X86-assembly/Instructions/rep

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X86-assembly

Instructions

rep

Description

Repeat string-operation until tested-condition

• rep: repeat while equal
• repnz: repeat while nonzero
• repz: repeat while zero

Each prefix causes the associated string instruction to repeat until the count register (CX) or the zero flag (ZF) matches a tested condition.

Examples

rep movsb

Repeat while equal: Copy the 8-bit byte from the DS:[(E)SI] to the ES:[(E)DI] register.

repnz scasl

Repeat while not zero: Compare the memory byte double-word addressed in the destination register EDL, relative to the ES segment, with the contents of the EAX register.

repz stosl

Repeat while zero:Transfer the contents of the EAX register to the memory double-word addressed in the destination register EDL, relative to the ES segment.

rep stosb

.text:004013E0 mov     edi, offset user_id ; memory location 0x40D020 (empty)
.text:004013E5 mov     ecx, 20h            ; size: 32
.text:004013EA mov     al, 4Fh             ; fill with value 0x4F
.text:004013EC rep stosb                   ; fill 32 bytes with 0x4F at memory location 0x40D020

rep movsd

At 0x0040127C, the mapped location of the file (named lpBaseAddress) is moved into EDI and adjusted by some offset using var_28. Next, ECX is loaded with 0x4E, the number of DWORDs to write (movsd). Therefore, the total number of bytes is 0x4E * 4 = 312 bytes in decimal. Finally, byte_409030 is moved into ESI at 0x401287, and rep movsd copies the data at byte_409030 into the mapped file.

.text:0040127C mov     edi, [ebp+lpBaseAddress]
.text:0040127F add     edi, [ebp+var_28]
.text:00401282 mov     ecx, 4Eh
.text:00401287 mov     esi, offset byte_409030
.text:0040128C rep movsd