section .data
lol: db "lol",10
global _start
section .text
_start:
mov rcx, 5
mainloop:
mov rax,1
mov rdi,1
mov rsi, lol
mov rdx, 4
syscall
loop mainloop
mov rax, 60
syscall
nasm linux x86_64
I wanted to output 5 times, but it ended up being an infinite loop.
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The syscall instruction overwrites rcx and r11, see https://www.felixcloutier.com/x86/syscall.
So if you’re using rcx as a loop counter, you’ll need to save it in another register, or on the stack, before the syscall, and then restore it afterwards.
However, the loop instruction is actually quite inefficient, and not really a good way to implement a loop: Why is the loop instruction slow? Couldn’t Intel have implemented it efficiently?. So it’s probably better to rewrite the code with a decrement and conditional jump, and then you’ll be free to use a different register for your loop counter.
For example, EBX, using mov ebx, 5 and dec ebx / jnz mainloop. RBX is call-preserved so would work even if you made a function-call inside the loop. R9D is a good choice for a call-clobbered register since this system call doesn’t need all 6 operands. If you were writing a function which returned, the calling convention would require you to save/restore RBX if you used it. But you just exit via a system call so it doesn’t matter which registers are modified.