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04/24/2024 |
This is a type of loop that is used to repeat a specific block of code an unknown number of times until a specific condition is met. In other words, it will continue to loop through the section of code as long as the predetermined condition remains true.
A counter is not required in a while loop. The number of iterations (counter) is not known and unspecified in a while loop.
Below is a while loop in c. This will be specifying an incrementing value.
while.c:
#include <stdlib.h>
#include <stdio.h>
int main() {
int i = 0;
while (i < 10){
printf("Hi\n");
i++;
}
return 0;
}Next, we will be doing this in ASM:
{% hint style="info" %}
Note: you can still set this up to either increment or decrement, it's up to you. I'll challenge you to set up a variation that decrements 😏. If you want to see the variation instead, view the last code snippet at the bottom of this blog.
{% endhint %}
while.s:
.global _start
.section .data
msg: .ascii "While loop lol\n"
len = . - msg
end_msg: .ascii "You hit the end of the while loop!\n"
len2 = . - end_msg
.section .text
_start:
mov x15, #0 // Initialize counter (i = 10)
while_loop:
cmp x15, #10 // Compare counter with 0
beq while_end // If counter is less or equal to 0, end loop
// "do something" goes here
mov x8, #64
mov x0, #1
ldr x1, =msg
mov x2, len
svc 0
add x15, x15, #1 // Increment counter (i--)
b while_loop // Jump back to start of loop
while_end:
// Continue with rest of program...
mov x8, #64
mov x0, #1
ldr x1, =end_msg
mov x2, len2
svc 0
// Exit program
mov x8, #93
mov x0, #0
svc 0
Assembling and Linking:
aarch64-linux-gnu-as while.s -o while.o && aarch64-linux-gnu-ld while.o -o while
while.s:
.global _start
.section .data
msg: .ascii "While loop lol\n"
len = . - msg
end_msg: .ascii "You hit the end of the while loop!\n"
len2 = . - end_msg
.section .text
_start:
mov x15, #10 // Initialize counter (i = 10)
while_loop:
cmp x15, #0 // Compare counter with 0
b.le while_end // If counter is less or equal to 0, end loop
// "do something" goes here
mov x8, #64
mov x0, #1
ldr x1, =msg
mov x2, len
svc 0
sub x15, x15, #1 // Increment counter (i--)
b while_loop // Jump back to start of loop
while_end:
// Continue with rest of program...
mov x8, #64
mov x0, #1
ldr x1, =end_msg
mov x2, len2
svc 0
// Exit program
mov x8, #93
mov x0, #0
svc 0
Assembling and Linking:
aarch64-linux-gnu-as while.s -o while.o && aarch64-linux-gnu-ld while.o -o while