Questions tagged [undocumented-opcodes]
Opcodes available in a processor that are not part of its documented instruction set.
14 questions
13 votes
1 answer
2k views
Did x86 CPU vendors like Intel, NEC, AMD, and Cyrix provide their own debugger for DOS with better CPU support and was it free?
MS-DOS's debugger DEBUG.EXE did only support the assembly/disassembly of 8086 opcodes. DR-DOS's debugger SID86.EXE, SID.EXE and Novel's debugger DEBUG.EXE did support 80286 opcodes and more, but no ...
- 1,282
10 votes
1 answer
696 views
Undocumented ModR/M byte combinations
The reason I am asking this question is because I was trying to compile a list of all 8086 opcodes, regardless of whether or not they are documented. The list is just for fun, the documented opcodes + ...
- 2,407
25 votes
5 answers
4k views
Undocumented instructions in x86 CPU prior to 80386?
I have questions regarding some x86 instructions that were documented for the 80386 and later x86 CPUs, but not for earlier chips. "OR reg/mem16, immed8" (0x83/1) "AND reg/mem16, ...
- 251
15 votes
3 answers
4k views
What 8086 instructions accept REP?
I tried this code in my assembler, set to 16 bit mode: bits 16 rep mov ds, ax Surprisingly, no error was thrown. Is this even valid? Wasn't rep only supposed to work with string instructions? Is it ...
- 2,407
8 votes
1 answer
270 views
Is scratchpad register 15 directly addressable on the F3850 (except as QL)?
The Fairchild F8 CPU, the F3850, has 64 scratchpad registers. The first 12 of these are directly addressable by several instructions. For example, the opcodes $CX add the contents of scratchpad ...
- 1,579
7 votes
2 answers
1k views
Does the NES handle unlisted processor instructions differently from the Famicom?
As you may be aware, developers used unregistered machine codes in their games, as discussed in this thread. My question isn't why they would do this, but instead if these instructions yielded a ...
- 4,161
17 votes
2 answers
2k views
How did the Trap65 work?
The Wikipedia page on the MOS 6502 mentions a hardware device called the Trap65 which apparently sat between the 6502 and its socket to trap any undocumented opcodes. The wiki page has the usual ...
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