cmpsb
Compare string bytes
Compares the byte at [rsi] with the byte at [rdi] and sets the flags accordingly.
8086 and later
- Arrays, strings and the string instructionsx86-64 assembly
- ● set from the result
- – not changed
- Reads DF
Forms
cmpsb
The b suffix selects an element of 1 byte. After each element, the instruction increases rsi and rdi by 1 when DF = 0, or decreases rsi and rdi by 1 when DF = 1. In 64-bit mode the default addresses use rsi and rdi. The a32 address-size prefix instead uses esi and edi; 16-bit string addressing is unavailable.
repe / repz repeats while ZF = 1, and repne / repnz repeats while ZF = 0. Each stops when the counter reaches zero; a nonzero initial counter permits the first comparison before testing its result. The counter is rcx by default or ecx with a32. If it starts at zero, a repeated instruction accesses no element. The data width is independent of this address and counter width.
global _start
section .data
source: db 1, 2, 3, 4, 5, 6, 7, 8
destination: times 8 db 0
section .text
_start:
cld ; count upwards
mov rsi, source
mov rdi, destination
mov rax, 7
cmpsb ; compares [rsi] with [rdi] and sets the flags
mov rax, 60 ; syscall 60: exit
xor rdi, rdi
syscall