A Simple Assembly Input Question

Question

This is my first post on this site. I am taking an X86 assembly class and I am having a bit of trouble with my second project. The project is very simple. The program needs to take in a simple string from the user and display it back. I have gotten the program to take input from the user but I can't seem to store it. Here is what I have so far:

BITS 32
global _main
section .data

prompt db "Enter a string: ", 13, 10, '$'
input resd 1 ; something I can using to store the users input.

name db "Name: ******", 13, 10,'$'
StudentID db "********", 13, 10, '$'
InBoxID db "*************", 13, 10, '$'
Assignment db "************", 13, 10, '$'
version db "***************", 13, 10, '$'

section .text
_main:

mov ah, 9
mov edx, prompt
int 21h
mov ah, 08h
while:
 int 21h
            ; some code that should store the input.
 mov [input], al
 cmp al, 13
 jz endwhile
 jmp while
endwhile:

mov ah, 9
    ; displaying the input.

mov edx, name
int 21h
mov edx, StudentID
int 21h
mov edx, InBoxID
int 21h
mov edx, Assignment
int 21h
mov edx, version
int 21h
ret

I am assembling this using NASM. Thanks for the help in advance!

Answer 1

It looks like you aren't using a proper buffer to store the users input.

This site has a large x86 tutorial split up into 23 sections, one for each day you are suppose to do that section.

Here on day 14 he shows an example of reading in a string from the user and storing it into a buffer, then printing it back out again.

Answer 2

You're only reading the characters without storing them. Instead of storing into that 'input', you should store AL either directly into StudentID/InBoxID/Assignment/Version. You could take advantage of their relative positions in memory and write a single loop to fill all of them, as in a contiguous space.

It could go like this:

; For each string already padded with 13, 10, $
; at the end, use the following:
mov ah, 08h
mov edi, string
mov ecx, max_chars
cld
while:
        int 21h
        stosb         ; store the character and increment edi
        cmp ecx, 1    ; have we exhausted the space?
        jz out
        dec ecx
        cmp al, 13
        jz terminate  ; pad the end
        jmp while
terminate:
        mov al, 10
        stosb
        mov al, '$'
        stosb
out:
        ; you can ret here if you wish

I didn't test, so it might have mistakes in it.

Or you could use other DOS functions, specifically INT21h/0Ah. It could be more optimal and/or easier.