Assembly Language for Beginners

.1. X86

CMPSB/CMPSW/CMPSD/CMPSQ(M) compare byte/ 16-bit word/ 32-bit word/ 64-bit word from the ad-
dress which is in SI/ESI/RSI with the variable at the address stored in DI/EDI/RDI. Set flags asCMP
does.

Together with the REP prefix, it is to be repeated in a loop, the counter is stored in the CX/ECX/RCX

register, the process will run until the ZF flag is zero (e.g., until the compared values are equal to

each other, hence “E” in REPE).

It works like memcmp() in C.

Example from the Windows NT kernel (WRKv1.2):

Listing 3: base\ntos\rtl\i386\movemem.asm

; ULONG

; RtlCompareMemory (

; IN PVOID Source1,

; IN PVOID Source2,

; IN ULONG Length

; )

;

; Routine Description:

;

; This function compares two blocks of memory and returns the number

; of bytes that compared equal.

;

; Arguments:

;

; Source1 (esp+4) - Supplies a pointer to the first block of memory to

; compare.

;

; Source2 (esp+8) - Supplies a pointer to the second block of memory to

; compare.

;

; Length (esp+12) - Supplies the Length, in bytes, of the memory to be

; compared.

;

; Return Value:

;

; The number of bytes that compared equal is returned as the function

; value. If all bytes compared equal, then the length of the original

; block of memory is returned.

;

;--

RcmSource1 equ [esp+12]

RcmSource2 equ [esp+16]

RcmLength equ [esp+20]

CODE_ALIGNMENT

cPublicProc _RtlCompareMemory,3

cPublicFpo 3,0

push esi ; save registers

push edi ;

cld ; clear direction

mov esi,RcmSource1 ; (esi) -> first block to compare

mov edi,RcmSource2 ; (edi) -> second block to compare

;

; Compare dwords, if any.

;

rcm10: mov ecx,RcmLength ; (ecx) = length in bytes

shr ecx,2 ; (ecx) = length in dwords

jz rcm20 ; no dwords, try bytes

repe cmpsd ; compare dwords

jnz rcm40 ; mismatch, go find byte

;

; Compare residual bytes, if any.

;