Jupiter CIS Instructions Oct80

This interoffice memorandum, dated October 7, 1980, from Sara Murphy of Technical Languages, provides an update to the Commercial Instruction Set (CIS) specification.

The primary motivation for these instructions is to significantly improve COBOL object code performance, particularly on the JUPITER system and KL-10 Model B machines. The most significant change in this update is that all CIS instructions are now specified as EXTEND instructions, meaning they are executed via an EXTEND opcode and are multi-word (2, 3, or 4 words long).

The instructions are designed to facilitate a general software shift towards 9-bit bytes and include a packed decimal (2 digits/byte) data type. They fall into two main categories:

1. Character String Manipulation: Instructions that handle strings of 9-bit bytes (e.g., MOVC, CMPC, MOVCV, CMPCV).
2. Decimal Arithmetic: Instructions for numbers represented in 9-bit display format and packed decimal format (e.g., ADDND, SUBP, CVTPB, ASHP, CHOP). While many require 9-bit ASCII support (which COBOL currently does not plan to support), some can still enhance packed decimal and EBCDIC performance for COBOL.

General characteristics of these instructions include:

• They typically perform memory-to-memory operations involving two operands.
• They support five addressing modes for 9-bit byte strings: direct, deferred, direct subscripted, deferred subscripted, and immediate.
• Comparison results are stored in the flag word.
• Arithmetic faults (overflows) trigger traps, resulting in truncated values.
• Specific rules govern handling of overlapping operands and interrupt state (AC 0 stores state information).

Additionally, the document notes extensions to the Basic Instruction Set (BIS) in Appendix B, such as enhancements to MOVST and new extended compare instructions (CMPSO, CMPST). These BIS extensions are usable by COBOL regardless of 9-bit ASCII support. Appendix C provides a preliminary proposal for assembler notation for the CIS instructions.