I'm trying to understand the “rep scasb byte [edi]” instruction

Question

In this statement, is the value pointed to by edi the one that compares byte by byte, with the value pointed to by eax ?

rep scasb byte [edi]

Answer 1

See https://www.felixcloutier.com/x86/scas:scasb:scasw:scasd and https://www.felixcloutier.com/x86/rep:repe:repz:repne:repnz .

But note that rep scasb is not encodeable .

With scasb , the two rep prefixes are repe and repne . The prefix byte that means rep with stos / movs means repe with scas and cmps .

REPE keeps repeating while ECX.=0 and ZF==1. ie repeat while equal.

repne scasb implements memchr : find the first occurrence (if any) in a fixed-size buffer.

repe scasb finds the first byte,= AL. or stops at the limit set by ECX. So it's a bit like strspn , but still for an explicit size buffer, not a zero terminated C-string.

Answer 2

rep means repeat until ecx is 0. scasb means 'scan byte', ie: compare al with the byte at memory location edi , increment edi and decrement ecx .
What this means in effect is that before this line is run ecx has been set to a value that dictates how many times the rep will run for and if the characters match the zero flag ZF (which is also used for determining equality, hence why je and jz are equivalent) will be set.