I'm trying to learn assembly a bit. I use NASM instead of AT&T syntax.
This is my first program, it compares the two command line arguments and outputs which one is the smallest (favouring the first in case they're equal).
I think I'm doing this wrong. I noticed that the stack grows with each "call" of compare, so I could probably make this better with tail recursion optimization. However, I don't know how to do it correctly. Should I replace all occurrences of 'call' here with something else, like jmp? I read that the manner of jumping also depends on how you link with the linux kernel and/or libc regarding 'function' objects (I do not link with libc and so do not have a main 'function'), which confuses me so I thought I'd come here for simple short advice.
Also, another question I had is how foolish it is to do "jl" immediately followed by "jg", which might cause unwanted behaviour if the "jl" jump actually changed the flag contents so "jg" also jumps. Is there a two-way 'atomic' jump?
section .data
usage: db 'Please provide two strings',10
usageLen: equ $-usage
bigger: db 'It is bigger!',10
biggerLen: equ $-bigger
smaller: db 'It is smaller!',10
smallerLen: equ $-smaller
section .text
global _start
_start:
pop ebx ; argc
cmp ebx, 3
jne usage_exit
pop eax ; argv[0]
pop eax
pop ebx
call compare ;
usage_exit:
mov eax, 4 ; sys_write
mov ebx, 1 ; int fd = stdout
mov ecx, usage
mov edx, usageLen
int 80h ; call kernel
mov eax, 1 ; sys_exit
mov ebx, 0 ; int status = 0
int 80h ; call kernel
report:
mov ecx, eax
mov edx, ebx
mov eax, 4 ; sys_write
mov ebx, 1 ; int fd = stdout
int 80h ; call kernel
mov eax, 1 ; sys_exit
mov ebx, 0 ; int status = 0
int 80h ; call kernel (exit)
compare:
push eax
push ebx
mov eax, [eax]
mov ebx, [ebx]
test ax, ax
jz is_smaller
test bx, bx
jz is_bigger
cmp ax, bx
jl is_smaller
jg is_bigger
; if the same, try next positions
pop ebx
pop eax
inc eax
inc ebx
call compare
is_smaller:
mov eax, smaller
mov ebx, smallerLen
call report
is_bigger:
mov eax, bigger
mov ebx, biggerLen
call report
Tail recursion is defined as a recursive function in which the recursive call is the last statement that is executed by the function. So basically nothing is left to execute after the recursion call. For example the following C++ function print() is tail recursive.
(algorithmic technique) Definition: A special form of recursion where the last operation of a function is a recursive call. The recursion may be optimized away by executing the call in the current stack frame and returning its result rather than creating a new stack frame.
Anything you would use an imperative loop for should be tail recursive: traversing arrays, linked lists, ranges of integers for math calculations, etc. The exception is if you know the size is bounded.
In computer programming, a function that calls itself, either directly or indirectly, is a recursive function. When this call happens at the end of the function, it is called tail recursion.
Yes, you should replace the call
s with jmp
s. In general, you should use call
when you expect execution to resume on the following line when the call returns. In your code, execution never returns since compare
is just a loop, so jmp
is the correct way to go to the next iteration. The same is true for the two instances of call report
you have.
As for your second question, jl
doesn't change the flags, so there is no problem calling jg
after it.
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