cmpsd
Compare string doublewords
Compares the doubleword at [rsi] with the doubleword at [rdi], sets the flags, then advances rsi and rdi by 4, backward when DF is set.
386 and later
- ● set from the result
- – not changed
- Reads DF
- The SSE2 form, which compares two floating-point values, writes no flags.
Forms
cmpsd
This describes the no-operand string instruction; operand forms using XMM registers perform separate scalar floating-point operations.
The d suffix selects an element of 4 bytes. After each element, the instruction increases rsi and rdi by 4 when DF = 0, or decreases rsi and rdi by 4 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 .text
_start:
mov rax, 12
mov rbx, 5
mov rcx, 3
mov rsi, 9
cmpsd
mov rax, 60 ; syscall 60: exit
xor rdi, rdi
syscall