7 ms·
There were several NOPs - XCHG BX,BX and so on. Those were taken later to be prefixes for new classes of opcodes.
by JoeAltmaier 1mo ago
There were several NOPs - XCHG BX,BX and so on. Those were taken later to be prefixes for new classes of opcodes.
- bonzini 1mo agoNone was. XCHG AX,AX is special because XCHG AX,reg has a one-byte encoding. You're probably confusing with: - POP CS being broken and later becoming a prefix - some opcodes being "reserved NOPs", i.e. reserved without generating #UD. They are used for instructions that may be defined in the future while guaranteeing backwards compatibility, for example new kinds of prefetches. MPX bounds checking instructions were also encoded in reserved NOPs.
- JoeAltmaier 1mo agoWas I thinking of 0x86?
- bonzini 1mo ago0x86 and 0x87 (XCHG rm,r with byte- and word-sized operands) encode nops in the same way as 0x90, but were never redefined as prefixes. It's interesting that a lot of ALU operations occupy four opcodes (memory source/memory destination x byte/word) but 0x84/0x85 and 0x86/0x87 only need two because not only are they commutative, but also 0x84/0x85 do not write to any operands and 0x86/0x87 write to both. So there is no difference between memory as the source or destination operand.