Questions tagged [undocumented-opcodes]

For questions regarding opcodes within a processor that are not part of the documented instruction set.

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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 ...
8
votes
0answers
195 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: ...
11
votes
3answers
693 views

0xF1 opcode-prefix on i80286

On intel IAPX 80286, as I recall from experience, the undocumented - officially unassigned - "opcode" position 0xF1 was actually decoded as an instruction-prefix, thus counting for maximum allowed ...
3
votes
1answer
600 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 ...
5
votes
2answers
856 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
votes
1answer
606 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 ...
16
votes
2answers
1k 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 ...