Linux nasm assembly append character/s to a string

Question

On NASM in Arch Linux, how can I append the character zero ('0') to a 32 bit variable? My reason for wanting to do this is so that I can output the number 10 by setting a single-digit input to 1 and appending a zero. I need to figure out how to append the zero.

The desirable situation:

Please enter a number: 9
10

Using this method, I want to be able to do this:

Please enter a number: 9999999
10000000

How can I do this?

Thanks in advance,

RileyH

Answer 1

Well, as Bo says... but I was bored. You seem resistant to doing this the easy way (convert your input to a number, add 1, and convert it back to text) so I tried it using characters. This is what I came up with. It's horrid, but "seems to work".

; enter a number and add 1 - the hard way!
; nasm -f elf32 myprog.asm
; ld -o myprog myprog.o -melf_i386

global _start

; you may have these in an ".inc" file
sys_exit equ 1
sys_read equ 3
sys_write equ 4
stdin equ 0
stdout equ 1
stderr equ 2
LF equ 10

section .data
    prompt db "Enter a number - not more than 10     digits - no nondigits.", LF
    prompt_size equ $ - prompt
    errmsg db "Idiot human! Follow instructions next time!", LF
    errmsg_size equ $ - errmsg

section .bss
    buffer resb 16
    fakecarry resb 1

section .text
_start:
    nop
    mov eax, sys_write
    mov ebx, stdout
    mov ecx, prompt
    mov edx, prompt_size
    int 80h

    mov eax, sys_read
    mov ebx, stdin
    mov ecx, buffer + 1 ; leave a space for an extra digit in front
    mov edx, 11
    int 80h
    cmp byte [buffer + 1 + eax - 1], LF
    jz goodinput

; pesky user has tried to overflow us!
; flush the buffer, yell at him, and kick him out!
    sub esp, 4 ; temporary "buffer"
flush:
    mov eax, sys_read
    ; ebx still okay
    mov ecx, esp ; buffer is on the stack
    mov edx, 1
    int 80h
    cmp byte [ecx], LF
    jnz flush
    add esp, 4 ; "free" our "buffer"
    jmp errexit

goodinput:
    lea esi, [buffer + eax - 1] ; end of input characters
    mov byte [fakecarry], 1 ; only because we want  to add 1
    xor edx, edx ; count length as we go

next:
; check for valid decimal digit
    mov al, [esi]
    cmp al, '0'
    jb errexit
    cmp al, '9'
    ja errexit

    add al, [fakecarry] ; from previous digit, or first... to add 1
    mov byte [fakecarry], 0 ; reset it for next time
    cmp al, '9' ; still good digit?
    jna nocarry

; fake a "carry" for next digit
    mov byte [fakecarry], 1
    mov al, '0'
    cmp esi, buffer + 1
    jnz nocarry

; if first digit entered, we're done
; save last digit and add one ('1') into the space we left
    mov [esi], al
    inc edx
    dec esi
    mov byte [esi], '1'
    inc edx
    dec esi
    jmp done

nocarry:
    mov [esi], al
    inc edx
    dec esi
    cmp esi, buffer
    jnz next

done:
    inc edx
    inc edx
    mov ecx, esi ; should be either buffer + 1, or buffer
    mov ebx, stdout
    mov eax, sys_write
    int 80h
    xor eax, eax ; claim "no error"

exit:
    mov ebx, eax
    mov eax, sys_exit
    int 80h

errexit:
    mov edx, errmsg_size
    mov ecx, errmsg
    mov ebx, stderr
    mov eax, sys_write
    int 80h
    mov ebx, -1
    jmp exit
;-----------------------------    

Is that what you had in mind?