Assembler MASM, printing in JNZ loop when value is 0 or below

Question

I'v got a a task to write a program which would create below as output:

ABCD******
   ABCD***
      ABCD

My code is:

kod segment
org 100h
assume cs:kod

start:
mov cx,3
mov bx,3
mov ax,6

mov ah, 02h

    PETLA_ZEW:

    push cx
    mov cx,4
    mov dl, 'A'

        PETLA_CIAG:

            int 21h
            inc dl
            dec cx

        jnz PETLA_CIAG

    push ax
    mov cx, ax
    mov dl, '*'

        PETLA_GWIAZD:

            int 21h
            dec cx

        jnz PETLA_GWIAZD

    sub ax, 3

    mov dl, 0ah
    int 21h
    mov dl, 0dh
    int 21h

    push bx
    mov cx, bx
    mov dl, ' '
        PETLA_SPACJE:

            int 21h
            dec bx

        jnz PETLA_SPACJE

    pop bx
    add bx, 3

    pop cx
    dec cx

    jnz PETLA_ZEW

koniec:
    mov ah, 4ch
    int 21h
kod ends
end start

but the problem is that in loop called PETLA_GWIAZD when I subtract ax 2nd time it's value is 0, which is causing the infinite loop. Is there any other loop I should use when subtracting? Maybe some validation check just before the loop? I just started working with Assembly and still do not know a lot...

Thanks!

Answer 1

The problem is the registers you are using : AX and CX are used for interrupts and loops, so you can use other registers, for example, SI instead of CX and DI instead of AX :

kod segment
org 100h
assume cs:kod

start:
mov si,3        ;◄■■ INSTEAD OF CX.
mov bx,3
mov di,6        ;◄■■ INSTEAD OF AX.

mov ah, 02h

    PETLA_ZEW:

    mov cx,4
    mov dl, 'A'

        PETLA_CIAG:

            int 21h
            inc dl
            dec cx

        jnz PETLA_CIAG

    cmp di, 0   ;◄■■ IF COUNTER == 0 ...
    je  koniec  ;◄■■ ... NO MORE ASTERISKS.

    mov cx, di  ;◄■■
    mov dl, '*'

        PETLA_GWIAZD:

            int 21h
            dec cx

        jnz PETLA_GWIAZD

    sub di, 3        ;◄■■

    mov dl, 0ah
    int 21h
    mov dl, 0dh
    int 21h

    push bx
    mov cx, bx
    mov dl, '_'
        PETLA_SPACJE:

            int 21h
            dec bx

        jnz PETLA_SPACJE

    pop bx
    add bx, 3

    dec si       ;◄■■
    jnz PETLA_ZEW

koniec:
    mov ah, 4ch
    int 21h
kod ends
end start