Questions tagged [undocumented-opcodes]

Opcodes available in a processor that are not part of its documented instruction set.

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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 + ...
8
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3answers
281 views

Has there been any effort to research the undocumented Thumb instructions of ARM7TDMI?

Many CPUs have "holes" in the ISA, where the bit pattern is left undefined. Considering the Z80 and the 6502, many people have mapped out what these undocumented instructions actually do. ...
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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, ...
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2answers
567 views

Why doesn't Visual6502 simulate RRA in the way I expect?

As everyone knows, the 6502 has many many undefined opcodes. One of these is RRA, which essentially performs both ROR and ADC. From what I understand of hardware (really not much) and the explanation ...
12
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3answers
2k 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 ...
8
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0answers
310 views

Behavior of lesser known illegal M6800 opcodes

The M6800 had a few illegal opcodes, including the (in)famous HCF (Halt and Catch Fire). The following illegal opcodes were revealed in BYTE Magazine, December 1977: $14: NBA $87: STAA immediate $C7: ...
8
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1answer
145 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 ...
13
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3answers
845 views

0F1h opcode-prefix on i80286

On Intel iAPX 80286, as I recall from experience, the undocumented – officially unassigned – opcode position 0F1h was actually decoded as an instruction prefix, thus counting for maximum allowed ...
5
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2answers
993 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 ...
10
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1answer
711 views

Can we express the instructions to the Analytical Engine in terms of assembler or machine code?

On a recent trip to the London Science Museum I saw Babbage’s Analytical Engine. Apparently this had an ALU (or equivalent). I can build an ALU out of logic gates but I can’t conceptualise how to do ...
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2answers
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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1answer
654 views

Why won't the game "Ultima I" work on my Apple IIe system? [closed]

I'm trying to complete the original Ultima I on an Apple //e. These instructions explain space flight, but I can't seem to destroy enemy ships in order to become a Space Ace, even when I should be ...