DEC STD 168

This document, DEC Standard 168 (Revision A, dated January 1979), titled "PDP11 Extended Instructions," provides the architectural definition and control for PDP11 instructions that utilize reserved and extended opcode spaces.

Authored by Lloyd Dickman for Digital Equipment Corporation, its primary purpose is to ensure architectural consistency, promote software transportability across the PDP11 family, facilitate migration to the VAX11 family, and reduce associated support problems.

The standard defines:

• A framework for extending the PDP11 instruction set, including opcode utilization, instruction groupings, operand types, trap mechanisms, and rules for suspendable instructions.
• Extended-instruction data types, such as character, decimal string, and long integer.
• Detailed descriptions and definitions of various extended instructions, particularly those forming a commercial instruction set (character string, decimal string, load descriptor operations) and processor-specific instructions.
• Reinterpretations of certain traditional PDP11 instructions, like the multiprocessing memory lock.
• Waivers and exceptions for specific PDP11 models regarding the implementation of these instructions.

It serves as a central repository for the specifications of PDP11 extended instructions and necessary reinterpretations of traditional ones.