In Assembly, a loop decrements ECX register. When ECX==0, loop will end.
We can make a loop using either control instructions like jmp, jnz etc (like in last page) or we can also use the loop instruction as well!
Make sure we preserve ecx throughout the program. Use stack to do this.
Question: Write a loop to display "hello world" 10 times using a loop.
; Self-written program for loops
global _start
section .text
_start:
xor rax,rax
mov rax,10
Display:
push rax ; storing rax's state
mov rax, 1 ; 1 for syscall write
mov rdi, 1 ; 1 for int fd=1 to stdout
mov rsi, message ; message hello world
mov rdx, len ; length of the message
syscall ; Calling to print message
;Now loop should be decremented until 10 times (until it becomes 0)
pop rax
dec rax
jnz Display
Ending:
;program should exit safely
mov rax,60
mov rdi,0 ; error code is 0
syscall
section .data
message: db 'Hello World',0xa
len equ $-message
As we can see here, I am using the rax technique as in last article to loop. Let us use the instruction "loop" to re-write this:
; Self-written program for loops
global _start
section .text
_start:
xor rcx,rcx
mov rcx,10
Display:
push rcx ; storing rcx's state
mov rax, 1 ; 1 for syscall write
mov rdi, 1 ; 1 for int fd=1 to stdout
mov rsi, message ; message hello world
mov rdx, len ; length of the message
syscall ; Calling to print message
;Now loop should be decremented until 10 times (until it becomes 0)
pop rcx
loop Display
Ending:
;program should exit safely
mov rax,60
mov rdi,0 ; error code is 0
syscall
section .data
message: db 'Hello World',0xa
len equ $-message
Using loop instruction, we are utilizing rcx, avoiding the risk of spoiling rax. We are also reducing the hassle of decrementing rax on loops. Finally, we are simplifying some logic too.
Please note, if we don't preserve rcx here, after the write syscall, rcx will become all f's. This would become an infinite loop. So we have to preserve this value manually using a stack.
Rep can also be used to loop through. Majorly used in string operations
