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I am looking at asmdb and the intel docs to try and find out what some of these mean from asmdb:

add  |  x:r8/m8, ib/ub  |  80 /0 ib
add  |  X:r64/m64, id   |  REX.W 81 /0 id
add  |  x:~r16/m16,~r16 |  66 01 /r
and  |  x:al, ib/ub     |  24 ib
btc  |  x:r16/m16, ib/ub | 66 0F BA /7 ib

What do the /0 and /r and /7 mean? I assume there might be more "slash x" values, what do they mean, how do I find them in the Intel docs?

Then what does the ib and id stuff mean, those aren't hex values and hard to search for in the Intel docs. 66 and 80 are hex values as far as I can tell, but the ib, iw, etc. aren't.

I want to eventually build a machine code generator, so need to be able to understand what these basic components of the machine code opcodes are.

Lance
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    The Intel Manuals have an entire chapter right at the beginning to explain what all these abbreviations mean. – fuz Jan 27 '21 at 11:46
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    The old NASM instruction reference has an explanation too: https://ulukai.org/ecm/doc/insref.htm#iref-opc – ecm Jan 27 '21 at 15:28

1 Answers1

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You'll want to see Intel® 64 and IA-32 Architectures Software Developer Manuals.

"slash x" denotes that part of the instruction is encoded in the opcode (reg) part of the modr/m byte. See Vol 2A Chapter 2 INSTRUCTION FORMAT.

"ib" and "id" mean "immediate byte" and "immediate dword" respectively. You can see all the abbreviations in Vol 2A Appendix A.2 OPCODE MAP / KEY TO ABBREVIATIONS.

user786653
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  • Some explanation would be nice. I see the "One-byte Opcode Map" and such, but I have no idea what that means, is there more explanation buried in here? I don't see one. – Lance Jan 27 '21 at 11:47
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    Which part? The abbreviations are listed before the opcode map (in A.2 in the version I linked above). The manuals also go through some examples (in both chapters). – user786653 Jan 27 '21 at 12:15
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    @LancePollard: Related: [How to read the Intel Opcode notation](https://stackoverflow.com/q/15017659) and [How to determine if ModR/M is needed through Opcodes?](https://stackoverflow.com/q/55312459) – Peter Cordes Jan 27 '21 at 20:04
  • [This](http://www.c-jump.com/CIS77/CPU/x86/lecture.html#X77_0010_real_encoding) is particularly good. – Lance Jan 27 '21 at 22:30