atoi in Assembly - Convert a number string to a 64 bit integer.

atoi.asm

    global atoi
    
    section .text
    
atoi:
    mov rax, 0              ; Set initial total to 0
     
convert:
    movzx rsi, byte [rdi]   ; Get the current character
    test rsi, rsi           ; Check for \0
    je done
    
    cmp rsi, 48             ; Anything less than 0 is invalid
    jl error
    
    cmp rsi, 57             ; Anything greater than 9 is invalid
    jg error
     
    sub rsi, 48             ; Convert from ASCII to decimal 
    imul rax, 10            ; Multiply total by 10
    add rax, rsi            ; Add current digit to total
    
    inc rdi                 ; Get the address of the next character
    jmp convert

error:
    mov rax, -1             ; Return -1 on error
 
done:
    ret                     ; Return total or error code

Negative numbers are not supported in this snippet. You'd have to check for a leading - and negate the final result if negative.

Negative numbers are not supported in this snippet. You'd have to check for a leading - and negate the final result if negative.

That's what I did, thx

Nice code ;)

quick (somewhat unrelated) newbie question, if the code starts at the atoi subroutine, how does the rest of the code get called?