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bonzini
on Nov 14, 2023
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Reptar
ModRM and SIB are not a prefix, they're part of the opcode (second and third byte after all the prefixes and the 0Fh/0F38h/0F3Ah opcode map selectors)
EarlKing
on Nov 14, 2023
[–]
More specifically, they're affixed to
certain
opcodes that require them. There are a number of byte-sized opcodes that do not require a ModRM or SIB byte (although a number of those got gobbled up to make the REX prefix, but that's another story).
TL;DR Weeee! Intel machine language is crazy!
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