Assembly: REP MOVS mechanism

Question

Looking at the following assembly code:

MOV ESI, DWORD PTR [EBP + C]
MOV ECX, EDI
MOV EAX, EAX
SHR ECX, 2
LEA EDI, DWORD PTR[EBX + 18]
REP MOVS DWORD PTR ES:[EDI], DWORD PTR [ESI]
MOV ECX, EAX
AND ECX, 3
REP MOVS BYTE PTR ES:[EDI], BYTE PTR[ESI]

The book I got the code excerpt from explains the first REP MOVS as copying over 4-byte chunks, with the second REP MOVS copying the remaining 2-byte chunk, if it exists.

How do the REP MOVS instructions operate? According to MSDN, "The instruction can be prefixed by REP to repeat the operation the number of times specified by the ecx register." Wouldn't that just repeat the same operation over and over again?

Answer 1

For questions about particular instructions always consult the instruction set reference.

In this case, you will need to look up rep and movs. In short, rep repeats the following string operation ecx times. movs copies data from ds:esi to es:edi and increments or decrements the pointers based on the setting of the direction flag. As such, repeating it will move a range of memory to somewhere else.

PS: usually the operation size is encoded as an instruction suffix, so people use movsb and movsd to indicate byte or dword operation. Some assemblers however allow specifying the size as in your example, by byte ptr or dword ptr. Also, the operands are implicit in the instruction, and you can not modify them.