What does the "rep stos" x86 assembly instruction sequence do? -

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i stumbled across following assembly instruction sequence:

rep stos    dword ptr [edi]

for ecx repetitions, stores contents of eax edi points to, incrementing or decrementing edi (depending on direction flag) 4 bytes each time. normally, used memset-type operation.

usually, instruction written rep stosd. experienced assembly coders know details mentioned above seeing that. :-)

eta completeness (thanks phis): each iteration, ecx decremented 1, , loop stops when reaches zero. stos, thing observe ecx cleared @ end. but, scas or like, repz/repnz prefixes used, ecx can greater 0 if operation stopped before exhausting ecx bytes/words/whatevers.

before ask, scas used implementing strchr-type operations. :-p


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