Undocumented 8086 instructions, explained by the microcode

Post by Solvency » Mon, Jul 17, 2023, 2:59 AM UTC

donw wrote: post #37

_ugfj wrote: post #15

> The undocumented C9 opcode is identical to the documented CB, far return instruction. I remember this. Once ... gosh, it's hard to believe how long ago that was now, but once I knew the entire Z80 opcode table off head. I could read the Z80 machine code and disassemble it. High school crowded that stuff out of my head, I went to a special math school, it was very very hard. Except... C9 was RET (and the Z80 is an e…

As a total aside, domain expertise is kind of amazing. Like, you can look at a string of opcodes from a processor that you haven’t touched for a decade-plus, and still recognize what’s going on. It’s important to remember that applies in all fields. That bumpkin that grew up on a farm? Sure, he might not be able to write an award-winning essay, but he could probably tell you everything you need to know about your loc…

Post by kens » Mon, Jul 17, 2023, 4:23 AM UTC

pwg wrote: post #49

kens wrote: post #12

Earlier quoted context omitted.

When I'm done with the 8086, I might look at the 68000 but I don't have any specific plans.

Ken, before you do, you have a typo in the 5th paragraph of this post: The 80_68_'s microcode ROM

Post by djmips » Mon, Jul 17, 2023, 4:55 AM UTC

userbinator wrote: post #27

_ugfj wrote: post #15

> The undocumented C9 opcode is identical to the documented CB, far return instruction. I remember this. Once ... gosh, it's hard to believe how long ago that was now, but once I knew the entire Z80 opcode table off head. I could read the Z80 machine code and disassemble it. High school crowded that stuff out of my head, I went to a special math school, it was very very hard. Except... C9 was RET (and the Z80 is an e…

I have most of the single-byte x86 opcodes still memorised - as many others have independently discovered, it's much easier in octal. 95 ba 07 01 cd 21 c3 48 65 6c 6c 6f 20 77 6f 72 6c 64 21 0d 0a 24

Post by sedatk » Mon, Jul 17, 2023, 5:35 AM UTC

greenyoda wrote: post #5

kens wrote: post #3

Earlier quoted context omitted.

There's a reason why every 8086 opcode matches something. If an instruction didn't match anything, I think the microcode engine would spin idle and the instruction would never terminate. (You need a RNI micro-instruction to end microcode processing.) Having your processor lock up due to a bad opcode is something that the designers presumably explicitly avoided. The 6502 on the other hand, didn't take such precautions…

Also the 6800, with its famous HCF (Halt and Catch Fire) instruction: > With the advent of the MC6800 (introduced in 1974), a design flaw was discovered by programmers. Due to incomplete opcode decoding, two illegal opcodes, 0x9D and 0xDD, will cause the program counter on the processor to increment endlessly, which locks the processor until reset. Those codes have been unofficially named HCF. During the design proce…

Post by nvy » Mon, Jul 17, 2023, 5:56 AM UTC

Solvency wrote: post #51

donw wrote: post #37

Earlier quoted context omitted.

As a total aside, domain expertise is kind of amazing. Like, you can look at a string of opcodes from a processor that you haven’t touched for a decade-plus, and still recognize what’s going on. It’s important to remember that applies in all fields. That bumpkin that grew up on a farm? Sure, he might not be able to write an award-winning essay, but he could probably tell you everything you need to know about your loc…

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