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

This works for numbers bigger than 9?

No. As the code is stated ; Anything greater than 9 is invalid

I did one that can do it for numbers greater than 9 so you can check it out. It has been such a long time that I commented here so weird to see that you actually replied lol

this code does work for strings that represent numbers greater than 9. Line 16 means any ASCII character greater than 57 (i.e., 9), is invalid--b/c it doesn't represent a number.

This is correct. These lines are for error handling in case the caller supplied a string containing characters other than the digits 0-9. It does not mean only strings representing 0-9 are supported.

This error handling code is for the current character in the string, so it will reject any characters that are not digits.

    cmp rsi, 48             ; Anything less than 0 is invalid
    jl error
    
    cmp rsi, 57             ; Anything greater than 9 is invalid
    jg error

Hey, how to act when we enter a negative number to convert ? Don't we need to compare rsi, 45 ? Otherwise we just get -1 returned, not what we want

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 ;)