes.
JMP

JUMP
01

‘0

I0 I

1.2US

dst

I 1 I

I
6

Operation:

dst + (PC)

Conditioned

Codes: not affected

I
5

dst

1
0

Description:
Register mode is illegal in JMP instructions
and will cause an
“illegal instruction”
condition.
(Program control cannot be transferred
to a
register.)
Register deferred mode is legal and will cause program control to
be transferred
to the address held in the specified register. Note that instructions are word data and must therefore
be fetched from an even-numbered

address.
A “boundary
error”
condition
will result
tempts to fetch an instruction
from an odd address.
Deferred
address

when

the

processor

at-

index mode JMP instructions
permit transfer
of control
to the
contained
in a selectable
element
of a. table of dispatch
vectors.

SUBROUTINES-The
subroutine
call in the PDP-II
provides for automatic
nesting of subroutines,
regntrancy,
and multiple
entry points. Subroutines
may call other subroutines
(or indeed themselves)
to any level of nesting
without
making special provision
for storage of return addresses
at each
level of subroutine
call. The subroutine
calling mechanism
modifies no fixed
location in memory and thus also provides for reentrancy.
This allows one
copy of a subroutine
to be shared among several interrupting
processes.

I 4

-Q

I,
6

dsi

I,
6

Operation:

dst + (tmp)
Oes) &
(W -+ (rea)
_ (tmp) + Up

(tmp.is an internal
(push reg contents
(PC holds location
now put in reg)

Condition

Codes: not affected

I,,
0

processor register)
onto processor stack)
following JSR; this address

Description:
Execution time for JSR is the sum of instruction
and destination
times. In execution
of the JSR, the old’contents
of the specified-register,
(the “linkage
pointer”),
are automatically
pushed onto the ‘processor stack
and new linkage information
placed in the register. Thus subroutines
nested
within subroutines
to any depth may all be called with the same linkage
re&ter.
There is no need either to plan the maximum
depth at which any
particular
subroutine
will be called or to include instructions
in each routine
to save and restore the linkage pointer. Further, since all linkages are saved
in a reentrant
manner-on
the processor stack-execution
of a subroutine
may be interrupted,
the same subroutine
reentered
and executed
by an
interrupt
service routine.
Execution
of the initial subroutine
can then be
resumed when other requests are satisfied. This process (called nesting) can
proceed to any level.
A subroutine
called with a JSR reg, dst instruction
can access the arugments
following
the call with either autoincrement
addressing,
(reg) +, (if arguments are accessed sequentially)
or by indexed addressing,
X(reg), (if accessed in random
order). These addressing
modes may also be deferred,
@(reg)+
and @X(reg)
if the parameters
are operand addresses rather than
the operands themselves.
JSR PC, dst is a special case of the PDP-11 subroutine
call suitable
subroutine
calls that transmit
parameters
through
the general registers.
register except the program counter is modified
by this call.

for
No

Another special case of the JSR instruction
is JSR PC, (SP)+ which exchanges the top element
of the processor
stack and the contents
of the
program counter.
Use of this instruction
allows two routines to swap program control and resume operation
when recalled where they left off. Such
-routines
are called “co-routines.”
Return from a subroutine
is done by the RTS instruction:
RTS reg loads the
&tents
of the reg into the PC and pops the top element of the processor
stack into the specified register.

ReTurn

from

3.5&S

Sutwoutine

Operation:
Condition

(reg) + (PC)
T (ret3
Codes: not affected

Description:
Loads content of reg into PC and pops the top element of the.
Processor stack into the specified
register. Execution time for RTS is equal
to the basic instruction
time.
Return from a subroutine
is typically made through
the same register that
was used in its call. Thus, a subroutine
called with a JSR PC, dst exists
with a RTS PC and a subroutine
called with a JSR R5, dst, picks up parameters with addressing
modes (R5)+,
X(R5);or
@X(R5)
and finally exists
with a RTS R5.

Programming

Examples

of the Use of Subroutines-

1. Passing
arguments
in subroutine
calls-The
subroutine
TOLER
checks each element
in an array of unsigned
integers to determine
whether
any elements
are outside specified
limits. If all are within
tolerance,
the value 0 is returned
in the register RO. If TOLER find
an element
out of tolerance,
it returns the address
of the bad
element + 2 in RO. The calling sequence for TOLER is:
JSR R5, TOLER
. WORD ARRAY

‘I

. WORD -LENGTH
. WORD HILIM
. WORD LOLIM
;Tolerance Check-Array Elements
TOLER: ,
MOV (R5)+,
MOV (R5)+,
MOV (R5)+,
MOV (R5)+,
TLOOP:
MOV (RO)+;
CMP R4, R2
BHI TEXIT
CMP R4, R3
BLO TEXIT
INC Rl
;
TEXIT:

BNE TLOOP
CLR RO
RTS R5

Within
RO
Rl
R2
R3
R4

.
;address
of array ts be
:checked
(*WORD expresision-defines
a word equal
;to the value of the expres;sion)
;minus # of items in array
;upper limit of tolerance
;lower limit of tolerance
;subroutine
returns here
Limits?
;get array address
;get minus the length
:get high tolerance
limit
;get low tolerance
limit
;get next element of array
;check it against high limit
;leave routine if higher
;check it against low limit
;leave routine if lower
;increment
count,
check
;whether
at end of array
;contihue
if not at end yet
;exit with RO = 0 if all ok
;return,
RO holds
poirter
:or 0

The instruction
INC Rl increases
CLR RO zeroes the register RO

the contents

of Rl by 1 and the instruction

2. Saving and restoring registers on the stack-This
RO-R5 onto the stack. It is called by:
JSR R5, SAVE
MOV R4, -(SP)
MOV R3, -(SP)

SAVE:
\

MOV R2, -(SP)
MOV Rl, -(SP)
MOV RO, -(SP)
JMP

subroutine

pushes

;R5 was pushed by the JSR
;R5 will be at the bottom
;of the stack
;R4, R3, R2, Rl, and RO
;in order
;will be above it
;RO is at the top of the
;stack
;R5 holds ‘the return
ad;dress

The TST operation

is equivalent
to comparing
the operand
TST opr = CMP opr, #0
The only effect is to set the appropriate
condition codes.

with 0, i.e.,

to restore

RO-R5 from

The following
stack.
REST:

example

illustrates

a subroutine

TST (SP) +
MOV (SP)+,
MOV (SP)+,
MOV (SP)+,
MOV (SP)+,
MOV (SP)+,
RTS R5

RO
Rl
R2
R3
R4

the

;this increments the SP by 2
;the registers are restored
:in reverse order to that in
which
;they were put on the stack
;R5 is loaded into the PC
and the old R5 restored

The operation
TST (SP)+ removes the top element on the stack. At the time
it is used, the top element holds the contents of R5 that were saved by the
call to REST. Since R5 is to be loaded with the value saved on the stack
by SAVE, this information
is not needed.
3. Stacks, recursion,
and nesting -The
following
subroutine
converts
an unsigned binary integer to a string of typed ASCII characters.
In
the routine, the remainders
of successive divisions by 10 are saved
and then typed in reverse order.
The operation
of the subroutine
is to call a part of itself (beginning with DECREM) repeatedly
until a zero quotient
is calculated
by an integer divide subroutine,
IDIVR. At each iteration, the dividend
is divided by 10, the resulting quotient
replaces the dividend,
and
the remainder
is pushed onto the processor stack. The processor
stack thus holds interleaved
data (remainders)
and control information (return addresses
from calls to DECPNT and DECREM) when
the quotient
finally comes up as 0 and the branch
is made to
DECTIY. The portion of the routine beginning
at DEClTY then pops
a remainder
from the stack, converts it to an ASCII character, types
it and then returns control to DECTTY (with RTS PC) until the stack
is reduced finally to its state immediately
after the call to DECPNT.

At this
program.

point

execution

RTS

PC returns

control

the

main

A character
is typed in DECTY by loading
the teleprinter
buffer
(TPB) and waiting for the teleprinter
READY flag, the most significant
bit of the low-order byte of the teleprinter
status word (TPS),
to be set.
The symbols
CR and
sentations
for carriage

LF are assumed
equal to the
return and line feed respectively.

This’subroutine
types the unsigned
the use of stacks.
,

DECPNT:
DECREM:

integer

MOV #lo.,
R2
JSR PC, IDIVR

in RO. It illustrates

ASCII

repre-

recursion

and

;set up divisor of 10
;subroutine
divides (RO) by

;W’)
MOV

Rl,

-(SP)

TST RO
BEQ DECTTY

l-rYOUT:

JSR PC, DECREM
MOV (SP)+,
RO
ADD #60,
RO
MOV RO, TPB

l-rYLUp:

TST TPS

DECTTY:

BPL

’

ll-YLlJP

CMP. #CR,

BEQ TI-YLF
RTS PC
T-IYLF:

MOV ‘#LF, TPB
BR TTYLUP

;quotient
is in RO, remain;der is in Rl
;after
pushing
remainder
;onto stack test quotient
;if the quotient
is 0, we‘re
;done getting remainders
;if not try again
;get next remainder
;make an ASCII character
;type the ASCII character in
;RO
;wait for the teleprinter
to
;be done
;TPS is negative
when the
;TP is done
;was the character of a car;riage return
;if not: return, if so; get a
;line feed
;returns
either to DEClTY
;or main program
;type a line feed
;and wait for it to be corn;pleted

4. Multiple entry points-In
the example that follows, the subroutines
described
above are used to type out all the entries in a table of
unsigned
integers. that are not within specified tolerance.
The subroutine
TOLER is entered at TOLER for initialization
and at
TLOOP to pick up each bad entry of the array after the first one.
’

The subroutine
DECPNT is entered at DECPNT to print the value of
the unsigned binary number held in RO and at TlYOUT to print the
ASCII character
held in RO. TTYOUT prints the carriage return, line
feed sequence when it sees the carriage return character.
This routine
The program

types
starts

all out-of-tolerance
at TYPOUT.

elements

of an integer

array.

TYPFIN:

;suspend
processor
opera;tion, wait for key continue
;get address
of bad item:
;initialization
entry
;address of array
;-length of array
;high limit
;low limit
;Z-bit is set if no more out
;of limits
;an element is out of limits,
;save registers
;RO holds address + 2, get
;operand into RO
Tprint out number
;type CR, LF
mote use of second entry
;point
;restore registers
;continue
searching
array,
;alternate entry
;another
bad element?
Operand
Instructions
are repre-

HALT
JSR R5, TOLER

TYPOUT:

. WORD ARRAY
. WORD -LENGTH
. WORD HILIM
. WORD LOLIM
BEQ TYPFIN

TYPCHK:

JSR R5, SAVE
MOV -(RO),

JSR PC, DECPNT
MOV #CR,
RO
JSR PC, TTYOUT
JSR
JSR

R5, REST
R5, TLOOP

BR TYPCHK
SINGLE OPERAND
sented as:

INSTRUCTIONS-Single

OPeRotion

OPR
operation

I’

dst

Instruction

code

Time

dst

I
6

5’

I
0

The execution
time for single operand
instructions
is the sum of the
instruction
time and destination
a$dress time for the operation.

General

OperationsCLMR
0

CLR
1

2.3~

101

Operation:

0 + (dst)

Condition

Codes:

Description:

Z:
N:
C:
V:

destination.
2.3~

Operation:

(dst) + 1 + (dst)

Condition

Codes:

Description:

drt
I

I
6

Z:
N:
C:
V:

1
0

Hcdst

set
cleared
cleared
cleared

Zeroes the specified

INCrement

-I

set if the result is 0; cleared otherwise
set if t&e result is < 0; cleared otherwise
not affected
set if (dst) held 077777; cleared otherwise

Adds 1 to the contents

of the destination.
31

basic

DECrment
01

DEC drt

10,

15‘

2.3~

13,

I
6

‘0

Operation:

(dst) -

1 + (dst)

Condition

Codes

2: set if the result is 0; cleared otherwise
N:-set if the result is < 0;cleared
otherwise
V: not affected
C: set if (dst) was 100000; cleared otherwise

Description:

Subtracts

1 from

the contents
NEG

NEGate
1

of the destination.
2.3~

dst

I 5 t

I4 I

drt

Operation:

(dst) + (dst)

Condition

Codes: as in SUB dst, #0
z:
set if the result is 0; cleared otherwise
N:
set if the result is < 0; cleared otherwise
c:
cleared if the result is 0; set otherwise
v:
set if the result is 100000; cleared otherwise

Description:
Replaces the contents
of the destination
address by their two’s
complement.
(However,
100000.
is replaced by itself-in
two’s complement
notation the most negative number has no positive counterpart.)
To-ST
0

TST
1

dst

t 5 1. I

2.3~s

, 7 I

I
6

dst

I’1

I
0

Operation:

0 -

(dst)

Condition

Codes: as in CMP #0, dst
Z: set if the result is 0; cleared otherwise
N: set if the result is < 0; cleared otherwise
C: cleared
V: cleared

Description:
Sets the condition
codes
the destination
address.
COMplement
COM dst

Z and N according

to the contents
2.31~

dst
01

IO,

15,

I’,

Operation:

Condition

Codes:

I
0

(dst) + (dst)
Z: set if result is 0; cleared otherwise
N: set if most significant
bit. of result
wise
C: set
V: cleared

set; cleared

other-

Description:
Replaces:. the co.nten)s. of tl$’ destination
address
by their
logical complement
(each: bit .equal- to. 0 is .set.and
each bit e&al to1
is
cleared).
l

See

the

note

for

the

CMP

instruction.

Multiple Precision Operations-It
is sometimes
on operands considered
as multiple words.‘The
vision for such operations
with the instructions
(SuBtract Carry).
ADC dst
ADd carry
01

IO,

convenient

to d6 arithmetic
proADC (ADd Carry) and SBC

PDP-11 makes special
2.3YS
dst
1

1
6

Operation:

(dst) + (F) + (dst)

Condition

Cobes:

I
0

Z: set if result = 0; cleared
N: set if result < 0; cleared
c: set if (dst) was 177777
wise
V: set if (dst) was 077777
wise.

otherwise
otherwise
and (C) was 1; cleared

other-

and (C) was 1; cleared

other-

Description:
Adds the contents of the C-bit into the destination.
This permits
the carry from the addition of the two low-order words to be carried into the
high-order result.
Double
quence:

precision

addition

ADD AO, BO
ADC Bl,
ADD Al,Bl
suatract Carry
0

,..

may
;
;
;

be done

with

the

SEC
5

dst

Condition

Codes:

se-

2.3~

dst
I

I
6

(dst) -

instruction

add low-order parts
add carry into high-order
add high-order parts

Operation:

following

otherwise

< 0; cleared

otherwise

C: cleared if the result is 0 and C = 1; set otherwise
V: set if the result is 100000; cleared

Description:

Subtracts

the contents

of the

C-bit

permits the carry from the subtraction
of two
tracted from the high-order part of the result.
Double precision

subtraction

from

otherwise

the-destination.

low-order

wdtds

This

to be sub-

is done by:-

SUB AO, BO
SBC Bl
SUB Al, Bl
Double precision

negation

NEG BO
SBC Bl
NEG Bl

Rotates-Testing
are aided

with

ROL (ROtate
appended

is accomplisheg

low-order

part; sets C unless BO = 0

;makes
;negate

“NEG Bl”
high-order

= “COMB
part

of sequential
rotate

Left)

bith:

;negate

bits of’s

operations.

The

cause

the C-bit

to the destination

operand

Bl”

word and detailed
instructions

in circular

bit shifting.

BO = 0

bit manipulation

ROR (RDtate

of the status

unless

Right)

and

to be effectively

ROtaA

ROR

Right

2.3~

dst
dst

01
15

Condition

lOI

I
6

Codes:

Z: set if all bits of result = 0; cleared otherwise.
N: set if the high-order
bit of the result is set; cleared
otherwise
C: loaded with the low-order bit of the destination
V: loaded with the Exclusive OR of the N-bit and C-bit
(as set by the completion
of the rotate operation).

Description:
Rotates all bits of the destination
right one place. Bit 0 is loaded
into the C-bit of the status word and the previous contents
of the’C-bit
are
loaded into bit 15 of the destination.
’
ROtato
0

ROL

Left
1

2.3~

dst

16,

I 1I

Condition

I
6

Codes:

1 I

dst

1 I

1
0

Z: set if all bits of the result word = 0; cleared otherwise
N: set if the highlorder
bit of the result word is set;
cleared otherwise
C: loaded with the high-order bit of the destination
V: loaded with the Exclusive OR of the N-bit and C-bit
(as set by the completion
of the rotate operation)

Description:
Rotates all bits of the destination
left one place. Bit 15 is loaded
into the C-bit of the status word and the previous contents
of the C-bit are
loaded into bit 0 of the destination.
SWAO

Bytes

10,

SWAB

10,

2.3~

dst
dst

Condition

131
6

Codes:

I
0

Z: set if low-order byte of result = 0; cleared otherwise
N: set if high-order
bit of low-order byte (bit 7) of result
is set: cleared otherwise
C: cleared
V: cleared

Description:
Exchanges high-order
byte and low-order
word (dst must be a word address).
Shifts-Scaling
data by factors of 2 is accomplished
ASR-Arithmetic
Shift Right
ASL-Arithmetic
Shift Left
The sign bit (bit 15) of the operand
low-order bit is filled with 0 in shifts
are lost.

byte of the destination
by the shift

instructions:

is replicated
in shifts to the right. The
to the left. Bits shifted out of the C-bit

/
34

Arithmetic

Shift

ASR

Right

2.3 us

dst
dst

Condition

I
6

Z: set if the result = 0; cleared otherwise
N: set if the high-order
bit of the result is set; cleared
otherwise
C: loaded from the low-order bit of the destination
V: loaded from the Exclusive OR of the N-bit and C-bit
.(as set by the completion,of
the shift operation)

Codes:

Shifts all bits of the destination
right one ‘place. Bit 15 is rePlicated. The C-bit is loaded from bit 0 of the destination.
ASR performs signed
division of fhe destination
by 2.

Description:

Arithmetic
01

Shift
,

dst

1
6

Condition

2.31~

ASL dst

Left

Codes:

I
0

Z: set if the result = 0; cleared otherwise
N: set if the high-order
bit of the result is set; cleared
otherwise
C: loaded with the high-order bit of the destination
V: loaded with the Exclusive OR of the N-bit and C-bit
(as set by the completion
of the shift operation)

Description:
Shifts all bits of the destination
left one place. Bit 0 is loaded
with a 0. The C-bit of the status word is loaded from the most significant
bit
of the destination.
ASL performs
a signed multiplication
of the destination
by 2.
Multiple

precision

shifting

is done with a sequence

of shifts

and rotates.

Double Precision Right Shift:
ASR Al;
low-order bit of Al to C-bit
ROR AO; C-bit to high-order bit of.AO
Double Precision Left Shift:
ASL AO;. high-order bit of A0 to C-bit
ROL Al; C-bit to low-order bit of Al
Normalization
of operands
(scaling of the operand
until the operand taken
as a 15-bit fraction with sign is in the range l/e < operand
< l/e) proceeds as follows:
NORM:

ASL

BEQ

NFtNy.3 i ’
: 1 “‘ 1 ?
NDONE:

BVC
ROR
BR
ROR
,ASR
. . .

A
NFIN
NORM
A
NDONE
‘A ”
A,

; shift O’s into low-order bit
; if the result is 0, the operation
; complete
’ ; if the sign did not change, continue
;restore the sign
; normalization
complete
: &store the sign: 000000 or 100000
f atiiYmplicate
R?OOOOOO or 140000

Double

precision

norma’lization
ASL
ROL

DNORM:

BVC
ROR
BR \
TST
BNE
ROR
ASR
:. .

DNDONE:

similarly:

; double

A0
Al
DZERO
DNORM
Al
DNDONE
A0
DN0RM
Al
Al

BEQ

DZERO:

proceeds

precision

left shift

; high order result O?. if so, check.10~
; if the sign did not change, continue
; restore the sign
; normalization
complete
; low order zero, too?
; if not, continue normalization
; restore the sign; 000000 or 100000
; and replicate it; 000000 or 140000

The following example illustrates the use of shifts and rotates in a 16-bit unsigned
integer
multiply
subroutine.
Access of operands
through
address
parameters
following
the subroutine
is also shown. The multiplication
takes
115170
&s in in-line code. The entire subroutine
as shown below takes
approximately
200 ps and requires
16 words. The calling sequence
is JSR
I
R5, MULT.
. WORD MCAND
. WORD MPLIER
. WORD PROD
CLR RO
MOV @ (R5) +, Rl
MOV @ (R5) +, R2
MOV #-169
R3
ASL RO
ROL Rl
BCC NOADD
ADD R2, RO
ADC Rl
INC R3
BNE MLOOP
MOV (R5) +, R2
MOV RO, (R2) +
MOV Rl, (R2)
RTS R5

MULT:

M LOOP:

NOADD:

; address
; address
;.address

of multiplicand
of multiplier
of product

; get multiplier
into Rl
; get multiplicahd
into R2
; set counter
; double prec shift
; shift and add multiply
; most significant
bit governs add
; if set add in multiplicand
; keep 32-bit product
; done?
; if not continue
; get address to store prod.
; put low-order away, move to high
; put high-order away
; return to calling program

Bni
OPERATIONS--The
PDP-11 processor
includes a full complement
of
instructions
that manipulate
byte operands.
Addressing
is byte-oriented
so
that instructions
for byte manipulation
are straightforward.
In addition,
byte
instructions
with autoincrement
or autodecrement
direct addressing
cause
the specified register to be stepped by one to point to the next byte of data.
Byte operations
in register mode access the low-order byte of the specified
register. These provisions enable the PDP-11 to perform as either a word or
byte processor.
Timing of byte instructions
is the same as for word instructions
except
an additional
0.6 ps is required for access of bytes at odd addresses.
Double

Operand
Move
1

Byte InstructionsMOVB

!sytr
,

WC, dst

2.3~

I
t2

dst
I

I’,
6

-36

I
5

I
0

that

Operation:

(src) + (dst)

Condition

Codes: Set on the byte result as in MOV

Description:
Same as MOV instruction.
The MOVB instruction
in register mode
(unique among byte operations)
extends the most significant
bit of the byte
register (sign extension).
Otherwise MOVB operates on bytes exactly as MOV
operates on words.
Coware Byte
‘I

CMPS
12

-I
12

src,dst

src
I

2.3us*
dst

1
6

Operation:

(src) -

Condition

Codes: Set on the byte result as in CMP

Description:

I
0

; in detail (src) + -

(dst)

(dst) + 1

Same as CMP instruction.

811Set Byte

BISB

2.3~

src,dst
dst

SW
t1

,s,-

Operation:

(src) V (dst) + (dst)

Condition

Codes: Set on the byte result as in BIS

Description:

I
0

Same as ,BIS.
,

Bll CIm3rByte

BICB

2.31~

crc .dat
SK

114,

I
12

Operation:

Condition

Codes: set on the byte result as in BIC

Description:
Bit

(src)

dst

(dst) i, (dst)

Same as BIC.
Tart

Byte

BIT6
3,

m,drt
UCI

2.31~~
1

drt
I

Operation:

(src)

Condition

Codes: Set on the byte result as in BIT

Description:

I
0

(dst)

Same as BIT.

The following
subroutine
scans a packed character
string of variable length
lines, removes blanks and unpacks the string to left-justified
length lines.
INSTRING is the address of the INput STRING, OUTSTRING
is the address
of,the
OUTput String. EOLCHAR. SPCHAR.‘and
EORCHAR are the End Of
Line CHARacter,
Space CHARacter,
and End of Record CHARacter
respectively.
* See

the

note

for

the

CMP

instruction.

LNLINE

is the Length

EDIT:

of uNpacked

LINES. The routine

MOV # INSTRING, RO
MOV #OUTSTRING,
Rl
MOV #EOLCHAR,
R2
MOV #SPCHAR, R3
MOV #LNLINE,
R4
MOVB (RO) + ,R5
CMP R5, R2
BEQ FILINE
CMP R5, R3
BEQ NXTCHR
DEC R4
MOVB R5, (Rl) +
BR NXTCHR
CLRB (Rl) +
DEC R4
BNE FILINE
CMPB (RO), #EORCHAR
BNE NULINE

NOLINE:
NXTCHR:

FILINE:

CHKEND:

Single Operand
CLeaR

requires

24 words.

; set up input byte pointer
; set up output byte pointer
; put high use constant in reg.
; to save time in loop
; R4 holds # char left in line
; get next byte
; end of line?
; if yes, fill line
; blank?
; if yes, skip character
; decrement
# of characters
left in line
; move byte to output string
; continue
; put a blank byte in output
; decrement
# char left
; continue if not end
; end of record?
; if not EOR, start next line

Byte Instructions-

Byte

CLRB

dst

2.3~
dst

‘I

lOI

15,

rot

Operation:

0 + (dst)

Condition

Codes: Set on the byte result

Description:

I
0

as in CLR

Byte

‘I

INCB

101

15,

2.3~

dst

121

Operation:

(dst) + 1 + (dst)

Condition

Codes: Set on the byte result as in INC

Description:
byte.

Same as INC. The carry from

OECrcmcnt
1

a byte does not affect

any other

2.3~
dst

I 3 I

I
6

dst

OECBdst

Byte

bperation:

(dst)

Condition

Codes: Set on the byte result as in DEC

Description:

Same as CLR

INCrement

1 + (dst)

Same as DEC.

I
0

~.
NEGote

Byte

NEGB

23ns

-dst
dst

‘I

101

Ii,

141

;

(dst) f. 1 + (dst)

-(dst)

Condition

Codes: Set on the byte result as NEG

Description:

+ (dst)

in detail, -

Operation:

Same as NEG.

T&T

TSTB

Byte

2.3~(1*

dst
dst

‘I
15

lOI

17,
6

Operation:

0 -

Condition

Codes: Set on the byte result as TST

101

COMB

151%

dst
I

2.3~

I’,

Operation:

Condition

Codes: Set on the byte result as COM

dst

(dst) + (dst)

Description:

Same as COM.

AOd

Carry

‘I

Byte

ADCB

lOI

dst

2.3~

15,

dst

15
‘\

(dst)

Description:
Same as TST.
cohtp*m(Hlt Byte
‘I

I
6

Operation:

(dst) + (C) + (dst)

Condition

Codes: Set on the byte result as ADC

Description:

Same as ADC.

SuBtract
I

Carry

Byte

iBC0

15,

dst

2.3~~

I 6 I

I
6

Operation:

(dst) -

Condition

Codes: Set on the byte result as SBC

Description:

dst

Same as SBC.

ROtate

R’ght

Byte

RORB

2.3u+

dst

I
‘I

10,

16,

dst
I

,“I

I
6

Operation:

as in ROR.on byte operands

Condition

Codes: Set on the byte result as ROR

Description:
l

See

the

note

Same as ROR
for

the

CMP

instruction.

n
0

.
mtate

Left Byte

ROLB

drt

’

2.3~1 Q

I I t 1 I I .I 1 1 1 ’ ’ 1
dst

1
15

Operation:

as in ROL on byte operands

Condition

Codes: set on the byte results

Description:

as ROL

Same as ROL

Arithmetic

Shift

1 1

Riaht

Bvte

ASRB

16’1

2.3~7

dst

I
6

Operation:

as in ASR on byte operands

ConditionCodes:
Description:

set on the byte result as ASR

Same as ASR

Arithmetic

Shift

‘I

Left

Byte

101

ASLB

I ‘61

dst

2.3u**
dst

131

I
6

Operation:

as in ASL on byte operands

Condition

Codes: set on the byte results

Description:

I
0

as ASL

Same as ASL
code operators
set and clear conof these bits may be cleared or set

CONDITION CODE OPERATORS--Condition
dition code bits. Selectable combinations
together in one instruction.
Condition

Code

Qpemtors

Condition
code bits corresponding
to bits in the condition
code operator
(bits 3-O; N, Z, V, C) are modified
according to the sense of bit 4, the set/
clear bit of the operator.
The following
mnemonics
are permanent
symbols
in the assembler:

Mnemonic
CLC
CLV

Operation

Op Code

Mnemonic

Qperetion

Op Code

Clear
Clear
Clear
Clear

000241

SEC

Set C

000261

000242
oo(2244
000250

SEV
SEZ

Set V
Set Z

SEN

Set N

000262
000264
000270

C
V
Z
N

Timing for all condition
code operators
is the basic instruction
time (1.5~s)
for the operators.
(The codes 000240 and 000260 are the shortest “no+operation” instructions.)
T Shift

and

rotate

operations

r,equire an

additional

addresses.

0.6

etceee

bytes

odd

Combinations
of the above set or clear operations
may be ORed together to
form new instruction
mnemonics.
For example: CLCV = CLC ! CLV. The new
or” in PAL-11.)
instruction
clears C and V bits. (‘I!” signifies “inclusive
MISCELLANEOUS
RESet

CONTROL

ExTernol

IO,

iNSTRUCTIONS
20 ms

RESET

bus

101

lOI

I 5 ,
0

Condition

Codes: not affected

Description:
Sends an INIT pulse along the Unibus
devices on the bus are reset to their state at power-up.

by the

processor.

AlI

\
WAit

for

IntempT

WAIT

1.8 IL*

Condition
Codes: not affected
Description:
Provides a way for the processor to relinquish
use of the bus
while it waits for an external interrupt.
Having been given a WAIT command,
the processor
will not compete
for bus use by fetching
instructions
or
operands from memory. This permits higher transfer
rates between a device
and memory,
since no processor-induced
latencies will be encountered
by
bus requests from the device. In WAIT, as in all instructions,
the PC points
to the next instruction
following the WAIT operation.
Thus when an interrupt
causes the PC and PS to be pushed onto the processor stack, the address of the next instruction
following
the WAIT is saved.
The exit from the interrupt
routine (i.e. execution
of an RTI instruction)
will
cause resumption
of the interrupted
process at the instruction
following the
WAIT.

l.SlLS

HALT

HALT
101

IO,

,Ol

101

101
0

Condition Codes: not affected
Description:
Causes the processor operation
to cease. The console is given
control of the bus. The console data lights display the contents
of RO; the
console address lights display the address of the halt instruction.
Transfers
on the Unibus are terminated
immediately.
The PC points to the next instruction
to be executed.
Pressing the continue
key on the console causes
processor operation
to resume. No INIT signal is given.
Processor Traps
-Processor
Traps are internally
generated
interrupts.
Error conditions,
completion
of an instrustion
in trace mode (i.e. T-bit of
status word set), and certain instructions
cause traps. As in interrupts,
the
current PC and PS are saved on the processor stack and a new PC and PS
are loaded from the appropriate
trap (interrupt)
vector. See Appendix C for
a summary of Trap Vector Addresses.
,Trapf Instructions-Trap
monitors,
debugging

Instructions
provide
packages, and user-defined

for calls to
interpreters.

emulators,

i/O

EMulotw

EMT

Traps

‘I

101

,4,

Operation:

9.9 us

xycx
,

XXI

(PS) J, SP
WI
J SP
(30) + PC
(3?) + PS

Condition

Codes: loaded from trap vector.

Description:
Performs
a trap sequence
with a trap vector address of 30.
All operation
codes from 104000 to 104377
are EMT calls. The low-order
byte, bits O-7 of the EMT instructions,
may be used to transmit
information
to the emulating
routine (e.g., function
to be performed).
The trap vector
for EMT is at address 30. The new PC is taken from the Word at address 30;
the new central processor status (PS) is taken from the word at address 32.
TRAP

TRAP

.‘I

10,

xxx

9.9JLs

xxx
I

’

Operation:

as in E&IT except the trap vector

Condition

Codes: loaded from trap vector.

is located at 34.

Description:
Performs
a trap sequence
with a trap vector address of 34.
Operation
codes from 104400 to 104777 are TRAP instructions.
TRAPS and
EMTs are identical in operation,
except that the trap vector for TRAP is at
address 34.
l/O

Trap

9.9LS

IOT

IO,

101

141

Operation:
information

as EMT except the trap vector
is transmitted
in the low byte.

Condition

Codes: loaded from trap vector.

Description:

Used to call the I/O executive

No defined
“I_-,04

mnemonic

is located

at address

101

routine

IOX.

000003
I

20 and

9.9us
101

IO,

I
0

Operation:

Same as IOT

Condition

Codes:

except that trap vector

is located

at address

14.

loaded from trap vector.

Description:
Used to call debugging
employing
code 000003
in programs

aids. The user is cautioned
against
run under these debugging
aids.

R.Turn

from

Interrupt

,o,

4.Bus

RTI
I

IO,

101

a21
0

Operation:

SP T (PC), SP t

(PS).

Condition

Codes: loaded from

processor

stack.

Description:
Used to exit from an interrupt
or TRAP service
and PS are restored (popped)
from the processor stack.

routine.

The PC

to execute instruction
codes
Instruction
traps are also caused by attempts
reserved for future processor
expansion
(reserved
instructions)
‘Or instructions with illegal addressing
modes (illegal instructions).
Order codes not
corresponding
to any of the instructions
described
above are considered
to
be reserved instructions.
Illegal instructions
are JMP and JSR with register
mode destinations.
Reserved and illegal instruction
traps occur as described
under EMT, but trap through
vectors at addresses
10 and 04 respectively.
Stack Overflow Trap-Stack.Overflow
Trap is a processor trap through
the
vector at address 4. It is caused by referencing
addresses below 400, through
the processor stack pointer R6 (SP) in autodecrement
or autodecrement
deferred addressing.
The instruction
causing the overflow is completed
before
the trap is made.
Bus Error Traps-Bus

1. Boundary

Error Traps are:
Errors-attempts

to reference

word

operands

at odd ad-

dresses.
2. Time-Out
Errors-attempts
to reference
addresses on the bus that
made no response within 10 ps. In general, these are caused by attempts to reference
nonexistent
memory,
and attempts
to referf?nce
nonexistent
peripheral devices.
Bus error traps

cause processor

traps

through

the

trap vector

address

Trace Trap-Trace
Trap enables bit 4 of the PS word and causes processor
traps at the end of instruction
executions.
The instruction
that is executed
after the instruction
that set the T-bit will proceed to completion
and then
cause a processor trap through the trap vector at address 14.
The following

are special

cases and are detailed

in subsequent

paragraphs.

1. The traced instruction
2.
3.
4.
5.
6.
7.
8.

cleared the T-bit.
The traced instruction
set the T-bit.
The traced instruction
caused an instruction
trap.
The traced instruction
caused a bus error trap.
The traced instruction
caused a stack overflow trap.
The process was interrupted
between the time the T-bit was set and
the fetching of the instruction
that was to be traced.
I
The traced instruction
was a WAIT.
The traced instruction
was a HALT.

An instruction
that cleared the T-bit-Upon
fetching
the traced instruction
an internal flag, the trace flag, was set. The trap will still occur at the end
of execution
of this instruction.
The stacked status word, however, will have
a clear T-bit.

An instruction
that
again has no effect.

set the T-bit-Since

the T-bit

was already

set,

Settiflg

An instruction
that caused an Instruction
Trap--The
instruction
trap is
sprung and the entire routine for the service trap is executed.
If the service
routine exists with an RTI or in any other way restores the stacked status
word, the T-bit is set again, the instruction
following
the traced instruction
is executed and, unless it ,is one of the special cases noted above, a trace
trap occurs.
An instruction

that

caused

a Bus Error-This
is treated as in an Instruction
is that the error service is not as likely to exit
with an RTI, so that the trace trap may not occur.

Trap. The only difference

An instruction
that caused a stack overflow-The
instruction
COmpleteS
execution
as usual-the
Stack Overflow does not cause a trap. The Trace
Trap Vector is loaded into the PC and PS, and the old PC and, PS are pushed
onto the stack. Stack Overflow occurs again, and this time the trap is made.,
An interrupt
between setting of the T-bit and fetch of the traced instruction
The entire interrupt
service routine is executed and then the T-bit is set again
by the exiting RTI. The traced instruction
is executed (if there have been no
other interrupts)
and, unless it is a special case noted above, causes a trace
trap.
Note’that
no interrupts
are acknowledged
between the time of fetching
any
trapped instruction
(including
one that is trapped by reason of the T-bit being
set) and completing
execution
of the first instruction
of the trap service.
A WAIT-The
trap occurred
is saved on the stack.

immediately.

The address

of the next instruction

A HALT-The
processor
halts. When the continue. key on the console is
pressed, the instruction
following,the
HALT is fetched and executed.
Unless
it is one of the exceptions
noted above, the trap occurs immediately
following execution.
Trap priorities-In
ously the following
1.
2.
3.
4.

case multiple
processor trap conditions
occur simultaneorder of priorities is observed (from high to low):

Bus Errors
instruction
Traps
Trace Trap
Stack Overflow Trap

The details on the trace trap process have been described
in the trace trap
operational
description
which includes cases in which an instruction
being
traced causes a bus error, instruction
trap, or a stack overflow trap,
If a bus error is caused by the.trap
process handling
instruction
traps, trace
traps, stack overflow traps, or a previous bus error, the processor is halted.
If a stack overflow is caused by the trap process in handling bus errors, instruction
traps, or trace traps, the process is completed
and then the stack
overflow trap is sprung.

CHAPTER
ADDRESS

ALLOCATION

The PDP-11 provides for a very flexible addressing
structure.
Both 16-bit
words and 8-bit bytes can be directly addressed.
Addresses are 1Bbits long
allowing for direct addressing
of 32,768 words or 65,536 bytes.

ADDRESS MAP
As a result of the organization
functions.
A map of possible

of the PDP-11, bus addresses
serve several
PDP-11 bus address
allocation
is shown

CONTENT

BUS ADDRESS
0

4608

Program

Counter

Processor

Status Word

Stack Pointer

Overflow

Stacks, Program

Processor.
Trap Vectors
and Device
Interrupt
Vectors

Limit

and Data Storage

160bOOs
Typical
Registers for
Programmed
Transfer
Device

.
Status Register and
Data Buffer Register

.
Device Address Register
Word Count Register
Memory Address Register
Control and Status Registers

1777778
Simplified

Figure 5-1
Address Allocation

Map

Typical
Registers
for a
Block
Transfer
* Device

in Figure 5-1. Three areas of addresses of particular
interest to the Programmers are: 1) Interrupt and Trap VeMors; 2) Processor Stack and General
Storage; and 3) Peripheral Device Registers.
INTERRUPT
AND TRAP VECTORS-Addresses
location 4001 are generally reserved for interrupt

between lOCatiOn
and trap vectors.

zero

and

PROCESSOR STACK AND GENERAL STORAGE-Addresses
between 4001
and the limit of implemented
core are available for the processor stack or
other programs and data. The highest address in ttiis region is 157777*.
PERIPHERAL DEVICE REGISTERS-Addresses
above 160000,
generally are
reserved for peripheral device status, control, and data registers. The general
registers and the processor
status can be addressed
from the program
console using addresses in this area.
A more detailed

address

allocation

map can be found

in Appendix

CORE MEMORY
The three types of core memory that can be used in a PDP-11 system are:
1) Read-Write Core Memory: 2) Read-Only Core Memory; and 3) Wordlet
Memory. These memories can be located anywhere in address space provided
they do not overlap. They do not have to be in continuous
address locations.’
MMll-E
READ
specifications:

WRITE

CORE

MEMORY-The
,

MMll-E

has the

following

Capacity: 4,096 l&bit words or 8,192 8-bit bytes
Cycle Time: 1.2 microseconds
Access Time: 500 nanoseconds
Configuration:
Planer 3-wire, 3-D using 22 mil cores
Packaging: One standard PDP-11 System Unit
interface: Designed to work with PDP-11 bus, l-FL-compatible
MRll-A
READ-ONLY CORE MEMORY (ROM)-The
ROM has the
specifications:
Capacity: 1,024 l&bit words or 2,048 8-bit bytes
Access Time: 500 nanoseconds
Configuration:
P-piece core with wire braid, 256 wires, 64 cores
Packaging: 3/4 of one standard PDP-11 System Unit
Interface: Designed to work with PDP-11 bus, TTL-compatible

following

MWllA
WORDLET MEMORY-The
wordlet memory is used with ROM systems and provides read-write memory capacity for temporary
data and instruction storage.
Capacity: 128 16.bit words or 256 8-bit bytes
Cycle Time: 2.0 microseconds
Access Time: 1.0 miorosecond
Configuration:
5-Wire, 3D
Packaging: l/4 standard PDP-11 single System UnitInterface:
The wordlet
memory
will work with the ROM and interfaces
through the ROM System Unit to the PDP-11 bus.

’

CHAPiER 6
PROGRAMMING
OF PERIPHERALS
Programming
of peripherals
is extremely
simple in the PDP-11-a
special
class of instructions
to deal with input/output
operations
is unnecessary.
The Unibus permits a unified addressing
structure
in which control, status,
and data registers for peripheral
devices are directly addressed
as memory
locations.
Therefore
all operations
on these registers, such as transferring
information
into or out of them or manpulating
data within them, are performed by normal memory reference instruction.
The ability to use all memory
reference
instructions
on peripheral
device.
registers greatly increases the flexibility
of input/output
programming.
lnformation
in a device register can be compared
directly with a value and a
branch made on the result.
CMP
#lOl,
PRB
BEQ SERVICE
In this case the program
looks for 101, from the paper tape reader data
buffer, and branches if it finds it. There is no need to transfer the information into an intermediate
register for comparison.
When the character is of interest, a memory reference instruction
can transfer the character
into a user buffer in core or in another peripheral device.
This instruction
transfers
a user-defined
location.
All arithmetic
This instruction

operations

MOV
PRB,
LOC
a character from the paper tape
can be performed

on a peripheral

ADD
#lo,
-DSX
will add lo8 to a display’s x-deflection

reader

buffer

device

into

All peripheral
device registers can be treated as accumulators.
need to funnel all data transfers,
arithmetic
operations,
and
through
a single or small number of accumulator
registers.

There is no
comparisons

DEVICE REGISTERS
All peripheral devices are specified by a set of registers which are addressed
‘as core memory and manipulated
as flexibly as an accumulator.
There are
two types of registers associated with each device: 1) Control and Status.Registers (CSR); and 2) Data Registers.
CONTROL AND STATUS REGISTERS (CSR)-Each
peripheral has one or more
control and status registers which contain all the information
necessary to
communicate
with that device. The general form of a control and status
register is shown below.

, This general
as a format

General Control and Status Register
form does not necessarily apply to any device, but is presented
which could be used as a guideline
for designing
peripheral

devices. Many devices will require less than sixteen status bits. Other devices
will require more than sixteen bits and therefore will require additional status
and control

registers.

Device Function
Bits-These
three bits specify operations
that a device iS
to perform.
An example of one operation
for a paper tape read&r is read
one character. For a disk one operation would be read a block of words from
memory and store them on the disk.
Memory Extension
Bits-These
two bits are resewed
They will allow devices to ‘use a full 18 bits to specify

for future expansion.
addresses on the bus.

Done Enable and Error Enable Bits-These
two bits are independently
Programmable.
If bit 6 is set, an interrupt
will occur as a reSUlt of a function
done condition.
If bit 5 is set, an interrupt
will occur as the result of an
error condition.
This occurs when one or more of the error bits is sat to a
one. To initiate
an interrupt
routine to read from the paper tape reader,
the instruction
MOV
#lOl,
PRS
could be used. This sets bit 0 and bit 6 of the paper tape reader control and
status register.
Setting
bit 0 starts the read operation
and setting
bit 6
enables an interrupt
to occur when the read operation’is
complete.
Condition
Bits--The
CSR contains
a DONE bit, a READY bit, or a DONEBUSY pair of bits, depending
on the device. These bits are set and cleared
by the hardware,
but may be queried
by the program
to determine
the
availability
of the device. For example,
the teleprinter
status register (TPS)
has a READY bit (7) that is cleared on request for output and then set when
output
is complete.
The keyboard status register (TKS) has a DONE-BUSY
between
no input (DONE = BUSY
pair (Bits 7 and 11) that distinguishes
= 0), input under way (DONE = 0, BUSY = 1). and input complete
(DONE
= 1, BUSY = 0).
.
The DONE bit could be used to control an input loop for reading from- the

paper tape reader as follows:
LOOP:

TSTB
BPL

PRS
LOOP

;
;

test low byte of paper tape status register
branch back if DONE bit (bit 7) is not set

Unit Bits-Some
peripheral
systems have more than one device per control.
For example, a disk system can have multiple
surfaces per control and an
analog-to-digital
converter
can have multiple
channels.
The unit bits select
the proper surface or channel.
Error Bits--Generally
there. is an individual
bit associated
with a specific
error. When more bits are required for errors, they can be obtained
by expanding the error section in the word or by using another status word.
Example of Control and Status Register -The
control and status register (PRS) is as follows:

high-speed

paper

tape

reader

These bits may be read or set by instructions
which use the appropriate
effective address. Bit 0 of the PRS is the function
bit for reading one char-

acter. Incrementing
read. The instruction
performs
that
more word.

the

function.

PRS will set bit 0 and cause

one character

PRS
INC
MOV #l,
PRS does the same

thing

but takes

to be

one

DATA BUFFER REGISTERS-Each
device has at least one buffer register
for temporarily
storing data to be transfer into or out of the computer.
The
number and type of data registers is a function
of the device. The paper
tape reader and punch use single 8-bit data buffer registers.
A disk would
use I&bit data registers and some devices may use two ldbit
registers for
data buffers.

PROGRAMMING EtiMPLES
PROGRAM CONTROLLED

DATA TRANSFER WITH THE INTERRUPT

DISABLED

-Single
character
I/O devices (teletype,
paper tape reader/punch)
have an
addressable
register buffer through
which data is transferred.
For input, the
data buffer register is the source operand of the instruction
used to get the
data; for output,
it% the destination
operand.
For example assuming
the
paper tape reader interrupt
is not enabled,
character
input could proceed
as follows:
save R on the stack
pointer to input buffer into register R
start up reader
START:
LOOP:
test DONE and ERROR bits
branch back if none on yet
branch to error routine if minus
move byte from device buffer register to user’s buffer and increment
pointer
CMP
#LIMIT
R,
check for end of buffer
BGE START
get next character.
MOV
(SP)+,
R
restore R
Character output to the paper tape punch might be executed as follows:
MOV
RO, L(SP)
; save RO
MOV
Rl,
-(SP)
; save Rl
; *number
of characters
into RO
MOV
NCHAR,
RO
; user buffer address into Rl
MOV
BUFFER,
Rl
; test device ready and error bits
LOOP:
BIT
PPS,
#100200
; fall through
if on
BEQ LOOP
; branch on error
BMI
ERROR
; output character,
increment
pointer
MOVB
(Rl)+,
PPB
; decrement
character
counter
(and
DEC
RO
; set condition
codes)
BGT LOOP
; repeat if greater than zero
; restore RO
MOV
(SP)+,
RO
; restore Rl
MOV
(SP)+,
Rl
MOV
R, -(SP)
MOV
#8UFFER,
R
INC
PRS
BIT
PRS,
# 100200
BEQ LOOP
BMI
ERROR
MOVB
PRB,
(R)+

;
;
;
;
;
;
;
;
;
;
;
;

BLOCK

TRANSFER WITH THE INTERRUPT
DISABLED-High-speed
block
transfer
devices use the Unibus to make data transfers
between the device
and core memory.
These devices are provided
with addressible
registers
that control the flow of data.
49

A tvoical
set might be:
_.
1. Control-and
status register
2. Memory address register
3. Word count register
4. Device address register
Loading the device address register would in general initiate the transfer,
which then proceeds without
processor intervention.
The device issues nonprocessor
requests
for the Unibus that, when granted,
allow direct data
transfer
between
the device and memory.
These requests
are interleaved
with processor. requests for the bus. If very fast transfer
is required,
the
processor may execute a WAIT instruction
after starting the block transfer.
The DONE or appropriate
-error bits are set in the CSR with Completion
of
the transfer
or when an error occurs. These may be enabled to cause an
interrupt
or may be tested to determine
when the device needs assistance.
A block transfer
’

could be executed

MOV

#401,

MOV

#BUFADR,

DKMA

MOV
MOV

#BUFCNT,
#BLKNO,

DKWC
DKDA

as follows:

DKS,

;
;
;
;
;
;
;
;

read block of data (function
1)
from unit 1
buffer
address
to memory
address register
word count to word count register
block number
to device address
register, which starts the transfer

.
: when data is needed.
LOOP:

; data

INTERRUPT

;
BIT
#DKMSK,
DKS
BEQ LOOP
;
;
BIT
#DKEMSK,
DKS
;
BNE
ERROR
is now in buffer at BUFADR

test done bit and error bits
branch back if none on \
test for any error bits
branch if any on

STRUCTURE

If the appropriate
interrupt enable bit is on, in,the control and status register
of a device, transition
from 0 to 1 of the DONE or READY bit causes an
interrupt
request to be issued to the processor. Also if DONE or READY’ is a
1 when the interrupt
enable is turned on, an interrupt
request is made. If
the device makes the request at a priority greater than that at which the
processor is running and no other conflicts exist, the request is granted and
the interrupt sequence takes place:
a. the current program counter and processor status are pushed onto
the processor stack;
.b. the new PC and PS are loaded from a pair of locations (the interrupt
vector) in low core unique to the interrupting
device.
Since each device has a unique interrupt
vector which dispatches
control to
the appropriate
interrupt
handling
routine immediately,
no device polling is
required..
Furthermore,
since the PS contains
the processor
priority,
the
priority at which an interrupt
request is serviced can be set under program
control
and is independent
of the priority of the interrupt
request. The

ReTurn fmm Interrupt
instruction
is used to reverse the action of the
interrupt sequence. The top two words on the stack are popped into the PC
and PS, returning control to the interrupted
sequence.

PROGRAMMING
A

EXAMPLE

paper tape reader interrupt

service could appear as follows:

First the user must initialize the service routine by specifying
an address
pointer and a word count
; set up address pointer
INIF
MOV #BUFADR,
#0
; in third word of MOV instruction.
POINTR = . - 2
; set up character count in
MOV #CNTR, #0
; third word of MOV instruction.
CRCNT=.-2
MOV #lOl, PRS
; read a character with interrupt
_ ; enabled.
When the interrupt request occurs and is acknowledged,
the processor stores
the current PC and PS on the stack. Next it picks up the interrupt vector or
new PC and PS beginning at location 70~. The next instruction
executed is
the first instruction
of the device service routine at PRSER.
PRS
PRSER:
TST
test for error
BMI
ERROR
I branch to error routine if
bit 15 of PRS is set.
MOVB
PRB, @POINTR
; move character (byte)
from reader to buffer
\
INC
POINTR
increment pointer
DEC
CRCNT
decrement character count
BEQ
DONE
branch when input done
. INC
PRS
start reader for next character
DONE:
RTI
; return from interrupt

The DIGITAL M225 module contains 8 high speed general-purpose
registers. The M225 general registers
provide program
flexibility
when used as accumulators,
index registers, and pointers to data
words.

CHAPTER
TELETYPE

(MODEL

LT33-DC/DD)

The standard
Teletype Model 33 ASR (Automatic
Send-Receive)
can be used
to type in or print out information
at a rate of up to ten characters
per second, or to read in or punch out perforated
paper tape at a ten characters
per second rate. Signals transferred
between the 33 ASR and the control
logic are standard
serial, 11-unit code Teletype signals. The signals consist
of “marks”
and “spaces”
which correspond
to idle and bias current in the
Teletype serial line, and to O’s and l’s in the control and computer.
The
start mark and subsequent
eight bits are each one unit of time duration and
are followed by the stop mark which is two units.
The 8-bit-code
used by the Model 33 ASR Teletype unit is the America1
Standard Code for Information
Interchange
(ASCII) modified.
To convert the
ASCII code to Teletype code, add 200 octal (ASCII + 200s = Teletype).
The Model 33 ASR can generate
all assigned
codes except 340 through
374 and 376. The Model 33 ASR can detect all characters,
but does not
interpret all codes that it can generate as commands.
The standard
number
of characters
printed per line is 72. The sequence for proceeding
to the next
line is a carriage return followed by a line feed. Punched tape format is as
follows:
Tape Channel

654

Binary Code
(Punch = 1)

110

100

Octal Code
(S = Sprocket)

SIZE-

Floor space approximately
Cable length 8 feet

MODEL
LT33-DC
LT33-DD

TELETYPE

221/”

321

wide, 181/2” deep

POWER REQUIREMENTS

115 v 510%
230 V -t-iO%
CONTROL

(MODEL

60 kO.45 Hz
50 kO.75 Hz

KLll)

TELETYPE CONTROL-Serial
information
read or written by a Teletype unit
is assembled
or disassembled
by the control for parallel transfer
on the
Unibus. The control also provides the flags which cause a priority interrupt
and indicate the availability of the teletype.
KEYBOARD/READER-The
Teletype control contains
an 8-bit buffer (TKB)
which assembles
and holds the code ‘for the last character
struck on the
keyboard
or read from the tape. Teletype characters
from the keyboard/
reader are received serially by the 8-bit -shift register TKB. The code of -a
Teletype character
is loaded into the TKB so that “spaces”
correspond
to
binary O’s and holes, “marks,”
correspond
to binary 1’s. Upon program
command,
the contents
of the TKB may be transferred
in parallel to a
memory location or a general register.
A character is read from the low-speed reader by setting the Teletype reader
enable bit, (RDR ENB), to a 1. This sets the busy bit (BUSY) to a 1. When a
Teletype character
starts to enter, the control de-energizes
a relay in the

Teletype
unit to release the tape feed latch. When releasea
the latch
mechanism
stops tape motion
only when a complete
character
has been
sensed, and before sensing of the next character is statthd. When the character is available in buffer (TKB), the busy bit (BUSY) i$ cleared and the done
flag (DONE) is set. If the interrupt
is enabled, a request is made for the bus
at level 4 (BR4). The interrupt
vector is at location 60,. The DONE bit is
cleared by any instruction
which reads the contents of the buffer (TKB) into
the processor.
If the DONE flag is cleared before the interrupt
is granted,
no
interrupt
will occur. The keyboard
must be read within 18 milliseconds
of
DONE to ensure no loss of information.
Registers1
Teletype

Keyboard

Status

(TKS)

0*
l-BUSY

Bit
0

RDR ENB

INT ENB

DONE

BUSY
’

L- ROR
ENB

’ $&TENB

Requests that one character
be read from the
reader; set from the bus: (Note: Setting RDR
ENB causes tape to advance by one character
which is shifted into TK8 if DONE is cleared.)
Receipt of START bit on the serial input line sets
BUSY, clears RDR ENB and clears TKB.
O-No
interrupt;
l-Attach
the keyboard
and
reader to the priority interrupt
system at bus
request level 4.
Character
available; cleared by reading the buffer (TKB).
Character
is being read; set by RDR EN8 going
to a 1. Cleared by DONE going to a 1.

I The following
notation will be used throyghout
this chapter for describing
0 -A
power clear sets this bit to 0.
1 -A
power clear sets this bit to 1.
l -This
bit can only be read from the bus.
$-This
bit can only be set from the bus. If it is read, it will

as zero.

Teletype

Keyboard

Buffer

always appear

(TKB).
&BIT

I
15

registers.

CHARACTER
0

TELEPRINTER/PUNCH-On
program command,
a character
is sent in parellel from a memory location (or a general register) to the TPB shift register
for transmission
to the teleprinter/punch
unit. The control
generates
the
start “space,”
then shifts the eight bits serially into the Teletype u&,-and
then generates the stop “marks.”
This transfer of information
from the TPB
into the teleprinter/punch
unit is accomplished
at the normal Teletype rate
and requires 100 milliseconds
for completion.
The READY flag in the teleprinter/punch
indicates that the TP8 is ready to receive a new character.
A
maintenance
mode is.provided
which connects the TPB eutput to the TKB
input so that the parallel serial and serial parallel shifting may be verified.

Registers
Teleprinter

Status Word (TPS)
7

L I-

IN-f

CONTROL

READY

Bit
2

MAINT

INT ENB

READY

Teleprinter

Buffer (TPB)

Maintenance
function
which connects
TPB serial
output to TKB serial input.
the Teleprinter
to
O-No
interrupt;
1 -attaches
the priority interrupt
system at BR4.
Set by punch/printer
DONE; cleared by loading
the teleprinterbuffer
(TPB).

PRDGRAMMING
EXAMPLE-To
the printer:
ECHO:
INC TKS
c
TSTB TKS
BPL .-2
TSTB TPS
BPL .-2
MOVB TKB, TPB

a-BIT

OATA

read i character

from

tape

and echo

it on

; set RDR ENB
; test for DONE set
; test again if not set
; test for printer READY set
; test again if not set
; put input character into’output
buffer to be printed
; return for another character

ADDRESS ASSIGNMENTS
177560
177562
177564
177566

VECTOR ADDRESS
PRIORITY
key board

CHARACTER

BR ECHO
PERIPHERAL
TKS
TKB
TPS
TPB

L MAINTENANCE

EN6

LEVEL

Keyboard/ Reader
Teleprinter/ Punch
set to BR&Teletype

printer

MOUNTING-Requires
one small peripheral
of a DDll or one of two such spaces in KAll)

HIGH-SPEED PERFORATED
CONTROL (TYPE PCll)

60
64
is lower

controller

TAPE READER

than

the

mounting

Teletype
s,pace (l/4

PUNCH ‘AND

TAPE READER-This
device senses 8-hole perforated
paper or Mylar tape
photo-electrically
at 300 characters
per second. The reader control requests
reader movement,
transfers
data from the reader into the reader buffer
(PRB), and signals the computer
when incoming data is present. It does this

by setting a DONE bit. If the interrupt
is enabled and the interrupt
is granted,
the processor traps to location 70, and may immediately
begin executing the
service routine for the paper tape reader.
Reghters
Paper Tape Reader Status Word (PRS)
15

DONE

*
0

ERROR

BUSY

LINT

L ROR
RDR

EN6

ENB

Bll

RDR ENB

Requests
bus only

INT ENB

O-No
interrupt;
1-attached
to priority interrupt
system at BR4. (Note: Interrupt
occurs when INT
ENB is a 1 and either the error flag, ERROR, or
the done flag, DONE, becomes a 1.)

DONE

Set by character
available; cleared
paper, tape reader buffer (PRB).

BUSY

Set by RDR ENB going
going to a 1.

ERROR

Error Flag - Set or cleared
or off line switch.

read of next character;
if ERROR = 0. Clears

can be set from
PRB, sets BUSY.

by reading

to a 1; cleared

the

by DONE
J

by out-of-tape

sensor

Paper Tape Reader Buffer (PRB)

PROGRAMMING
READ:
TEST:

ERROR:

E-BIT

EXAMPLE/Gape

CHARACTER
0

reading

subroutine

(not

using

INCB PRS
; enable reader
BIT $100 200 PRS ; test for error or done
; branch back if not done
BEQ TEST
; branch if error = 1
BML ERROR
MOVB PRB. RO
;~ get character from buffer
; return to caller
RTS R
(message type out routine)
; wait for operator intervention
HALT
; try again when continue switch
JMP READ

interrupt):

is hit.

TAPE PUNCH-This
option of a Royal McBee paper tape punch that perforates B-hole tape at a rate of 50 characters
per second. Information
to be
punched on a line of tape is loaded in an B-bit punch buffer (PPB) from a
memory
location
or one of the general registers. The punch flag, READY,
becomes a 1 at the completion
of punching
action, signaling new information
may be transferred
into the punch buffer and punching initiated.

Registers
Paper Tape Punch Status Word (PPS)

Bit
6

INT ENB

O-No
Interrupt;
l-Attached
fo priority iflterrUFt
system. (Note: An interrupt
occurs when 1NT EN8
is a 1 and either the ERROR flag or the READY flag
i
becomes a ‘1.)

READY

ERROR

Set by punch done; cleared
tape punch buffer (PPB).
Error Flag-Set
by out-of-tape
off switch.

Paper Tape Punch Buffer

1
(5

the buffer

sensor:

the

paper

or unit power

(PPB)

by loading

initiates

&BIT

CHARACTER

DATA

I
0

punching.

PROCRAMMING
EXAMPLE
PUNCH:
BIT # 100200, PPS ; test for ready or error
BEQ PUNCH
BMI ERROR
MOV RO. PPB
RTS R
;
ERROR:
(message type out)
HALT; wait for operator to fix punch
JMP PUNCH; try again when Continue is hit.
PERIPHERAL
PRS
PRB
PPS
PPB

ADDRESS ASSIGNMENTS

177550
177552
177554
177556

VECTOR ADDRESSEGReader
70
Punch 74
PRIORITY

LEVEL-Set

to BR4. Punch is lower than

reader.

MOUNTING-Electromechanical
assembly-EIA
Standard
19” rack, lOI/,”
vertical mounting
space, by 171/2” deep.
PCll-M
Controller--One
small peripheral
controller
mounting
space (l/4
of DDll or one of two such places in KAll).

-\
57

ENVIRONMENTAL
55”-100°F
20% -95%
MODEL

PC11
PCllA
PRll

RH (without

condensation)

POWER REQUIREMENTS
115+10%
60 Hz
115+-10%
50 Hz
115-c10°h
50-60 Hz

DESCRIPTION
Reader, Punch & Control
Reader, Punch &Control
Reader 81 Control

LINE FREQUENCY

KWll-L)

CLOCK (TYPE

The KWll=L real timeclock
provides a method
of measuring
time intervals
at line frequency.
This clock consists of a frequency source and control logic.
When enabled the clock causes an interrupt
every 16.6 or 20 milliseconds,
depending
upon Iflie frequency.
Register
Line Time Clock Status

(LKS)
7

6
(I

Bit
6 INTR ENB
7 CLOCK

LINTR
CLOCK

I
ENB

When set, an interrupt
will occur every time CLOCK goes true.
Cleared by program or reset or start sequence.
Set to 1 every 16.6 milliseconds
Hz). Cleared by reading LKS,
switch.

(60 Hz) or 20 milliseconds
(50
RESET or pressing ttie START

PERIPHERAL ADDRESS ASSIGNMENTS
177546
LKS
VECTOR ADDRESS
100
PRIORITY LEVEL
BR6
MOUNTING-This

option

plugs into the KAll

processor.

.
58

CHAPTER
DESCRIPTION

OF THE UNIBUS

Communication
between all system units in a PDP-11 configuration
is done
by a single common
bus: the Unibus. All communication-both
instructions
and logical operations-is
defined by a set of 56 signals. This set of 56 sig
nals is used for program controlled
data transfers,
direct memory data transfers, priority bus control, and program interrupt.
This chapter

effect.data

presents the concepts
of the Unibus and how they affect proand interfacing
hardware.
The use of the 56 bus signals to
transfers and to control bus use is also described.

GENERAL

CONCEPTS

gram software

OF THE UNIBUS

There are five major aspects of the Unibus
hardware considerations
in the PDP-11.

that

affect

both

software

and

SINGLE BU!j-The
set of 56 signals that comprise the Unibus is the one
and only bus connecting
all peripheral
devices, memories,
and the central
processor. Thus, to every device there exists a single set of signals by which
it can be interrogated
by the processor or other devices, or be used by the
device itself to transfer data to and from memory.
The processor uses this same set of signals to communicate
with all memories and devices. The important
point here is that the form- of the communication
used by processor
and peripheral
devices is identical.
Consequently;
the same set of program
instructions
used to reference
memory
is used to reference
peripheral
devices. (A look at the PDP-11 instruction
set will reveal that there are no explicit l/O instructions.)
Peripheral devices in a PDP-11 system are designed to respond to the Unibus
in the same manner
as memory.
Device status
registers,
device control registers, and device data registers are each assigned unique “memory”
addresses.
For example, the instruction
MOVB RO, PUNCH would load the
punch buffer register with an 8-bit character
contained
in RO. Other instructions
would monitor
the punch status and the program
could determine when the punching operation was complete.
BIDIRECTIONAL
BUS-Unibus
bus signals are bidirectional-the
signal
ceived as an-input
can be driven as an output, as shown in Figure 8-1.

r---

-----

re-

Aurn7

RECEIVE.

DRtVE

BUS

SIGNAL

StGNAL

I
I

Figure 8-l

DEVtCE

-----------

Bidirectional

Nature

LDGtC

of the Bus

MASTER-SLAVE RELATION-At
any one point in time, there is one device,
called the master, that has control of the bus. The master device controls

the bus to communicate
with other devices, call,ed slaves, on the bus. An
example of this relationship
is the processor (master) fetching an instruction
from memory (which is always a slave).
INTERLOCKED
COMMUNICATION-For
erich control
signal issued by the
master device, there is a response from the slave; thus bus communication
is independent
of the physical bus length and the response time of the master and slave devices. Also, master-slave
relationships
can exist in nearly
any combination
between fast-responding
and slow-responding
devices.
DYNAMIC
MASTER-SLAVE
RELATION-Master-slave
relationships
are dynamic. The processor, for example, can pa&s bus control to a disk. The disk,
as master, could then communicate
with a slave memory bank.

UNIBUS

SIGNALS

The 56 Unibus signals can be divided into two major groups-the
interrupt
group and the non-interrupt
group. The interrupt
group can then be subdivided into two classes-the
request and control class and the grant class.
All bus signals except the grant class are bidirectional
in nature and are
connected
to every device (though they may not be used by every device).
The grant signals, because of their special nature in priority bus control
(to be explained later), are bussed through each device and are unidiiectional
in nature.
NON-INTERRUPT
SIGNALS
Data Lines (0 < 15:OO >)-(Note
that the notation
A <a:b>
specifies
b - a + 1 signal lines which are named Aa through
Ab.) The 16 data lines
are used to transfer
information
between master and slave. This is the bit
format:
I

HIGH

BYTE

Address Lines (A < 17:00 >)-The
device’to
select the slave (a unique
with which it will be communicating.

LOW

BYTE

18 address lines are used by the master
core memory or device register address)
This is the bit format of the 18 signals:

A < 15:Ol > are used to specify a unique 16-bit word group. In byte operations, A00 is used to specify the byte being referenced.
If a word is referenced at X (X must be even, since words can be addressed on even boundaries only), the low byte can be referenced
at X and the high byte at X + 1.
A < 15:00 > are supplied
by the software
as memory
reference addresses.
Al7 and Al6 are used as extended
memory
bits for relocation
and as protection schemes
in future systems.
In the PDP-11/20
and the PDP-ll/lO,
Al7 and Al6 are asserted or forced to 1 whenever an attempt
is made to
reference
a memory
location where A15’= Al4 = Al3 = 1. Thus the hardware converts the 18bit software address to a full Is-bit-bus
address.
An address

map is shown

in Figure 8-2.
60

oooo#O-017777
trt
El
.

scFrwAF&

AOORESS

HAROWARE

oooooo-own7

MEMORY

ADDRESS

BANK

02CWO-037777

02ooOO-037777

2nd

MEMORY

0Aw

I
I

t40000-t57777

t40000-157777
7th
MEMORY

4K
BANK

_
760000-777777

t6oooo-t77777
PERIPHERAL
BANK
El

Figure 8-2

Address

Map

The peripheral
bank is composed
of the processor’s
fast
register, console switch register, and all device registers.

memory,

status

device

Lines (C < 1:0 > )-These
two bus signals
to indicate to the slave one of four possible

are coded by the master
data transfer operations.

Master

Synchronization

(MSYN,

Control

and

Slave

Synchronization

SSYN)-MYSN

is a control
and control

signal used by the master to indicate to the slave that address
information
is present.
SSYN is the slave’s response- to MSYN

lnititiliiation

signal is a power clear signal asserted by the conwhich is used to reset peripheral
devices.

sole

(INIT)-This
and the processor

PA, PB. SPl.
PDP-ll/PO.

SP2-These

lines

are not implemented

INTERRUPT
SIGNALS
Bus Request
Lines (BR i: 7:4 > )-These
peripheral

devices to request

control

four
of the bus.

on the

PDP-ll/lO

signals

are

bus

used

Bus Grant Lines (BG < 7:4 >-)-These
signals are the processor’s
response
to a BR. They will be asserted only at the end of instruction
execution.
Non-Processor

Request

(NPR)-This

is a bus

request

from

-a peripheral

device to the processor.
Non-Processor
Grant (NPG)-This
occurs at the end of bus cycles

isthe processor’s
response to an NPR. It
within the instruction
execution.

Selection

Acknowledge
(SACK)-SACK
is asserted by a bus-requesting
device
that has received a bus grant. Bus control will pass to this device when the
current master of the bus completes
its operations.
INTERRUPT
interruption

(INTR)-This
signal
in the processor.

Bus Busy (BBSY)-This

signal

UNIBUS

DATA TRANSFER

Direction

of data transfers

is asserted

denotes

by the master

to start

bus in use by a master

program

device.

OPERATIONS

on the Unibus
61

is defined

in relation

to the master

device. A data transfer from processor to memory (always
out,” and a transfer from memory to processor is “data in.”
TYPES OF DATA TRANSFERS-The
master and slave is determined

a slave)

is “data

type of data transfer being made between
_
by the C lines coded as follows:

DATO AND DATOB-The
DATO and DATOB operations
are used to transfer
data out of the master to the slave. DATO is used to transfera
word to the
address specified by A < .17:01 >. The slave ignores A00 and the data ap
pears onD < 15:00>.
DATOB is used to transfer a byte of data to the address specified by A < 17:OO >. A00 = 0 indicates, the low byte, and data
appears on D < 07:OO >; A00 = 1 indicates the high byte, and data appears
on D < 15:08 >.
DATI AND DATIP-The
DATI and DATIP operations
transfer data from a slave
whose address is specified on A < 17:Ol > into the master. Both transfers
are made in words on D < 15:00 >. In destructive
read-out devices, DATI
commands
a read-write operation,
while a DATIP commands
a read operation
only and sets a pause flag. When the device receives the subsequent
DATO
or DATOB and its pause flag is set, the usual read cycle is skipped and an
immediate
write cycle is initiated.
Thus, DATlPs are immediately
followed
by a DATO or DATOB to effect a read-modify-write
data exchange.
In nondestructive
read-out devices, DATI and DATIP are treated identically.
This diagram

illustrates

the data flow in the four data transfers:
DATI OR DATIP
DATA= D<l5:00>

8.1
SLAM

~~EGISTER

I
HIGH BYTE ,
I
A

LOW BYTE
A

DAToBI\AOO
DATA=D<t5:08 >

DATOBhm

DATA=D <OX00

DATA=CKl5:00>
Figure

8-3

Data

Flow

Note that all transfers
into the master are word operations;
it is up to the
master to accept the appropriate
byte. On a DATOB, the master must place
the byte on the appropriate
data lines; the slave must accept the proper byte.
DATA TRANSFER EXAMPLES-The
bus operations
used by the processor for
a typical instruction
sequence
illustrates
how the data transfer
operations
are used. The “program”
starts at location 1000:
1000:
INCB @RO
ADD #3, @RO
where

RO contains

500

and location

500 contains
62

10023.

The result

of this

instruction
sequence
coding appears as:

will leave

10027

in location

500.

1000:

105210

1002:

062710

;INCB @RO
;ADD (PC)+,

000003

1004:

In binary

200:
This instruction

The processor

CMPB

is assembled

200:
202:
204:

compares
is 177560

Dais Transferred
105210

010923
000024
062710
000003
010024
010027
on D < 15:OB >;

the contents
of the Teletype
with the ASCII value for the

@#177560,

#301

in three words as follows:

123727
177560
000301

will execute

Processor Cycle
1. Fetch
2. Source
3. Source
4. Destination
5. Execute

as a consequence

3, it is inconsequential
what data appears
only the modified low byte.

A second example of bus ‘operation
keyboard data buffer whose address
letter “A.”

this

@RO

. The following
table lists the bus operations
that result
of these two instructions:
Bus Address
Bus Operation
Processor Cycle
(PC),= 001000
DATI
1. Fetch
(RO) = 000500
DATIP
2. Destination
(RO) = 000500
DATOB
3. Execute
(lqz) = 001002
4. Fetch
DATI
5. Source
DATI
(PC)= 001004
DATIP
(RO) = 000500
6. Destination
(RO) = 000500
7. Execute
DATO
Note that instep
the slave accepts

form,

;CMPB @@7)+,

WV+

;Address

buffer

;301

this instruction

Bus Operation
DATI
DATI
DATI
DATI
none - condition

Note that in step 3, the soware
the bus address 777560.

of data

with these cycles:
Bus Address
(PC) = 200

Data Transferred
123727
’

(PC)=202
777560

177560

(PC) = 204
codes set internally.

specified

address

177560

ASCII
000301

was converted

SIGNAL DESCRIPTION OF DATA TRANSFERS-Figure
8.4(a) shows the sig
nal flow between master and slave during a DATO operation.
(The sequence
is similar for DATOB except that only a byte of information
is transferred.)
The master sets Control for DATO, sets Address for the unique slave address,
and sets Data for the information
to be transferred.
The master then asserts
MSYN. This signal is received by the slave that recognizes
its address:
it
responds by accepting the data arid asserting SSYN. SSYN is received by the
master which then negates Control,
Address,
Data, and MSYN. The slave
sees MSYN negated
and negates SSYN. The master device continues
its
operation when it sees SSYN negated.

MASTER
OPERATION:
A.C,D
MSYN

‘4

rg------

A’C’D

SLAVE

DAl-0

SSYN

3
SSYN

Figure 8-4(a)
The flow of signals for DATI is shown in Figure 8.4(b).
(DATIP is similar
except that the internal operation
of the slave device is modified.)
The master
sets Control for DATI, sets Address for the slave to be se’lected, and asserts
MSYN. The selected slave responds
by setting Data for the information
requested
and asserts SSYN. The master sees SSYN, accepts the data, and
then negates Control,
Address, and MSYN. The slave sees MSYN negated
and negates SSYN. The master continues
when it sees SSYN negated.
A more detailed signal sequence for the DATI, DATIP,
operations
can be found in Appendix D.
MASTER
OPERATION:

DATO, and DATOB bus
SLAVE

/
DATI

W
MSYN
i
,

SSYNaD

v
$%

_
ssVN6

’

Figure 8-4(b)

UNIBUS CONTROL OPERATIONS
The following section will deal with how a device becomes master of the bus
.and how control of the bus is transferred
from one device to another.
TWO
additional
bus operations
will be presented-the
PTR (Priority Transfer) and
INTR (Interrupt).
In normal operation,
the processor is bus master, fetching
instructions
and
operands
from memory.
Other devices on the bus have the capability
of
becoming
bus master, and use the bus for one of two purposes:
l), to gain
direct memory
access or 2). to interrupt
program
execution
and force the
processor to branch to a specific address:
PRIORITY ARBITRATION-Transfer
is determined
by a priority scheme
First,

the

processor’s

priority

of bus control from one device to another
in which three factors must be considered.

is determined

by bits

7, 6, and 5 in the

pro-

cessor
granting

status register. These three bits
of bus requests-on
lower levels.

set

a priority

level

that

inhibits

Second, bus requests
from external devices can be made on oni of five
request lines. NPR has the highest priority, and its request is honored by the
processor between
bus cycles of an instruction
execution.
BR7 is the next
highest; BR4 is the lowest. These four lower level requests are honored by
the processor
between
instructions,
except when the instruction
currently
being executed causes an internal trap (either an error or trap instruction).
In this case, BR requests will not be honored
until completion
of the first
instruction
after the trap sequence.
Thus if two requests are made to the
processor for bus control, the higher of the two requests will be honored first.

Third, in response to a bus request, the processor may honor the request by
asserting a bus grant (BG) corresponding
to the line on which the bus request was made. This signal is passed serially through
each device in the
system.
If a device had made a request,
it would. block the grant signal
and prevent it from reaching the following
devices. Thus, in this “pass-thepulse” chain, the device that is closest to the processor
has the highest
priority on that request level.
This table lists device priorities:
Highest:
Devices on NPR
Processor when priority
Devices on BR7
Processor when priority
Devices on BR6
Processor when priority
Devices on BR5
Processor when priority
Devices on BR4
Processor when priority
Internal options
Processor when priority
Internal options
Processor when priority
Internal options
Lowest:
Processor when priority
When the processor’s
priority
level N and below are ignored.

= 111
= 110
=

101

100

011

010

001

= 000

is set at N, all requests

for

bus control

SELECTION OF NEXT BUS MASTER-The
signal sequence by which a device
becomes
selected as next bus master is the PTR (Priority Transfer)
bus
operation.
Note that this operation
does not actually transfer
bus control:
it only selects a device as next bus master. It takes one additional
condition
to complete
the transfer:
the current
bus master must complete
its bus
operations.
The signal that indicates this is BBSY. Thus, when a device makes
an NP,R or BR request to the processor for bus control, it waits until it first
becomes selected as next bus master by the PTR operation
and second, it
no longer senses BBSY, The negation
of the BBSY signal indicates
that
the current
master has completed
its bus operation.
The selected device
now becomes bus master and asserts BBSY itself.
INTERRUPT SEQUENCE---Once
the device has bus control and is asserting
BBSY itself, it‘is sole user of the bus until it releases its control. This release
of control can be made either actively or passively. Passive release is realized

by negating
BBSY. Bus control will then pass to either a device that was
selected in the meantime
by another PTR sequence or back to the processor,
which will continue
where it was interrupted.
Active release of bus control
is realized through the INTR bussequence.
The INTR (interrupt)
operation
is used by the bus master to transfer to the
processor a memory address (called the interrupt
vector). Two consecutive
words, the starting address of an interrupt
service routine and a new status
word, are stored at the interrupt
vector address. After the INTR sequence is
Complete, the Processor automatically
becomes bus master and begins a trap
sequence in which it stores the current value of the PC and PS on the stack
and fetches a new PC and PS from the location pointed to by the interrupt
vector. Thus, the next instruction
executed
is the Start of, the interrupt
service routine.
It is illegal to issue an INTR command
after gaining control of the bus by
requesting
on an NPR line. NPR requests
are granted
during instruction
execution
and external bus masters must restrict their bus use* to nonprocessor activities.
Interrupt
Servicing
INTR sequence.

Sequence

Example-The

following

When a peripheral
requires service and requests
BR signal, the operations
undertaken
to “service”

is an example

of the
,
_

control of the bus with a
the device are as follows:

l Gain Control of the Bus-When
the processor has no higher priority tasks
to complete,
it relinquishes
the bus to that device. Higher priority items are
(in order of priority):
1. Acknowledging
an NPR request
2. Handling
a processor
error (illegal instructions,
requirements
for nonexistent memory, etc.)
3. Completing
the current instruction
4. Acknowledging
a trace trap
5. Continuing
a higher priority process
6. Acknowledging
a higher level BR signal
7. Acknowledging
same level BR signals for devices closer to the processor
l Do INTR Sequence-when
the device has cpntrol of the bus, it initiates
an INTR sequence, transferring
to the processor the interrupt
vector address
which specifies two words in memory containing
the address and status of
the appropriate
device service routine.
0 Push Old Interrupt
Vector Onto Stack-The
processor then “pushes”first, the current central processor status (PS) and then the current program
counter (PC) onto the processor stack.
l Fetch New Interrupt
Vector-The
new PC and PS (the “interrupt
vector”)
are taken from the address specified by the device, and the device service
routine is begun. Note that those operations
all occur automatically
and that
no device polling is required to determine
which service routine to execute.
Example of NPR Operation-Disk
operation
gives an example of a device
‘which uses the bus for direct memory access. Under program.control,
the
processor would initialize registers in the disk control that specify word count
(WC, number of words in block of data to be transferred),
memory address
(MA, the address at which the block of /data is found or is loaded), and Track
Address (TA, the point on the disk where the block of data starts). Also, the

program would set certain function
bits in the disk’s command
register that specify a read or write function.
For this example,
disk was set to read. *

and status
assume the

Once the disk’s control registers are initialized, the disk control logic’starts
a search for the requested data. (fhe processor in the meantime
has continued in its program execution.)
When the disk has found the data, it
assembles the first l&bit word from the disk surface into its data register.
The disk now requests bus control via the NPR request line. The processor,
when it has completed
its current bus cycle of the current instruction
and
no higher NPR requests exist, grants control of the bus to the disk. The disk,
as bus master, effects a DATO bus operation,
transferring
the contents to
its data buffer to the core address held in its MA. The MA is now incremented
and the WC is decremented.
When the DATO operation is COftydete,
the disk
passively releases control of the bus.
When the second word has been assembled,
the disk again requests bus
control, does a data transfer, and then releases bus control. This cycle is
repeated until the WC reaches zero. At this point, the disk has completed
the transfer that was requested.
To notify the program that the transfer is finished, the disk initiates a request
for bus control at the BR level, gains control when higher priority requests
are satisfied, and does an immediate
INTR to the processor and causes the
program to branch to a specific service program (as described in the previous
example).
\
Details

of the INTR and PTR bus operations

can be found

in Appendix

,
,I

;

The plug-in console board with modular
construction
is supplied
in the basic 11/20 configuration.
In addition
to aiding programming, console contributes
to ease of maintenance
on the PDP-11.

CHAPTER

Interfacing
A typical device bus interface
as shown in Figure 9-l is composed
of five
major components:
1). Registers; 2), Bus Drivers and Receivers; 3). Address
Selector; 4). interrupt Control; and 5), Device Control Logic.

REGISTERS
Each device is assigned
bus addresses
at which the program
can interrogate and/or
load the device status, control, and data registers. The standardized
mapping
for these registers and the bit assignments
of the corn- .
mandlstatus
register (CSR) were given in Chapters 5 and 6.
As shown in Figure 9-1, all information
flow between the device logic and
the Unibus is done through
the registers.
In general, registers are designed
to be both loadable and readable from the bus. This allows the program to
use such instructions
as ADD RO, REG, or INC REG. However, registers can
be “one-sided,”
either “read-only”
or “write-only.”
Examples
of read-only
bits are the DONE and BUSY flags in the device’s CSR. These bits are derived from the internal state of the device log& and are not under direct
program
control.
Write-only
registers
are used when it is unnecessary
to
read back information.
Attempting
to read such a register would result in an
all-zero transfer.
The instructions
effective with this type of register are then
limited to those which load the register such as MOV RO, REG, or CLR REG
(as opposed to ADD REG, RO, or INC REG).

Figure 9.1

BUS DRIVERS AND

Typical

Peripheral

Device Interface

RECEIVERS

To maintain
the transmission-line
characteristics
of the Unibus, special circuits are required to pass signals to and from the bus. The majority of bus
signals (all except the five grant lines) are received, driven and terminated
as shown in Figure 9-2.

’

I
I
I
I
I
I
I
I
L-

M930
---

-J i

WI-DRIVER

Rl , R2=190fi
R3. R4 = 390A

5% 1/4W
5% iI4W

Figure 9.2

Typical

Unibus

Line

Information
is received from the bus using gates which have a high input
impedance
and proper logic thresholds.
High input levels must be greater
than 2.5 V with an input current less than 160 pa. Low level input must be
less than 1.4 V with an input current greater than 0 pa.
information
transmitted
on the bus must be driven with open collector drivers
capable of sinking 50 ma with a collector voltage of less than .8 V. Output
leakage current must be less than 25 ~a.
In PDP-11 systems, the bus signals are terminated
at both ends by resistor
dividers provided
on the M930 module.
Physically, an M930 is located in
the processor;
another
is located at the last unit on the bus. A bus signal
sits at logical “0” (inactive, or negated state) at a voltage of 3.4 V. A bus
line is at logical “1” (active, or asserted) when it is pulled to ground.
Drivers and receivers
M783, M784 and M785

meeting
modules

these specifications
as shown in Figures
70

are available
on
9-3, 9-4 and 9-5.

the

Ml05 ADDREsS SELECTOR
Tho ‘Ml05 Address Selector as shown in Figure 9-6 is used to provide gating
signals for up to four device registers. The selector decodes the 18-bit bus
address on A < 17:00 > as follows:

Figure

9.3

9.4

M783

M784

Unibus

Drivers

Receivers

Figure 9.5

M785

Unibus

Drivers

and Receivers

A00 is used for byte control. A01 and A02 are decoded to provide one of
four addresses. A < 1203 > are determined
by jumpers on the card. When
the jumper
is in, the selector
will look for a 0 on that address
lineA < 17:13 > must all be l’s-(this
defines the external bank). Other bus
inputs to the selector are C < 1:0 > and MSYN. The single bus output
is
SSYN. The user signals are SELECT 0. 2, 4, and 6 (corresponding
to the
decoding of A02 and AOl, one of which is asserted when A < 17:13 > are alll’s and A < 12:03 > compare with the state of the jumpers. Other user signals are OUT HIGH (gate data into high byte), OUT LOW (gate data into low
byte), and IN (gate data onto the bus). The equations
for these last three
signals are as follows:
.
OUT HIGH =
OUT LOW =
=
IN

DATO +
DATO +
DATI
+

DATOB;AOO
DATOB*m
DATIP

where “+” means a logical or and “*” means a logical and.
Use of the M105, drivers, receivers and a flip-flop register
ure 9-7.

is shown

in- Fig

EXT.

CAP
I

-J

SELECT

A62L

SELECT

A61 L
AmL
GIL
CaL

SELECT

OUTHl6HH
OUT LW
IN tl

Figure

9.6

Ml05

Address

Selector

M782 INTERRUPT CONTROL

The M782 Interrupt
Control module contains the necessary logic circuits to
allow a peripheral
device to gain bus control and perform a program
interrupt. The three circuits on this card are block diagrammed
in Figure 9-8.
Note that only signals relevant
to the user’s interface
are shown;
bus
signals SSYN, BBSY and SACK have been omitted for clarity.
The Master Control circuit is used to gain bus control. When INT and INT
ENB are asserted, a bus request is made on the request line to which BR
is jumpered.
When the processor
issues the corresponding
grant and other
bus conditions
are met, the MASTER signal is asserted, indicating
that this
device now has bus control. Note that this circuit also can be used to gain
bus control on an NPR line for a device which requests the bus for direct
memory access.

-i
I
L---

Figure 9.7

------2

Typical Peripheral

Device Regker

In addition to two Master Control circuits, a third logic network provides the
necessary signals and gating to perform the INTR bus operation.
When either
of the START INTR signals is asserted,
the INTR bus signal is asserted
along with a vector address qn D < 07:02 >. Bits 07:03 are determined
by
jumpers on the card. A jumper “in” forces a 0 in that bit. Bit 2 is controlled
by Vector Bit 2. When the processor responds to the INTR signal by asserting
SSYN, the INTR DONE signal is asserted.
This line is used to clear the
condition which asserted INTR START.

Figure

9.8

M782

Interrupt

Control

Figure 9-9 shows a possible interconnection
of the M782 to provide independent interrupts for two possible conditions
in a device: ERROR and DONE.
The ERROR and DONE signals shown in Figure 9-9 are signals from bits 15
and 7 in a device’s CSR. Likewise ERROR INT ENB and DONE INT ENB are
derived from the CSR. Both interrupts
in this example are tied to the BR4
level: the corresponding
grant line BG4 enters the ERROR Master Control and
is passed on to the DONE Master Control. Thus, ERROR ha% a slightly higher
priority interrupt level than DONE.
Both MASTER signals are tied to the INTR control. Thus, whenever
either
ERROR or DONE gains bus control,
an INTR operation
is initiated.
Note
that
Vector
Bit 2 is a 1 or 0 as a function
of which
master
control
_ is interrupting.
Also, INTR DONE is tied to MASTER CLEAR to clear the
master condition.

DEVICE CONTROL LOGIC
The type of control
logic for a peripheral
depends on .the nature of )h”
device. Digital offers a wide line of general-purpose
logic IllO~Ul~S for IITF
plementing
control logic. These modules are described in detail In another
Digital publication:
The Logic Handbook.

Figure 9.9

Typical‘lnterconnection

of M782

Interrupt

Control

CHAPTER 10
CONFlGURATtON AND INSTALLAflON
MODULAR

PLANNING

CONSTRUCTlbN

Physically, the PDP.11 is composed
of a number
of System Units. Each
System Unit is composed
of three 8-slot connector
blocks mounted
end-toend as shown in Figure 10-l. The Unibus connects to the System Unit at
the lower left and at the upper left. Power also connects to the unit in the
leftmost
black. A System Unit is connected
to other System Units only via
the Unibus.
UNIBUS

CONNECTION

p:*~~

Figure

10.1

System

Unit

The remainder
of the System Unit contams logic for the processor, memory
or an I/O device interface.
This logic is composed
of single height, double
height, or quad height modules which are 8.5 ” deep.
The use of System Units allows the PDP-11 to be optimally
packaged for
each individual
application.
Up to six System Units can be mounted
into a
single mounting
box. For a basic PDP-ll/PO
system, the processor/console
would fill 21/2 System Unit spaces and 4096 words of core memory would
fill one System Unit space. This leaves 21/, spaces for user-designated
op
tions. This would allow the user to add 8,192 words of additional
core
memory, a Teletype control, and a High-Speed
Paper Tape Control, or 4,096
words of core memory
and six Teletype
interfaces,
Larger systems
will
require a BAll-EC or BAll-ES Extension Mounting
Box which contains space
for six additional
System Units.
The use of System Units also facilitates
expansion
of systems in the field
and service. To add an additional
option to a PDP-11 system, the proper
System Unit is mounted
in the Basic or Extension
Mounting
80x and the
Unibus
is extended.
Servicing of the PDP-11 can be done by swapping
modules or by swapping System Units.

MOUNTING

BOXES AND CABINETS

The PDP-11 is available
as either a tabletop
or rack-mounted
configuration. The rack-mounted
configuration
may be installed in a DEC cabinet or
mounted
in a customer
cabinet. The PDP-11 mounts in an EIA standard
19inch cabinet.
The rack-mounted
PDP-11 has tilt-slides as standard mounting hardware.
1 The following

mounting

units

and cabinets

are‘available

for PDP-II

systems.

BOX AND POWER SUPPLY FOR 11/20, ll/lO
SYSTEMSfBAll-CC AND H720)-This
cover and box may be specified with a basic
i1/20 and ll/lO system and includes:
'1. H720 Power Supply

PDP-11 TABLETOP

2. 15’ of power cord with ground

wire
77

+
+

For 115 V standard,

3prong;

U-ground,

15-ampere

connectors

For 230 V pigtail leads on one end
3. Cooling Fans
4. Filter
5. Programmers

Console

Approximate
Size-11"
layout of this unit.

high,

with

11/20

or Turn-Key

Console

26”

wide,

24”

Figure

deep.

with

ll/lO

10-2 shows

the

I
*

Figure 10.2
Approximate

Power-12OV

Weight-100

Table Top PDP-11 Dimensions
Ibs. (including

+- 10%,47-63 Hz

and H720-A)
230V -c 10%,47-63 Hz
(BAll-CC and H720.B)

CP, console

and 4K core)

6 amps.

single phase

3 amps.

single phase

(BAll-CC

PDP-11BkC MOUNTINGBOXAND PGWERSUPPLY(BAll-CSAND H720)
basic mounting
box may be specified with a basic 11/20 or a ll/lO
and includes:
1. Tilt and Lock Chasis Slides
2. H720 Power Supply
3. 15’ of power cord with ground wire
+ For 115 V standard,
3-prong, Uground,
15-ampere
connector
+ For 230 V pigtail lead% on one end
I
4. Cooling Fans
5. Filter
6. Programmers
Console ‘with 11/20 or Turn-Key Console with lI/IO

-This
system

Approximate
Size-lbl/2”
high, 19" wide, 23” deep. Figures
lo-5 show the layout of this unit and give slide dimensipns.

16-3,

.
78

IO-4 and

Approximate

Power-12OV

Weight-90

Itis. (including

zflO%,47-63

Hz
@All-C5
and H720-A)
230V +10%,47-63 Hz
(BAll-C5 and H720-B)

Figure 10.3

Figure 10.5

CP, cdnsole
6 amps.
X
3 amps.

Rack Mountable

and AK core)
single

phase

single phase

PDP-11 Dimensions

Side View of Mounting

Hardware

PDP-11TABLETOP

UcFENSfdh
MOUNTING
BOX (BAll-EC)-The tabletop
Extension Box is supplied, when ordered, for mounting
of up to 6 additional
System Units which can not be contained
in the Basic Mounting
Box. This
unit is supplied with:
1. 15' of power cord with ground wire
+ For 115 V standard,
3-prong, U-ground,
15-ampere
connector
+ For 230 V pigtail leads on one end
2. Cooling Fans
3. Filter
4. Front Panel
5. Unibus Cable from Basic Mounting
Box, 8’6” long

Approximate
Power-120

Size-11"

high, 20” wide, 24” deep

V -C lo%, 47-63 Hz
6 amps.
(when H720-A is added)
230 V + 10%,47-63 Hz
3 amps.
(when H720-B is added)

single

phase

single

phase

‘)

PDP-Ll EXTENSIONMOUNTING

BOX(BAll-ES)-The Extension Box is sup
plied, when ordered, for mounting
of up to 6 additional
System Units which
can not be contained
in the Basic Mounting
Box. This unit contains:
1. Tilt and Lock chassis slides
2. 15’ of power cord with ground wire
+ For 115 V standard,
3-prong, U-ground,
15-ampere
connector
+ For 230 V pigtail leads on one end
3. Cooling Fans
4.. Filter
5. Front Panel
6. Bus Cable from Basic Box, 8’ 6” long
Approximate
Power-120

size--101/z”

high,

19” wide, 23” ,deep

V + lo%, 47-63 Hz
6 amps.
(when H720-A is added).
230V +10%,47-63 Hz
3 amps.
(when H720-B is added)

single

phase

single phase

PDP-11FREESTANDING

BASE CABINET (H969CA)This
optional
cabinet
can be used to mount
the BAll-CS
Basic Mounting
Box and a
BAll-ES Extension Mounting Box supplied with Tilt and Lock chassis slides
.
.
in addition to other PDP-11 equipment.

cabinet

Panel capacity is six lOI/”
high mounting
spaces, each of which
with black plastic panels if equipment
is not mounted-(5
panels,
supplied).
items supplied with the cabinet include:
1. H950-A Frame
2. H952-E Coasters
3. H-952-F Levelers
4. H-952-C Fan Assembly (in top of cabinet)
5. H-950-S Filter
6. PDP-11 Logo
7. H-950-B Rear Door
8. lOl/," Plastic Bezels, maximum
of 5 supplied
9. Two H952-A End Panels

is covered
maximum,

10. H955)-D Mounting
11.. H952-B Stabilizer
12. #7406782
13. #7005909
left side)

Panel Doors
Feet
Kick Plate
Power Distribution

Approximate

Size-22”

Approximate

height-150

e Voltage-115

Panel

(ac ?rrd dc, mounted

wide, 39” deep (including
Ibs. (without

stabilizer

feet),

on upper
711/”

high

computer)

V 60 Hz (for fans)
230 Y 50 Hz (for fans)

PDP-11 POWER SUPPLY SUBSYSTEM

H728-This
Power supply is used in
the Basic and Extension Mounting
boxes and supplies power to all devices
in. basic PDP-11 systems,
mounted
in one of these boxes. It is included
‘but must be ordered separately with a BAllES or BAllEC Extension Mounting Box.
Approximate

Size-161/,”

wide, 8” high, 6” deep

Approximate

Weight-25

Ibs.

Voltages-(specify
input voltage)
IN
108V +-lo%,
47-63
120V *lo%.
47-63
47-63
216V *lo%,
228V +-lo%,
47-63
240V *lo%,
47-63
OUT

+5v
-15v

Hz
Hz
Hz
Hz
Hz

amps
amps
amps
amps
amps.

(H720A)
(H720A)

gEE;
(H7208)

. 12 amps
10 amps

,*50/o
‘5%

+8RMS
-22v

6
6
3
3
3

(unregulated)
(unregulated)

1.5 amps,

1.0 amps

FREESTANDING PROGRAMMER’S TABLE (H952-HA)-This
freestanding
table
fits directly
below the programmer’s
console
in the Freestanding
Base
Cabinet and extends into the cabinet approximately
1”. The surface plate is
supported
by its own adjustable
height legs.
Approximate

Size-20”

extension

from

cabinet,

19”

wide,

27”

above

floor

SYSTEM UNITS AND CABLES
The following
items are available for mounting
eral device logic into a PDP-11 system.

standard

and special

periph-

DDll
is a prewired System
PERIPHERAL MOUNTING
UNIT (DDll-A)-The
Unit which allows standard
small peripheral
interfaces
to be mounted
in a
PDP-11 system.
It accepts standard
small peripheral
interfaces
(up to 4)
such as the KLll Teletype Control or the controller
portion (PCll-M)
of the
High Speed Reader/Punch.
For mounting,
it requires one-sixth
(l/6)
of a
BAll Mounting Box.
BLANK SYSTEM. UNIT (BBll)-The
BBll
consists of three 288-pin
connector blocks connected
end-to-end.
This unit Is unwired except for Unibus
and power connections
and allows customer-built
interfaces to be integrated
easily into a PDP-11 system. For mounting
it requires one-sixth
(l/6)
of a
BAll Mounting Box.

UNIBUS MODULE (M920)-The
M920 is a double module which connects
the Unibus from one System Unit to the next within a Mounting
Box. The
A single M920
printed circuit cards are separated ‘by 1” for this-purpose.
will carry all 56 Unibus signals .and 14 grounds.
UNIBUS CABLE (BCllA)-The
BCllA is a 120~conductor
flexprint cable used
to connect System Units in .different
mounting
boxes or a peripheral
device
which is removed from the mounting
boxes.
The 120 signals consist of the 56 Unibus
grou,nds alternate to minimize cross talk.

lines plus 64 grounds.,
.

Signals

and

2’
5’
8’6”
10’
15‘

BCl lA-2.

BCllA-5
BCllA-BA
BCllA-10
BCl l A-15
BCl lA-25

25’

CABLE REQUIREMENTS
When an Extension
Mounting
Box is used, an external cable, the @CIlA, is
the only signal.conn@ion
between mounting
boxes. This external bus cable
may also be. used to connect other peripherals
to the PDP11. The maximum
combined,
internal and external,
bus cable.length
is 50’.
-

’ PDP-11/20 POWER REQUlkEMENTS
Input Voltage and Current-105-125
peres, (single phase)
Line Frequency47-63

Pdwer Dissipation400

watts

Vat,

6 amperes,

210-260

Vat

3 am-

A standard
15-foot, 3-prong,- U-ground,
15-ampere,
line cord is provided on
the rear of the PDP-11 for connection
to the power source on 120 Vat
models. On 230 Vat models,
a 15-foot, S-conductor
cable with pigtails is
provided.

TELETYPE REQUIREMENTS
The standard
Teletype
requires
a floor space approximately
wide by 181/s inches deep. The Teletype cable length restricts
within 8 feet of the si#e of the computer.

221/2 inches
its location to

Input Voltage-115
-CO.75 Hz

*lo%,

Line Current

Drainl2.0

Power Dissipation-150

Vat

*lo%,

60 Hz kO.45

Hz, 230 Vat

50 Hz

amperes
watts

The Teletype plugs into the rear of the PDP-11 Basic Mounting
Box and is
turned ON and OFF by the~eower
s,witch on the front panel of the PDP-11.

’

ENVIRONMENTAL REQUIREMENTS
The PDP-11 is designed to operate from +lO to +5O”C
humidity
of from 20 to 95% (without
condensation).
82

and with

a relative

. INSTALLATION

.PROCEDlJRE

The PDP-11 is crated for shipment to the customer site to prevent
Installation
is provided by DEC personnel at the customers site.
Computer customers may send persorinel to instruction
courses
operation,
programming,
and maintenance
conducted
regularly
Massachusetts,
Palo Alto, California,
and Reading, England.

damage.

on camput&
in Maynard,

The PDP-11 has adopted
a moddlar
packaging
custom configuring
of systems, easy expansion

approach
to allow
and easy servicing.

CHAPTER

PAPER TAPE SiFTWARE
PAPER TAPE SOFTWARE SYSTEM (PTS)

SYSTEM‘
’

PTS is a compatible
group of software packages designed to aid development
of PDP-11 application
programs.
A brief description
of each item with its
major features is offered below with detailed programming
information
available in corresponding
software user’s manuals.
PTS FEATURES
l
l

0
0

4K Absolute Assembler
Symbolic Program Editor for editing of paper tape which is string oriented
On-Line Debugging Aid allowing rapid and accurate modification
of assembled programs
I/O Driver Routine.allowing
subroutine
level communication
with peripheral devices. and double buffered
input/output
operation
concurrent
with
running programs
Floating Point Math Package’using
both reentrant
and relocatable
code
General Utilities in.cluding loaders and dump routines

PALIlA ASSEMBLER-This

two- or three-pass
assembler
runs on a PDP-11
with 4K words bf core memory and an ASR-33. It will also accommodate
a
high-speed
reader/punch.
Optional outputs include the absolute object code,
an assembly listing containing
each sdurce statement,
and an indication
of
any errors detected
in the statement.
A symkol table may be alphabetically
listed.

ED11 EDITOR-The
PDP-11 Editor (EDll)
allows the user to type identified
portions
of source program
on the teleprinter
and to make corrections
or
additions.
This is accomplished
by typing simple commands
that cause the
Editor to-read, print, punch out on paper tape, search, delete and/or add to
the text of the program.
Use of the ED11 presupposes
no special knowledge
or technical skill beyond
that of the operation
of explicitly
defined
one-character
commands.
The
commands
are grouped
according
to function:
input, positioning
of the
current-character
location pointer, output, search (which is done by character string), insert, delete, and exchange of text portions.

ED11 uses 2,000 words of-core and requires an ASR-33 unit which includes
a printer, keyboard,
paper tape reader and paper tape punch. Alternatively,
a KSR-33 may be used in conjunction
with the high-speed
paper tape’reader
and punch.

ODT-11ON-LINE

DEBUGGING TECHNIQUE-ODT-11
is a core resident program which allows the user to debug his binary programs at the console by
running them in specific segments and checking for expected results at various points. If modification
of the program is needed, the user can alter the
contents of the appropriate
location by “opening’!
it and typing in new data.
Two versions of ODT are available,
one being a subset of the other. The
larger system uses 750 words of core and utilizes an ASR-33, or a KSR-33
and a high-speed
paper tape punch and reader. The smaller version uses the
same peripherals
and 500 words of core. Up to eight breakpoints
can be set
using the larger .version of ODT, while one breakpoint
is allowed in the
smaller version.

Debugging
operations
alternate between commands
to ODT and the running
of the program to be debugged. Breakpoints
are set in the user’s program by
ODT commands,
and a command
to run starts execution
of the program.
When a breakpoint
is encountered,
the program run is suspended,
and the
progress of its execution can be monitored and altered. This is accomplished
by using commands
to open memory locations of interest, as well as special
registers.
An Operator may examine and change the operating
priority of both ODT
and the user’s program, the mask and address range for searches, results
of logical and arithmetic operations,
the SP and PC, and the general registers.
Other commands
will search for values of specified bits of a word, or for
references to.an address within an address range, calculate 16-bit and 8-bit
offsets to an address and restart the running of the user’s program at any
address.
IOX input/Output
Utility ‘Peripheral Driver-lox
is a set of service routines
allowing sing&or
double buffered I/O processing on an ASR-33 and/or a highspeed paper tape reader and punch. This routme atlows the user to make
simple assembly language calls specifying devices and data forms to accomplish interrupt-controlled
data transfer concurrent
with execution of the running program. Multiple devices can be run simultaneously.
IOX frees the user from the details of dealing directly with the device and
allows development
of programs which may be run under the direction of a
monitor with minimum modification.
IOX also provides some degree of real-time control
to be executed at priority leaIs at the completion
data transfer.
MATH PACKAGE-A
simplify

programming.

number
These

of commonly
routines

by allowing pser programs
of some device action or

used subroutines

are reentrant

and

are available
relocatable

to pro-

vide maximum
flexibility.
Arguments
are treated as floating point numbers
with a signed 31.bit fraction and a signed 15-bit exponent. Subroutines
supplied include:
ADD
MULtiply
SUBtract
DlVide
SIN
cos
ATAN
FIX-FLOAT
FLOAT-FIX
NORmalize
(Integer MULtiply and DlVide are also supplied)
LOADiER%Two
l

A Bootstrap

ABSolute
jumps

loaders are used:
loader loads the ABSolute

loader and jumps to it.

loader loads PAL-1lA output, checks
to a user program or halts when done.

CORE DUMP ROUTINES-Routines
ranges of core locations on paper
printer in octal.

are provided
tape in absolute

for checksum
which
format

errors

and ,’

dump
specified
or on the tele-

’

CHAPTER 12

’

THE OPERATOR’S CONSOiE

‘The PDP-11 Operator’s
Console has been configured
to achieve convenient
control of the system. Through switches and keys on the console, programs
or information
can be manually inserted or modified. Also.indioator
lamps on
the console face display the status of the machine, the contents
of t’he Bus
Address Register and the data at the output of the data paths.
_
The consoie

is shown in Figure 12-1.

kllilaliltlalll

lcidial

Figure

CONSOLE

12-1

ELEMENTS

The console has the following
indicators
and switches:
1. A bank.of
8 indicators,
indicating
the following
conditions
or oper‘ations:
Fetch, Execute, Bus, .Run, Source, Destination
and Address
(2 bits).
2. An l&bit Address Register Display
3. .A 18bit Data Register Display
4. An l&bit Switch Register
5. Control Switches:
a. LOAD ADDR (Load Address)
b. EXAM (Examine)
c. CONT (Continue)
,d. .ENABLE/ HALT
e. SIINST-S/CYCLE
(Single Instruction/Single
f. START
g. DEP (Deposit)

Cycle)

INDICATOR
LIGHTS-The
indicators
signify
specific
machine
functions,
operations,
or states. Each is defined below.
1. Fetch-indicates
that thdcentral
processor
is in the state of fetching
an instruction.
2. Execute-indicates
that the central
processor
is in the state of
executing an instruction.
87

3. Bus-indicates
that a peripheral
is controlling
the bus. It is lit when
BBSY (Bus Busy) is asserted, unless the processor (which includes
the Console) is asserting BBSY.
4. Run-indicates
that the processor is running. It monitors the cdntrol
flip-flop for the internal clock.
5. Source&ndicates
that the central
processor
is. obtaining
source
data except from an internal register.
6. Destination-indicates
that the central processor
is obtaining
des
tination
data (except from an internal register).
7. Address-identifies
the source or destination
address cycle of the
’ central processor,
using two lights that are decoded zero, one, two,
or three. When references are made via the Unibus to the.addresses,
the lights tell the machine’s
source or destination
cycle. For an internal register reference,
there is a “zeroth”
addressing
operation.
REGISTER DISPLAYS-The
Operator’s
Console has an l&bit Address Register Display and a 16-bit Data Register Display. The Address Register Display
is tied directly to the output of an l&bit
flip-flop
register called the Bus
Address Register. This register displays the address of data examined
or
deposited.
*The l&bit data register is divided on the face of the console by a line into
two 8-bit bytes. This register is tied to t,he output of the processor data paths
and will reflect the output of the processor adder.

SWITCH REGISTER-The
PDP-ll/lO’and
PDP-ll/PO
can reference 216 bytes
addresses.
However, the Unibus ,has expansion
capability
for 218 byte addresses.
In order that the console can access the entire
l&bit
address
scheme, the switch register is 18 bits wide. These bits are assigned as 0
through
17. The highest two are used only as addresses.
A switch in the
“up” position is considered
to have a “1” value and in the “down”
position
to .have a “0” value. The condition
of the 18 switches can be loaded into the
bus address register or any memory location by using the appropriate
control
switches which are described below.
CONTROL
SWITCHES-The
switches
listed in item 5 of the “Console
Elements”
have these specific control functions:,
1. LOAD ADDR-transfers
the contents
of the l&bit
switch register
into the bus address register.
2. EXAM-displays
the contents
of the location
specified by the bus
address register.
the contents of the low 16 bits of the switch register
3. DEP-deposits
into the address then displayed in the address register. (This switch
is actuated by raising it.)
or prevents running of programs.
For a pro4. ENABLE/HALT-allows
gram to run, theswitch
must be in the ENABLE position (up). Placing
the switch in the HALT position (down) will halt the system.
5. START-starts
executing
a program when the ENABLE/HALT
switch
is in the ENABLE position. When the START switch is depressed,
it
asserts a system initialization
signal; the system actually starts when
the switch is released.
The processor
will start executing
at the
address which was last loaded by the LOAD ADDR key.
6. CONT-allows
‘the machine
to continue
without
initialization
from
whateyer state it was in when halted.
whether
a single
instruction
or a
7. S/ INST-S/CYCLE-determines
single bus cycle is performed
when the CONT switch is depressed
while the machine is in the halt mode.

88
.

When the system is running
a program,
the LOAD ADDR, EXAM, and DEPOSIT functions
are disabled to prevent disrupting
the,program.
When the
machine
is to be halted, the ENABLE/HALT
switch is thrown
to the halt
position.
The machine will halt either at the end of the current instruction,
or at the end of the current bus cycle, depending
upon the position of the
S/ INST-S/CYCLE switch.

-OPERATING

THE CONTROL

SWlTCHES

When the PDP-11 has been halted, it is possible to examine and update bus
locations. To examine a specific location, the operator
sets the switches of
the switch register to correspond
to the location’s address. The operator then
presses LOAD ADDR, which will transfer the contents
of the switch register
into the bus address register. The location of the address to be examined
is
then displayed in the address register display. The operator then depresses
EXAM. The data in that location will appear in the data register display.
If the operator then depresses EXAM again, the, bus address register will be
incremented
by 2 to the next word address and the new location will be
examined.
In the PDP-11, the bus address register will always be pointing to
the data currently
displayed in the data register,.The
incrementation
occurs
when the EXAM switch is depressed,
and then the location is examined.
The examine function
has been designed
so that if LOAD ADDR and then *
EXAM are depressed,
the address register will not be incremented.
In this
case, the location
reflected
in the address
register
display is examined
directly. However, on the second (and successive) depressings
of EXAM, the
bus address register is incremented.
This will continue
for successive depressings as long as another control switch is not depressed.
If.the operator finds an incorrect entry in the data register, he can enter new
data there by putting it in the switch register and raising the DEP key. The
address register will not increment
when this.data
is deposited.
Therefore.
when the operator
presses the EXAM key, he can examine the data he just
deposited.
However, when he presses EXAM again, the system will increment.
If the operator
attempts
to examine data from, or deposit data into, a nonexistent memory location, the “time out” feature will cause an error flag. The
data register will then reflect location 4, the trap location, for references to
nonexistent
locations.
To verify this condition,
the operator
should try to
deposit some number other than four in that location;
if four is still indicated, this would indicate that either nothing is assigned to that location, or
that whatever
is assigned to that location is not working properly.
When doing consecutive
examines or consecutive
deposits,
the address will
increment
by 2, to successive word locations. However, if the programmer
is
examining
the fast registers (the “scratch
pad” memory),
the system only
increments
by 1. The reason for this is that once the switch register is set
properly, the programmer
can then use the four least significant
bitsof
the
switch register in examining
fast memory registers from the front panel.
To start a PDP-11 program,
the programmer
loads the starting
address of
the program
in the switch register, depresses
LOAD ADDR, and after ensuring that the ENABLE/HALT
switch is in the ENABLE position,
depresses
ST;:.
The program will ,start to run as soon as the START switch is reThe Run indicator
lamp is driven off the flip-flop that controls
the clock.
Normally, when the system is running,
not only will this light be on, .but the

other lights; (Fetch, Execute, Source, Destination;
the.Address
lights, and the
Address and Data registers) will be flickering.
If the run light is on, and none
ofthe
other indicators are flickering,
the system could. be executing a “wait”
instruction
which waits for an interrupt.
While in the halt mode;if
the operator wishes to do a siqgle instruction,
he
places the S/INST-S/CYCLE
switch in the S/lNST position
and depresses
CONT. When CONT is depressed,
the console momentarily
passes control to
the processor, allowing the machine to execute one instruction
before regain.ing control.
Each time the CONT switch is depressed,
the machine
will
execute one instruction.
Similarly,
if the operator
wishes to have the machine perform
a single bus
cycle, he places the S/INST-S/CYCLE
switch in the S/CYCLE position and
presses CONT. The machine will then perform
one complete
bus cycle and
halt. The operator
cannot do an,examine
or deposit function
at the end of a
single bus cycle unless the cycle ends coincidental
with the end of an instruction.
This prevents altering machine flow. Only when the machine is at
the end of an instruction
and in the halt mode’can
the examine or deposit
functions
operate.
To start the
ENABLE/HALT
switch.

machine
switch

running
in the

its program
again, the operator
ENABLE position,
and depresses
‘.

places the
the CONT

APPENDIX

A-PDFIi

INSTRUCTION

instruction
Operation

Mnemonic

DOUBLE OPERAND GROUP: OPR scr;dst
MOV(B)

MOVe (Byte)
(src) + (dst)
CoMPare (Byte)
Cm) - (dst)
Blt Test (Byte)
(src) A (dst)
Blt Clear (Byte)
- (src) A (dst) + (dst)
Blt Set (Byte)
(src) v (dst) + (dst)
ADD
’
suBtra(@
+ (dst) -, (dst)

CMP(B)
/
BIT(B)
BIG(B)
BIS(B)
ADD
SUB

’

CONDITIONAL
BR

BNE

BEQ
BGE
BLT
BGT
BLE
BPL
BMI
BHI
BLOS
BVC
BVS
, BCC
(or BHIS)
BCS
(or BLO)

(dst)‘-

BRANCHES:

REPERTOIRE

OP Code

Codes
ZNCV

Timing

.lSSDD

/ d-0

2.3

-2SSDD

/ r’ fI r/

2.3;

.3SSDD

4 i-0

2.9*

4SSDD

,,’ i-0

2.9

.5SSDD

/ d-0

2.3

06SSDD

r/ r’ / ,/

2.3

16SSDD

/ / / /

2.3

’

(src) + (dst)

Bxx lot

BRanch

(unconditionally)
lad+ (PC)
-.
Bratch if Not Equal (Zero)
lot + (PC) if Z = 0
Branch if Equal (Zero).
loc+(PC)ifZ=l
Branch‘if Greater or Equal (Zero)
loc+(PC)ifNVV=O)
Branch if Less Than (Zero)
loc+(PC)ifNYV=l
Branch if Greater Than (Zero)
loc+(PC)ifZv(NVt~O)
Branch if Less Than or Equal (Zero)
loc+(PC)ifZv(NVV)=l
Branch if PLUS
loc+(PC)ifN=O
b
Branch if Minus
loc+(PC)ifN=l
Branch if Higher
lot-*
(PC) ifCvZ=O
Branch if Lower-o? Same
loc+(PC)ifCvZ=l
Branch if overflow Clear
- loc+(PC)ifV=O
Branch if overflow Set
lot+
(PC) if V= 1
Branch if Carry Clear
lot+
(PC) ifC=O
Branch if Carry Set
lot + (PC) if C = 1
91

0004xx

2.6-

0010xx

2.6-

0014xX

2.6-

002oxx

2.6 -

0024Xx

2.6-

003oxx

2.6 -

0034xX

2.6 -

ioooxx

i.6-

1004xX

2.6 -

1010xx

2.6 -,

1014xX

2.6 -

102OXX

2.6 -

1024Xx

2.6 -

103oxx

2.6 -

1034xX

2.6 -

’

SUBROUTINE

1JSR

CALL: JSR reg,dst
Jump to SubRoutine
(dstb
(tmp), 0%) J
(PC> + (w3), Ww) + (PC)

SUBROUTINE RETURN: RTS reg
RTS
ReTurn from Subroutine
(w) + PC. t (w3)

004RDD

4.2

00020R

3.5

.050DD

1000

2.3

-051DD

4 400

2.3

SINGLE OPERAND GROUP: OPR dst
CLR(B)
<

CLeaR

(Byte)
0 + (dst)
COMplement
(Byte)
- (dst) + (dst)
INCrement
(Byte)
(dst) ,+ 1 + (dst)
DECrement (Byte)
(dst) 1
(dst)
NEGate (Bvte)
1 + (dst)
ADd Carry (Byte)
(dst) + (Q --, (dst)
SuBtract Carry (Byte)
(dst) - (C) + (dst)
TeST (Byte)
0 - (dst)
Rotate Right (Byte).
rotate right 1 place with C
ROtate Left (Byte)
rotate left 1 place with C
Arithmetic
Shift Right (Byte)
shift right with sign extension
Arithmetic
Shift Left (Byte)
shift left with lo-order zero
JUMP
GM) + (PC)
SWAP Bytes
bytes of a word are exchanged

COM(B)
INC(B)
DEC(B)

i 4
+‘(dstj +

NEG(B)
ADW)
SBC(B)
TST(B)
ROR(B)
ROL(B)
A5R( B)
ASL( B)
JMP
SWAB

-052DD
.053DD
.05,4DD
.055DD
.056DD
.057DD
.060DD

.061DD
.062DD

r’ r’/

2.3”

.063DD

4’4 r/ 4

2.3”

OOOlDD

1.2

0003DD

,.‘t/OO

2.3

1.5
CONDITION
CODE OPERATORS: OPR
Condition
Code Operators
set or clear combinations
of condition
code bits.
Selected bits are set if S = 1 and cleared otherwise.
Condition code bits corresponding
to bits set as marked in the word below are set or cleared.
CONDITION

CODE

OPERATORS;
0

0
1

2
I

4SNZVC
I

i
54324

Thus SEC ‘= 000261 sets the C bit and has no effect
code bits (CLC = 000241 clears the C Bit)
OPERATE GROUP: OPRHALT
HALT
000000
processor stops; (RO) and the HALT address
00000 1
WAIT
WAIT
processor releases l&s, waits for interrupt
92

on the other

in lights

condition

l.8

1.8

RTI
IOT
RESET
EMT
TRAP

ReTurn from Interrupt
000002
4 d / /
t Pa
f (W
Input/Output
Trap
000004
d/v’/
(PSI 4 s (PC) 4, (20) + (PC), (22) + (PS)
RESET
000005
an INIT oulse;s issued bv the CP
EMulator Trap’
104400--1104377
(PS) 4, (PC) 4, (30) + (PC), (32) + (PS)’
TRAP
104400-104777
4
PSI 4, (PC) 4 r (34) + (PC), (36) + (PSI

4.8
8.9
20 ms.

NOTATION:
1. for order codes
word/byte
bit, set for byte (+lOOOOO)
field,
. SS-source
DD-destination
field
XX-offset
(8 bit)
2. for operations
A
and,
V
or,
not,
contents of,
(’
XOR
“is pushed onto the processor stack”
&
“the contents
of the top of the processor
stack
popped and becomes”
+
“becomes”
3. for timing
*
0.4 ‘p.s less if not register mode
0.9 ps less if conditions
for branch not met
0
1.2 r.~smore if addressing odd byte
/
(0.6 r~s additional
in addressing
odd bytes otherwise)
4. for condition codes
f
~;a;~;$=W

-r

set

APPENDIX R-ADDRESSING

SUhMlARY

ADDRESSING. ‘MODES.

S,C or’

GENERAL

Mode
0
’

Description
register

registerdeferred

auto increment
auto increment
deferred
auto decrement
auto decrementdeferred
indexed
indexed deferred

5
6
7

dst

SymboJii

Timing
src
00’

$ R or (R)
CR) +
@ (RI +
- CR)
@ - 09
X CR)
@ X (RI or @ CR)

1.5
1.5
2.7
1.5
2.7
2.7
3.9

(ws)
dst
00
1.4

1.4
2.6
1.4
2.6
2.6
3.8

PC REGISTER ADDRESSING

f
.6
7

Timing

w)
dst

1.4
2.6
2.6
3.8

Symbolic

Description

Mode

immediate
absolute
relative
relative deferred

F?YA

2.7
2.7
3.9

INSTRUCTION FORMATS
DOUBLE

OPERAND

GROUP:

OPR

I PI CODE 4 11’15
15

CONDITIONAL

BRANCHES:

src,dsl
Y

I I I I I dsi,

Sax

lot

(loc=bffset-2)+

I
0

+2)

SUBROUTlNE

CALL:

JSR

r*g.dsl

SUBROUTINE

RETURN;

RTS

rr9

0 ,
15

1 r*g II
3

SINGLE

OPERAND

GROUP:

OPR

COOE

drt

CONDITION
0

CODE

OPERATORS:
0

0“

4SNZVC
5432t

APPENDIX C-ADDRESS
0
4

USER DEVfCE INTERRUPT VECTOR
BUS ERROR, ILLEGAL INSTRUCTION,
STACK
VECTOR
RESERVED INSTRUCTIONS
TRAP VECTOR
CODE 000003 AND TRACE TRAP VECTOR
IOT INSTUCTION TRAP VECTOR
POWER FAIL INTERRUPT VECTOR
.EMT INSTRUCTION
TRAP VECTOR
TRAP INSTUCTION TRAP VECTOR

10
14
i:

30
34
40
44
50,
54
z
70
74
.
.
.
.
.
.
400
.
.
.

OIKRFLOW

TRAP

SYSTEM SOFTWARE COMMUNQTION

1
TELEPRINTER INTERRUPT YECTOR
TELETYPE KEYBOARD
AND LOW SPEED READER INTERRUPT
VECTOR
HIGH SPEED PAPER TAPE PUNCH INTERRUPT VECTOR
HIGH SPEED PAPER TAPE READER INTERRUPT VECTOR

(additional

interrupt

vectors)

PROCESSOR STACK
PROGRAM AND DATA
RESIDENT SYSTEM SOFTWARE

*
.
.

(ABSOLUTE
(end of implemented
160000

LOADER,
storage)

BOOTSTRAP,

I/O EXECUTIVE)

.
.

SMALL

READ-ONLY

STORAGE

UNITS

.
.

OTHER

PERIPHERAL

DEVICE

REGISTERS

177550

MAP

HIGH SPEED READER AND PUNCH
REGISTERS

DEVICE STATUS AND BUFFER

177560

TELElYP;- KEYBOARD
REGISTER
-’

AND PUNCH

DEVCE STATUS AND EUFFER

.
.
.

*
177576
17-7600
.

RESERVEP

FOR EXPANSION

177677
177700
.
.

GENERAL

REGISTERS

1777;6

CENTRAL

RO -

OF PROCESSOR

PROCESSOR STATUS REGISTER (PS)

REGISTERS

APPENDIX l+UNlBUS

OPERATIONS

There are ‘six bus operations:
four to effect data transfers,
on9 to transfer
bus control, and one to effect a program
interrupt.
This appendix describes
the signal interaction
on the Unibus to perform these six operations.

DATA TRANSFERS
The four data transfers

use the C lines coded as follows:

Cl
0
1

co
0
1
0

DATI-DATa In
DATIP-DATa In, Pause
DATO-DATa Out
DATOB-DATa Out, Byte

DATI AND DATIP-These
two bus operations
transfer
data from a Slave
whose address is specified by A < 17:Ol > into the master. Both transfers
are made in words on D, < 15:OC >.
In destructive
read-out
devices,
DATI commands
a read-restore
operation,
while DATIP commands
a readpause operation
and the setting of a pause flag. DATlPs are to be followed
by a DATO or DATOB to effect a read-modify.write
data exchange.
In non
_
destructive
read-out devices, DATI and DATIP are treated
identically.
The
sequence of operations
is as follows:
1. Master puts address on A, 0 or 1 on C, and waits 150 nanoseconds.
(75 nanoseconds
for deskewing
address + 75 nanoseconds
for ad.
dress decoding).
2. Master asserts MSYN.
3. Slave decodes address, sees 0 er 1 on C, and MSYN and ldegins read
cycle (flip-flop register would simply gate flop outputs to bus).
4. Slave completes
read cycle, outputs
data to D lines, and asserts
SSYN. If the slave is a destructive
read-out device, it now restores
data on a OATI: it sets a pause flag on a DATIP.
Figure D-l shows the signals

for a DATI operation.

a
SIGNALS

MASTER

ADtRESS-CONTROL

MSYN

SSYN
SIGNALS

DATA

SLAVE
IR

ADDRESS-CONTROL

DATA p

MSYN

SSYN
MEMORY

CYCLE
T= SIGNAL

TFIANSMITTEO

R*SIGNAL

RECEIVED

Figure D-l DATI Operation
99

5. Master sees SSYN and waits 75 nanoseconds,
minimum
(data deskewing + internal gating deskewing).
data, drops MSYN, and waits 75 nanoseconds
(des6. Master.strobes
kew address).
7. Masfer drops A and C and waits for SSYN to fall.
8. Slave sees MSYN fall and drops SSYN and D lines.
9. Master sees SSYN fall, signaling-end
of bus operation.
NOTES:
1. Step 1 of the
DATI P.
‘2. Step 2 of the
DATIP.

data

transfer

may

begin

at step

7 of the

current

DATI

data

transfer

may

begin

at step

9 of the

current

DATI or

MT0
AND DATOB-These
two bus operations
transfer data out of the master to the slave. DATO is used to transfer
a word to the address specified
by A < 17:Ol >. The slave ignores A00 and the data appear. on D < 15:00 >.
DATOB is used to transfer a byte to the add<ess specified by A < 17:00 >.
A00 = 0 indicates the low byte and data appears on D < 07:OO >; A00 f 1
indicates high byte and data appears on D < 15:08 >. The sequence of op
eration is as follows:
1. Master puts address on A, data on D, 2 or 3 on C, and waits 150
nanoseconds
(75 nanoseconds
for deskewing
address + 75 nanoseconds for address decoding).
.
2. Master asserts MSYN.
3. Slave decodes address, sees 2 or 3 on C and MSYN and strobes in
word or byte. When slave has taken data, it asserts SSYN. If the slave
is a destructive
read-out device and its pause flag is set (by DATIP),
slave begins write cycle; if .not, slave must first do a read cycle to
clear the memory cell and then a write.
sees SSYN and drops MSYN and waits 75 nanosecondi
(des4. Master
kew address).
e
Master drops A, D, and C, and waits for SSYN to fall.
2: Slave sees MSYN fall and drops SSYN.
7. Master sees SSYN fall, signaling end of bus operation.
Figure D-2 shows the signals

for a DATO operation.

DATO
SIGNALS

AT MAST&
.

DATA
I

IT
IT

MSYN

SSYN

SIGNALS

AT SLAVE

ADDRESS-CONTROL

BATA IR
IR

MSYN
SSYN
MEMORY

‘T
-1

CYCLE

Figure D-2 DATO Operation
100

\
NOTES:
1. Step 1 of the next-data transfer-may begin at step 5 of the current DATO or
DATOB.
2. Step .2 of the next data transfer may begin at step 7 of the current DATO or
DATOB.
\

PTR-PRIORITY

TRANSFER

This bus operation
is used to pass control of the bus from
another.
The steps which fbllow are performed
simultaneously
transfers:

one master to
with the data

0. Current master device always has BBSY asserted.
device asserts its assigned BR line.
1. Requesting
determines
which BR is highest, and
2. Processor sees BR asserted,
asserts the corresponding
BG line if the processor’s
Current Prior@
level’allow that level of bus request.
3. Each device that receives the BG passes it on to the next
device
.
unless it itself is requesting.
4. The BG is propagated
a!ong the priority chain until it reaches the
first .requesting
device. This device becomes
selected as next bus
master and does not allow the BG to pass to succeeding
devices.
5. The selected device asserts SACK and drops its BR, .and waits for
BBSY,,BG, and SSYN to drop.
.
6. The processor sees SACK and drops BG.
The device which is current
master
completes
its data transfers,
. 7.
drops BBSY, and ceases to be bus master.
.
8. The selected device sees BG, BBSY, and SSYN drop, becomes bus
master, asserts BBSY, drops SACK, and begins data transfers.
9. New master relinquishes
bus control, either to the processor or to a
requesting
device, by dropping
BBSY at the end of its last bus op
eration. This is termed a passive release of bus control.
NOTES:
1. NPR bus requests ore handled as above.
2. Processor defers action on BR <7:4>
until last bus cycle of an instruction
execution or interrupt sequence, NPR is acted upon immediately.
.
3. Processor becomes bus master and asserts BBSY whenever it sees BBSY = 0
end no other’ device has been selected or is being selected as next bus master.
4. Processor will not execute step 2 if SACK is asserted. See note 2 under INTR.
Figure D-3 shows the signals

for a PTR operation.

PTR
SIGNALS

AT DEVICE
JT

SIGNALS

II3

SACK

AT PROCESSOR
JR

‘0G
IR

SACK
T=

SIGNAL

R * SIGNAL

TRANSMITTED

AS RECEIVED

Figure D-3

PTR Operation
101

INTR-lNTerRupt
This bus operation
is initiated by a master immediately
after receiving bus
control to effect a program interrupt in the processor. It proceeds as follows:
0. Device has become bus master via PTR and BBSY is as&ted.
Master puts interrupt vector address on D and asserts INTR.
:: Processor sees INTR and waits 75 nanoseconds
(deskewdata).
Processor strobes data and asserts SSYN.
:: Master sees SSYN, drops INTR, -D, and BBSY. The master has now
relinquised
bus control directly to the processor. The INTR sequence
is termed an active release of bus control.
- 5. Processor sees INTR drop and drops SSYN and. enters interrupt
sequence to update PC and PS.
1. Step 1 must be made simultaneously
with step 8 of FTR; that-is,
SACK cannot
be dropped
until INTR i,s asserted.
2.’ When the processor’sees
SACK drop. it waits 75 nanoseconds
(deskew).
If, at
that time, INTR = 1. the processor
issues no SG’s until the interrupt
sequence
is complete.

Figure D-4 shows the signals for the INTR operation.
~
SIGNALS

MASTER
BBSY

DATA

INTR

SMN

-%NALS

AT FIWCESSOR
BBSY -i
MTA

UT
I

-R
jR

INTR

1
IT

SBYN
T.

SIGNAL

TRANSMITTED

R * SIGNAL

RECEIVED

Figure D-4

GENERAL

INTR Operation

NOTES ON THE BUS OPEliATlONS

1. A master device doing a read-modify-write
operation
must keep bus
control BBSY asserted for both bus transactions
(both the DATIP
. and the DATO or DATOB). This is the one case where an NPR request
will not be honored between bus transactions.
2. A device becomes master by the PTR operation.
If ‘the request for
bus control was made on the NPR line, bus control must be released
passively (by dropping BBSY). Bus control is then passed either back
to the processor to execute the next bus cycle of the instruction
or
to another device requesting
on the NPR tine. If a device becomes
master via a,BR request line, control may be passed actively back
to the processor by using the INTR operation
or passively (by drop

pitig BBSY); If control is given up actively, only NPR~requests will‘be
honored during the interrupt
sequence of.updating
the PC and PS.
If control
is given up passively, control
may pass either to +he
processor
to fetch the next instruction
or to an NPR requesting
device.

io3

’

The PDP-11provides
Direct Device Addressing.
All memory-and
devices on the Unibus are directly addressable
and may be op
erated upon by all computer
instructions.
Direct device to device
transfers are possible.

,
.
104