Intel x86 Assembly Language & Microarchitecture IA-32 assembly, GAS, cdecl calling convention


Example

# make this routine available outside this translation unit
.globl string_to_integer

string_to_integer:
    # function prologue
    push %ebp
    mov %esp, %ebp
    push %esi

    # initialize result (%eax) to zero
    xor %eax, %eax
    # fetch pointer to the string
    mov 8(%ebp), %esi

    # clear high bits of %ecx to be used in addition
    xor %ecx, %ecx
    # do the conversion
string_to_integer_loop:
    # fetch a character
    mov (%esi), %cl
    # exit loop when hit to NUL character
    test %cl, %cl
    jz string_to_integer_loop_end
    # multiply the result by 10
    mov $10, %edx
    mul %edx
    # convert the character to number and add it
    sub $'0', %cl
    add %ecx, %eax
    # proceed to next character
    inc %esi
    jmp string_to_integer_loop
string_to_integer_loop_end:

    # function epilogue
    pop %esi
    leave
    ret

This GAS-style code will convert decimal string given as first argument, which is pushed on the stack before calling this function, to integer and return it via %eax. The value of %esi is saved because it is callee-save register and is used.

Overflow/wrapping and invalid characters are not checked in order to make the code simple.

In C, this code can be used like this (assuming unsigned int and pointers are 4-byte long):

#include <stdio.h>

unsigned int string_to_integer(const char* str);

int main(void) {
    const char* testcases[] = {
        "0",
        "1",
        "10",
        "12345",
        "1234567890",
        NULL
    };
    const char** data;
    for (data = testcases; *data != NULL; data++) {
        printf("string_to_integer(%s) = %u\n", *data, string_to_integer(*data));
    }
    return 0;
}

Note: in some environments, two string_to_integer in the assembly code have to be changed to _string_to_integer (add underscore) in order to let it work with C code.