Print numbers diagonally in assembly

Question

I'm trying to display 0-9 diagonally in assembly, but the output puts my diagonally printed numbers in the middle of the window.

Here is the code:

start:
mov ah, 02h 
mov cl, 0Ah ;counter (10)
mov dx, 02h
;mov bx, 02h
mov dl, 30h ;start printing 0-9
mov dh, 02h ;start row
mov al, 02h
int 21h

again:

int 10h
int 21h
;add dx, 01h
inc dh
inc dx
inc al

loop again
mov ax, 4c00h
int 21h

The output should be:

0
  1
    2
      3
        4
          5
            6
              7
                8 
                  9

The current output prints that, but on the middle of the window. I've tried adding a new register bh and use it to put my cursor on my current position when I execute the file. How can I display it starting on my cursor? Should I put it on a loop and increment register ah ?

Answer 1

Your current program fails because you horribly mix 2 system functions that happen to have the same function number 02h, but that expect to receive totally different information in the DL register. The DOS OutputCharacter function expects a character code and you set it at 48, but the BIOS SetCursor function will interpret the same value 48 as the column. That's why the results are displayed in the middle of the screen!

Since you say you want to start from the current cursor position, which will nearly always be at the left edge of the screen at program start, there's no need to set the cursor position at all.

    mov     ah, 02h
    mov     dl, "0"
Next:
    push    dx          ;Preserve current character
    int     21h
    mov     dl, " "     ;Your desired output shows this space?
    int     21h
    mov     dl, 10      ;Linefeed moves the cursor 1 line down
    int     21h
    pop     dx          ;Restore current character
    inc     dl
    cmp     dl, "9"
    jbe     Next

Instead of using a separate counter, you can decide about looping back by looking at the value in the incremented DL register.

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Notice that you used the loop instruction which depends on the CX register but that you only initialized the CL bottom half of it! That's often a reason of program crashing.

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EDIT

Given that DOSBox emits both Carriage return and Linefeed when asked to display character 10 (brought to my attention in this comment by Michael Petch), I wrote this next little program that I tested for accuracy in the latest DOSBox available which is version 0.74.

    ORG     256          ;Create .COM program

    mov     ah, 02h      ;DOS.DisplayCharacter
    mov     dx, "0"      ;DH is spaces counter, DL is current character
    jmps    First        ;Character "0" has no prepended spaces!
Next:
    push    dx           ;(1)
    mov     dl, " "
Spaces:
    int     21h
    dec     dh
    jnz     Spaces
    pop     dx           ;(1)
First:
    int     21h          ;Display character in DL
    push    dx           ;(2)
    mov     dl, 10       ;Only on DOSBox does this do Carriage return AND Linefeed !
    int     21h
    pop     dx           ;(2)
    add     dx, 0201h    ;SIMD : DH+2 and DL+1
    cmp     dl, "9"
    jbe     Next

    mov     ax, 4C00h    ;DOS.TerminateWithExitcode
    int     21h