How do I perform a while loop in assembly code?

-I'm just learning assembly code and I am not understanding how loops work very well.

I made the following program to print a few letters in the alphabet and it just keeps printing everything in the ASCII table over and over again until I terminate it with ctrl+c, but I'm not sure why. when I switch the values 'a' and 'b' it doesn't loop at all:



.section .data



msg: .ascii "A"

len: .int 1

a: .int 1

b: .int 5



.section .text

.global _start

_start:



mov len, %edx

mov $msg, %ecx

mov $4, %eax

mov $1, %ebx

int $0x80



loop_begin:

mov a, %eax

mov b, %ebx

cmp %ebx, %eax

jge loop_end

movb (msg), %al

incb %al

movb %al, (msg)

mov $4, %eax

mov $1, %ebx

int $0x80

inc %eax

jmp loop_begin



loop_end:



mov $0, %ebx

mov $1, %eax

int $0x80Hi,



The bug can be resolved by moving the label after the loop initialization code, as such:



mov a, %eax

mov b, %ebx

loop_begin:

cmp %ebx, %eax

jge loop_end



There's another bug there as well. You are writing over your counter register when you try to set up the INT 80h call. This can be fixed the following way:



push %eax

mov $4, %eax

mov $1, %ebx

int $0x80

pop %eax



Cheers,

BogdanYou keep comparing 1 to 5, rather than comparing something that changes inside the loop.



Update: These two lines are the problem:

mov a, %eax

mov b, %ebx

Those overwrite the value you are incrementing.
Assembly for what computer? x86? Sparc? Vax? PDP-11?



In any case, in all cases they will be:



top_of_loop:

If_some_condition_skip

jump_to_end_of_loop

(body of loop)

jump_to_top_of_loop

end_of_loop: