Assembly - CMPS Instruction

The CMPS instruction compares two strings. This instruction compares two data items of one byte, word or doubleword, pointed to by the DS:SI and ES:DI registers and sets the flags accordingly. You can also use the conditional jump instructions along with this instruction.

The following example demonstrates comparing two strings using the CMPS instruction −

section .text
   global _start            ;must be declared for using gcc
 
_start: ;tell linker entry point
   mov esi, s1
   mov edi, s2
   mov ecx, lens2
   cld
   repe  cmpsb
   jecxz  equal             ;jump when ecx is zero

   ;If not equal then the following code
   mov eax, 4
   mov ebx, 1
   mov ecx, msg_neq
   mov edx, len_neq
   int 80h
   jmp exit
 
equal:
   mov eax, 4
   mov ebx, 1
   mov ecx, msg_eq
   mov edx, len_eq
   int 80h
 
exit:
   mov eax, 1
   mov ebx, 0
   int 80h
 
section .data
s1 db 'Hello, world!',0      ;our first string
lens1 equ $-s1

s2 db 'Hello, there!', 0     ;our second string
lens2 equ $-s2

msg_eq db 'Strings are equal!', 0xa
len_eq  equ $-msg_eq

msg_neq db 'Strings are not equal!'
len_neq equ $-msg_neq

When the above code is compiled and executed, it produces the following result −

Strings are not equal!