scasq
Scan string for a quadword
Compares rax with the quadword at [rdi], sets the flags, then advances rdi by 8, backward when DF is set.
x86-64 and later
- ● set from the result
- – not changed
- Reads DF
Forms
scasq
The q suffix selects an element of 8 bytes. After each element, the instruction increases rdi by 8 when DF = 0, or decreases rdi by 8 when DF = 1. In 64-bit mode the default addresses use rdi. The a32 address-size prefix instead uses 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
scasq
mov rax, 60 ; syscall 60: exit
xor rdi, rdi
syscall