[Feature request] Allow a ModR/M indicator keyword to choose encodings not otherwise selected by the assembler

[ecm-pushbx]

As bugzilla.nasm.us is down and the nasm.us page now links to this github tracker rather than the bugzilla, here's a suggestion:

Allowing to select either the "load form" or the "store form" of an instruction is occasionally reported as a need, eg recently in https://stackoverflow.com/questions/79793821/can-i-somehow-tell-gas-to-change-direction-of-operands

As I commented there:

My debugger's line assembler allows choosing either form, xor di, di and xor modrm di, di give 31 FF whereas xor di, modrm di gives 33 FF. NASM does not have such a keyword, hence why it's documented as a difference from NASM.

Obviously NASM may not want the "naked" modrm keyword, as it could clash with existing label names, but I think that some way of selecting these encodings would be useful. Perhaps {modrm} ?

[hpax]

It is a surprisingly large request, given that there are a LOT of ways that some instructions can be encoded. This is thus part of something that is much, much larger in scope, which makes it a lot harder to address.

[ecm-pushbx]

I agree that the expanded topic is larger, but this request is specifically about a ModR/M keyword on an operand. I've found that to be fairly easy to add to my debugger's line assembler, simply by rejecting encodings where the modrm operand isn't encoded using a ModR/M. The logic of the patch for this is fairly simple, requiring no changes to the assembler tables.