Why does incrementing not work the same for larger numbers as it does for a single digit? - linux

Question

section .bss
num1 resb 4
result resb 4
section .data
SYS_EXIT equ 1
SYS_READ equ 3
SYS_WRITE equ 4
STDIN equ 0
STDOUT equ 1
INCREMENT equ 1
msg1 db 'Please enter an integer here:',0xA
len1 EQU $- msg1
msg2 db 'Your integer after being incremented is:',0xA
len2 EQU $- msg2
section .text
global _start:
_start:
mov eax, SYS_WRITE ; Prompting user to enter a number.
mov ebx, STDOUT
mov ecx, msg1
mov edx, len1
int 0x80
mov eax, SYS_READ ; Reading users number.
mov ebx, STDOUT
mov ecx, num1
mov edx, 4
int 0x80
mov eax, SYS_WRITE ; Writing second message.
mov ebx, STDOUT
mov ecx, msg2
mov edx, len2
int 0x80
mov eax, [num1] ; incrementing the user's number.
mov ebx, INCREMENT
add eax, ebx
mov [result], eax
mov eax, SYS_WRITE ; Printing out incremented number.
mov ebx, STDOUT
mov ecx, result
mov edx, 4
int 0x80
Just started learning basic assembly code today and I'm trying to increment user input numbers by 1. I am getting weird results when the number is any bigger than a single digit. What's the difference?

Answer 1

The problem is that your treat your input as numbers and not as strings. When you receive your input (string), you get an (ASCII) string as your return value. Increasing the first char(digit) by add eax, INCREMENT (compacted) does work, because the first ASCII char value of [num] contained in AL(lowest byte of EAX) is increased by one. This happens because the ASCII value of 0 is 48 and the ASCII value of 9 is 57.
If you think you increase these 'digits', you would in fact increase their ASCII values, which works well until you reach the one-digit-limit(0-9). Increasing 9 by one would result in : and not 10, as your may have expected because the ASCII value of 9 is 57 and adding one to it results in 58, which is the ASCII value of :.
So you would have to 'normalize' your 'numbers' before you operate on them with integer arithmetic. Transform them from ASCII strings to integer values.
Luckily for you, others have done this before you and optimized these approaches to the practical maximum. Look and search for atoi(ASCII to integer) and itoa(integer to ASCII). Then surround your arithmetical operations
mov eax, [num1] ; incrementing the user's number.
mov ebx, INCREMENT
add eax, ebx
mov [result], eax
with these functions.