6502 Illegal Opcodes in the Siemens PC 100 Assembly Manual (1980)

Summary

The 6502’s “illegal” opcodes were of intense interest to home computer enthusiasts, and analyses were published in various magazines. But one would have never expected a company like Siemens to document illegal opcodes in a programming manual from 1980. The PC 100 Assembly Manual The Siemens PC 100 is basically a Rockwell AIM-65 single-board computer in a case, featuring a 6502 processor, integrated keyboard, LED display, and thermal printer, tailored for educational and development purposes with localized documentation and modified ROMs. Siemens’ German-language manuals were largely based on Rockwell’s originals, but shuffled the contents. Of particular interest is the assembly manual: Siemens Assembler-Handbuch Personal-Computer PC 100, Ausgabe 1980/1981(124 pages, 17 MB) It consists of: “Special Instructions” The 6502 Reference has three extra pages at the end describing “Sonderbefehle” (“special instructions”) that were not in the original MOS/Rockwell 6502 reference that this chapter is a translation of. We can assume this was original research by Siemens. Here is the translated transcription: 10.3 Special Instructions The microprocessor recognizes a number of special instructions that are largely unknown but can provide valuable assistance to the user in program development. The chosen mnemonics in the following tables are merely recommendations for effectively representing these instructions. Note: These instructions are not part of the specification and may be changed at any time without notice. The special commands cannot be decoded by the assembler program and must be programmed using the .BYT directive (see Chapter 10.2). AAX Logical “AND” operation between the accumulator and X-register, with result storage. Operation A ∧ X → M with Zero Page und (A) ∧ X ∧ $02 → M with Absolute Flags Addressing Mode Assembler Mnemonic OP CODE No. Bytes Zero Page AAX Oper 87 2 Zero Page, Y AAX+16 Oper 97 2 X-Reg. ∧ $02Absolute AAX+23 Oper 9E 3 X-Reg. ∧ Accu ∧ $02Absolute ...