Chuck Guzis schrieb: It does not help not to document those instrs in the hope that they can later be reused when more transistors become available. Those "undocumented instructions" *were* used in any software product where the programmers became aware of ., and be it only for the reason to make it harder to understand the code by competitors (thus have disassemblers stumble on them). Typical example were the Z80 "mov xx,reg" instructions where xx is IXL,IXH,IYL,IYH which are side effects of adding a DD/FD prefix to "mov L,reg" or "mov H,reg" which were, from my own disassembly activities frequently used in Z80 CP/M code. Admittedly, the inplace moves like "mov B,B" are often only useful for alternative NOPs, but I remember having seen also creative uses of such codes as part of some data structure which happened to have a suitable pattern to execute as some code - again to obscure the program logic to interested hackers. Typical was also to use a three byte instr like "lda xxyy" where xx,yy formed another instruction like a short jump which is executed on PC+1, but skipped on PC+0 (location of lda). Almost every 8 bit CPU of that time had one or more "undocumented opcodes" resulting from don't cares in the instruction decoding logic. -- Holger