Question

I’m trying to learn Assembler and following a tutorial, and the first examples worked perfectly. I know a bit of the basics, but I’m having problems with variables. Here’s the code I’m trying to compile:

leftbr db "("
rightbr db ")"
input db

start:
mov ah,08
int 21h
mov input,al

output:
mov dl,leftbr
mov ah,02
int 21h
mov dl,key
int 21h
mov dl,rightbr
int 21h

exit:
mov ah,4ch
mov al,00
int 21h

It crashes at “input db” saying “invalid argument”. If I change it to “input db “” ” then it crashes at “mov input,al” claiming “invalid operands”. I changed it to the following and it now works.

start:
mov ah,08
int 21h
mov [input],al

output:
mov [leftbr], "("
mov [rightbr], ")"
mov dl,[leftbr]
mov ah,02
int 21h
mov dl,[input]
int 21h
mov dl,[rightbr]
int 21h

exit:
mov ah,4ch
mov al,00
int 21h

leftbr db 0
rightbr db 0
input db 0

Answer 1

The line mov input, al tries to move al into the value defined by the line input db 0, e.g. the compiler translates it into mov 0, al. What you want to do, is move al to the position “input”, so I guess (ASM coding was some time ago for me) mov [input], al or mov byte ptr:[input], al would work better.

Edit: this is what displays “(a)” for me. Running CrunchBang Linux/Wine/FASM for windows.

format MZ
org 0x100

jmp start
leftbr db "(", 0
rightbr db ")"
input db "a"

start:
xor ax,ax
mov ah,08
;int 21h ; commenting this line because wine doesn't seem to like it
;mov [input],al

output:
mov dl,byte [leftbr]
mov ah,02
int 21h
mov ah,02 ; not sure if ah gets modified, probably not
mov dl,[input]
int 21h
mov ah,02
mov dl,[rightbr]
int 21h

exit:
mov ah,4ch
mov al,00
int 21h