Question

For science fair, I need three programs that count to 50,000 and output each number as they do, I need one in c++, one in java, and one in assembly. I have the c++ and java programs, but I don’t see what I’m doing wrong for my assembly code:

    [org 0x100]
    [bits 32]

    mov ax, 0
    mov bx, target
    jmp start

    start:
    mov cx, 0x01
    add ax, cx
    mov dx, ax
    mov ah, 09
    int 0x21
    mov ax, dx
    cmp ax, bx
    jg term
    jmp start

    term:
    mov dx, msgT
    mov ah, 09
    int 0x21
    mov ah, 00
    int 0x21

    msgT db 'Terminating'
    target dw 50000

I am using the assembler NASM, and right now, it counts to 50,000 but doesn’t output each number as it calculates them.

Answer 1

Copied from my comment:

If you’re trying to make a 16-bit MS-DOS com file, you should be using [bits 16]. And as @Vlad says AH=09h takes a string in DX not a number (see e.g. here on how to convert a number to string, also note that you’ll have to $-terminate the string instead of NUL-terminate it).

Some other things:

• mov bx, target moves the address of target to bx. You wanted: mov bx, [target].
• jg term is branching based on signed comparison (you’re effectively comparing ax to -15536). You want ja term.
• You need to make sure to preserve the number, keeping it in a register requires extreme care. It’s easier to just save it on the stack.

The basic structure of your program should be something like this:

    [org 0x100] ; DOS .COM files are loaded at CS:0100h
    [bits 16]   ; And are 16-bits

start:
    mov ax, 0          ; The current count
printloop:
    push ax            ; Save current number 

    call myconvertfunc ; Some function to convert a number in ax and return a '$'-terminated string in dx
    mov ah, 0x09
    int 0x21

    mov dx, newline    ; Point dx to newline string
    mov ah, 0x09       ; Print $-terminated string in dx
    int 0x21

    pop ax             ; Restore current number

    inc ax             ; Next number 
    cmp ax, 50000      ; Compare number to the maximum number 
    jbe printloop      ; Notice branching based on unsigned comparison is needed

    mov ax, 0x4c00     ; Return 0. AH=4Ch AL=Return value
    int 0x21

newline: db 13, 10, '$' ; String containing "\r\n$"