or we can say:
"123" -> starting from 1

1 + 0 * 10 = 1
2 + 1 * 10 = 12
3 + 12 * 10 = 123

This will match to atoi function as below:

atoi:

push %ebx # preserve working registers
push %edx
push %esi

mov $0, %eax # initialize the accumulator
nxchr:
mov $0, %ebx # clear all the bits in EBX
mov (%esi), %bl # load next character in BL
inc %esi # and advance source index

cmp $'0', %bl # does character preceed '0'?
jb inval # yes, it's not a numeral jb:jump below
cmp $'9', %bl # does character follow '9'?
ja inval # yes, it's not a numeral ja:jump above

sub $'0', %bl # else convert numeral to int
mull ten # multiply accumulator by ten. %eax * 10
add %ebx, %eax # and then add the new integer
jmp nxchr # go back for another numeral

inval:
pop %esi # recover saved registers
pop %edx
pop %ebx
ret







This is a subroutine about string to int

Any help would be appreciated!

source address：
https://stackoverflow.com/questions/19461476/convert-string-to-int-x86-32-bit-assembler-using-nasm

That is AT&T syntax, and it is the worst possible syntax.

To convert remove all % and $ symbols, reverse the order of operands. change () to [], and # to ;

That should get most of it.

There is no check for something like: "-123".