Assembly - (NASM 32-bit) Printing a triangle of stars not working properly

Question

EDIT: This problem is already solved. Many thanks to mbratch.

My code outputs:

But it should display this:

I think problem is in the innerloops but I can't fix it, it works properly on the first loop but not on the succeeding ones.

Here's my code:

innerloop1: ;;for(j=n-i;j>0;j--)
mov bl, [i]
sub byte [num], bl
mov dl, [num]
mov byte [j], dl
cmp byte [j], 0
jle printStar

mov eax, 4
    mov ebx, 1
    mov ecx, space
    mov edx, spaceLen
    int 80h
dec dl
    jmp innerloop1

printStar:
mov eax, 4
    mov ebx, 1
    mov ecx, star
    mov edx, starLen
    int 80h

innerloop2: ;;for(k=0;k<(2*i)-1;k++)
mov al, [i]
mul byte [two]
dec al
cmp byte [k], al
jge printMe

mov eax, 4
    mov ebx, 1
    mov ecx, space
    mov edx, spaceLen
    int 80h

    inc byte [k]
    jmp innerloop2

printMe:
mov eax, 4
    mov ebx, 1
    mov ecx, star
    mov edx, starLen
    int 80h

    mov eax, 4
    mov ebx, 1
    mov ecx, newLine
    mov edx, newLineLen
    int 80h

    inc byte [i]
    jmp outerloop

    printSpace:
mov eax, 4
    mov ebx, 1
    mov ecx, space
    mov edx, spaceLen
    int 80h

Answer 1

There are lots of inefficiencies in your code and it could be much more clearly and concisely written. However, I'll just address the areas that are causing a functional problem.

There are a couple of problems with innerloop1 . You are modifying [num] every time through the loop. Instead, you want to do it prior to the loop as an initializer for j . Secondly, you are counting on the value of dl being intact through the execution of the loop, but your mov edx, spaceLen clobbers it, as might the call to int 80h . So you can correct it by this:

    mov dl, [num]       ; INITIALIZE j=n-i
    sub dl, byte [i]
innerloop1: ;;for(j=n-i;j>0;j--)
                        ; REMOVED modification of 'num' here
    mov byte [j], dl
    cmp byte [j], 0
    jle printStar

    mov eax, 4
    mov ebx, 1
    mov ecx, space
    push dx             ; SAVE dx
    mov edx, spaceLen
    int 80h
    pop dx              ; RESTORE dx
    dec dl
    jmp innerloop1

In your second inner loop ( innerloop2 ) you are relying on the pre-initialized value of k every time you enter the loop, which is no longer valid after the first time the loop is encountered.. So you must initialize it each time:

    mov byte [k], 0   ; INITIALIZE k=0
innerloop2: ;;for(k=0;k<(2*i)-1;k++)
    mov al, [i]
    mul byte [two]
    dec al
    cmp byte [k], al
    jge printMe

This all makes the code work. Some additional comments:

• You need to be cautious about counting on the values of registers and watch where they may get altered
• You should probably not rely on pre-initialized declarations to initialize loop variables or other data that can vary for that matter. It's generally good practice to explicitly initialize a value in code if you're going to change it regularly
• Think about how to optimize your code (make it more concise and clear) now that it basically works.
• Be more consistent in variable usage. Your asm program used num for what was n but also defined a value n , which was a little confusing.
• Be consistent in your code indentation and spacing. It will make it much easier to read.
• When doing constructs, such as for loops, try to maintain a consistent approach to doing them every time. It will reduce the chance of errors. For example, manage the loop variables in the same or similar ways.