CODEWITHSUNDEEP
loopsassemblyx86
Rox
Rox

Reputation: 2917

Infinite loop in my simple assembly program

I am learning loops and jumps in assembly and I try to make a simple loop. I want the printf command to be called 10 times. I have set the counter variable to 1. I have also set %edx to 1 and then I increment it for every iteration. If it is equal to 10, then we should exit the loop. But now the loop is infinite. I have debugged with gdb and %edx seems to be overwritten in the printf function. That is why I push %edx to the stack and the pop it back after the printf call, but it doesn't work. What have I missed?

.section .data
output:
    .asciz "Value is %d\n"
val1:
    .int 123
counter:
    .int 1
.section .text
.globl _start
_start:
    nop
    movl counter, %edx   # start at 1
gohere:
    movl val1, %ebx      # move value 123 to %ebx
    pushl %edx           # push %edx to stack
    pushl %ebx           # push %ebx to stack 
    pushl $output
    call printf          # call printf
    popl %edx            # pop %edx value
    inc %edx
    cmp $10, %edx        # if %edx is less than 10...
    jl gohere            # ... go to gohere, otherwise exit

    movl $0, %ebx
    movl $1, %eax
    int $0x80

Upvotes: 1

Views: 2770

Answers (1)

Behnam Safari
Behnam Safari

Reputation: 3081

you pushed output as the last push so the first pop will pop output. it is Stack and it is LIFO. in your code output will be in edx after you pop it. to solve it put two pops before popl edx:

popl output
popl ebx

Upvotes: 2

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