I'm trying to create a triangle of dots in assembly but it isn't working

Question

I'm trying to create a triangle of dots on the screen by taking a user-entered value (to vary the size of the resulting triangle) and using it to write decrementing lines of dots.

Here is the code:

section .data
        global  _start

        char    db      ' '

        prompt_text     db      "Enter triangle size (2-99) "
        prompt_length   equ     $-prompt_text

section .bss
        tri_size        resb    3
        tri_size_length equ     $-tri_size

section .text
_start:
        call    prompt
        call    insert_size

        mov     rax,    [tri_size]

outer_loop:
        mov     rbx,    [tri_size]

inner_loop:
        call    dot
        dec     bx
        cmp     bx,     0
        jg      inner_loop

        call    linefeed

        call    dec_length

        dec     ax
        cmp     ax,     0
        jne     outer_loop

        call    linefeed
        call    exit

prompt:
        mov     rax,    4
        mov     rbx,    1
        mov     rcx,    prompt_text
        mov     rdx,    prompt_length
        int     80h
        ret

insert_size:
        mov     rax,    3
        mov     rbx,    0
        mov     rcx,    [tri_size]
        mov     rdx,    tri_size_length
        int     80h
        ret

dot:
        mov     [char], byte '.'
        call    print_char
        ret

linefeed:
        mov     [char], byte 10
        call    print_char
        ret

print_char:

        push    rax
        push    rbx
        push    rcx
        push    rdx


        mov     rax,    4
        mov     rbx,    1
        mov     rcx,    char
        mov     rdx,    1
        int     80h

        pop     rdx
        pop     rcx
        pop     rbx
        pop     rax
        ret

dec_length:

        push    rax
        push    rbx
        push    rcx
        push    rdx

        mov     rax,    [tri_size]
        dec     ax
        mov     [tri_size],     rax

        pop     rdx
        pop     rcx
        pop     rbx
        pop     rax
        ret

exit:
        mov     rax,    1
        mov     rbx,    0
        int     80h

Problem:

• On running the program, I want the user-inputted number to be used for the number of dots on the first line. However, when I type any number, loads of lines are printed with a single dot on each, then after about a second, a line is printed which contains 32768 dots. This is followed by a line with 32767 dots etc. The number of dots on each line continue to decrease until the line which has 1 dot.

I've noticed that 32768 is hexadecimal for 10000000_00000000, but other than that I'm completely stuck and I'd REALLY appreciate any help!

P.S. I'm using x84-64 linux and assembling with YASM

Answer 1

There are two problems in your code, both when you read the input. First, the fix. Then, an explanation of the current results.

insert_size:
    mov     rax,    3
    mov     rbx,    0
    mov     rcx,    [tri_size]          ; issue 1
    mov     rdx,    tri_size_length
    int     80h
    ret                                 ; issue 2 (sort of)

The Fix

First, rcx should contain the address of a buffer, but you are getting the contents of tri_size, not its address. Since tri_size is in the bss section, it is initialized with 0s, so you are telling the OS to read into a NULL buffer. If you were to check the result of the system call, you would see an error code because of this.

Second, when you read input, you are reading a string, not a number. If you want to use it as a number, you need to convert it first. Here is the code with both issues fixed:

insert_size:
    mov     rax,    3
    mov     rbx,    0
    mov     rcx,    tri_size           ; 1
    mov     rdx,    tri_size_length
    int     80h
    mov     dh,     0                  ; 2
    mov     ah,     0
    mov     dl,     [tri_size]         ; 3
    mov     ah,     [tri_size+1]
    sub     dl,     '0'                ; 4
    cmp     al,     '0'                ; 5
    jb      done
    cmp     al,     '9'                ; 6
    ja      done
    imul    dx,     10                 ; 7
    sub     al,     '0'                ; 8
    add     dx,     ax                 ; 9
done:
    mov     [tri_size], dx             ; 10
    ret

The first issue is an easy fix, just remove the brackets to get the address instead of the contents. The second is more complicated. First, we are going to use 16 bit registers but only read into 8 bits, so step 2 puts 0 in the high byte for each. Then, we read the first two bytes of the string. Next, we convert the first character from a character to a number. Since the digits in ASCII are sequential, we can do this by subtracting the character '0'. Note that this assumes the first character is a valid digit.

We cannot assume the second character is a valid digit, since there might have been only one entered. Therefore, steps 5 and 6 check to see if it is less than '0' and greater than '9', respectively, and jump to the end if either is true. If we get past both, then the second character is a digit. This means the first digit should be in the 10's place, so we multiply it by 10. Then we convert the second character to a number and add it to the first. Now that both characters have been tested, we can store the result back where it is expected and return.

---

The Explanation

As explained in the fix, you are passing a NULL buffer, so it returns an error. Your tri_size variable is never changed, so it still contains 0. This means both of your counters start at 0. Since you don't check for this, the first dot is printed and bx is decremented, resulting in -1. Since -1 is not greater than 0, the inner loop exits, a newline is printed, your counter and ax are decremented, resulting in -1. This is not zero, so it loops back to the outer loop. This happens 32768 times, until your counter gets to -32768.

When you decrement bx at this point, it becomes -32769, but this number is not representable using 16-bit 2's complement. Instead, it overflows, and the number you get is 32767. This is greater than 0, so you continue in the inner loop until it gets to 0, printing a total of 32768 dots. After the inner loop exits and the newline is printed, the counter and ax are both decremented, resulting in 32767. From this point on, your program works as if the input was 32767, meaning you get a triangle from there to 0 dots.

Interestingly, if your inner loop tested if bx was equal to 0 instead of greater than, you would simply get a triangle from 65536 dots to 0, with no 1-dot lines before it.