Messages in this thread | | | Date | Tue, 15 Dec 1998 15:39:46 +0100 | From | Caffeina et Nutella <> | Subject | NOP instruction |
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>I've always wondered... A much used NOP "instruction" is >OR AX,AX (or similar); >But doesn't this, as well as MOV DI,DI, set the zero flag??? >As a NOP instruction is supposed to do nothing, would this then be wrong? >(Nothing meaning not setting any flags?)
>Hmmm... Where's the time from the good ol' Z80 when you HAD a real NOP >instruction? *Sigh* :-)
From my very old TASM quick reference:
OR AX,AX -> Sets flags MOV DI,DI -> Doesn't set flags (ALL MOV's doesn't affect the status)
XCHG AX, AX (opcode 0x90) was used as a synonym of NOP (NOP was/is accepted as NOP -- at least by TASM and is assembled as XCHG AX,AX) XCHG doesn't set flags so it's good
BUT, something should be wrong in my manual: XCHG is rated for 3 clock ticks on a 486 (Pentium weren't invented yet...) but his 'synonym' NOP is rated for only 1 tick.... -- Lorenzo 'Caffeine' Marcantonio <lomarcan@tin.it>
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