Question

I’m trying to make this program in assembly.
It should just convert a string to a decimal

int main (int argc, char **argv)
{
    int result = 0; 
    char *c = argv[1];  
    while(*c != 0)
    { 
        result *= 10;           
        result += (*c - 48);
        c++;    
    }
return result;
}

but without getting argv from the stack… so I wrote a little code in asm that I think should do this but it does not work.

Can anyone point me to what I’m doing wrong?
I commented the code so you will know what I think is going on, please correct me with anything.

section .bss
        buf resb 8  ; define an array of 8 uninitialised bytes
section .text

global _start
_start:
    mov rax, 3  ; system call number (sys_read)
    xor rbx, rbx    ; file descriptor 0 (stdin)
    mov rcx, buf    ; buffer to store data
    mov rdx, 8  ; Lenght of buffer
    int 0x80

    xor rax,rax ; rax = 0
    mov rbx, buf    ; rbx = &buf
    xor rcx, rcx    

StrToDec:
        mov cl, [rbx]   ; cl = *rbx 
        cmp rcx, 0      ; check for nul THIS IS WRONG
        cmp rcx, 10     ; we have to check for NL ascii code 
        je end          ; if rcx == 0 goto end
        imul rax, 10    ; rax *= 10
        sub rcx, 48     ; rcx -= 48 (48 is acii for '0')
        add rax, rcx    ; rax += rcx
        inc rbx         ; rbx++
        jmp StrToDec

end:
    mov rbx, rax    ; rbx = rax
    mov rax, 1      ; rax = 1
    int 0x80

When i run this program and type 100 for example
and then type echo $? to the terminal it should print 100

Thanks.

Answer 1

FIXED:

I found out what was wrong with the program =D

First of all you need to check for value 10 so

cmp rcx, 0      ; check for nul 

is wrong because were looking for the new line ascii code not a null

cmp rcx, 10      ; check for NL

is right, I don’t think it works if your getting the args from the stack, when getting args from stack you should check for 0.

Second: For numbers bigger than 255, echo $? would give me wrong results, but that’s ok because echo $? can only show numbers up to 255, so even tho echo may show a wrong number the register rbx holds the right value.

I debugged it and it works fine now.