.org 22h
dw @vbi

.org 80h

; %al, overwrites ;
abs:
    test %al 1h
    jz @.L2
    neg %al
    .L2:
    ret

drawpixel:
    push %a
    push %b
    push %c

    mov %a [%sp + 8h]

    mov %b %al
    mul %b 12h
    add %b @T1

    mov %c %ah
    mov %a %ah

    div %c 8h ; todo: add mod ;
    mul %c 8h

    sub %a %c

    div %c 8h
    add %b %c

    mov %c 7h
    sub %c %a
    mov %a %c

    mov %c 1h
    shl %c %a

    mov %a [%b]
    or %a %c
    mov [%b] %a

    pop %c
    pop %b
    pop %a
    ret

; %ch - x1, %cl - y1, %dh - x2, %dl - y2 ;
drawline:
    push %a
    push %b
    push %c
    push %d
    push %e

    mov %al %dh
    sub %al %ch
    call @abs
    mov %ah %al ; ah - dX ;

    mov %al %dl
    sub %al %cl
    call @abs
    neg %al ; al - -dY ;

    mov %bh FFh ; bh - signX;
    mov %bl FFh ; bl - signY ;

    cmp %ch %dh
    jgz @.S1
    neg %bh
    .S1:

    cmp %cl %dl
    jgz @.S2
    neg %bl
    .S2:

    mov %el %ah
    add %el %al ; el - error ;

    push %d
    call @drawpixel
    pop %d

    .loop:
        cmp %ch %dh
        jnz @.L1
        cmp %cl %dl
        jnz @.L1
        jmp @.loop_end
        .L1:

        push %c
        call @drawpixel
        pop %c

        mov %eh %el

        cmp %eh %al
        jng @.L2
            add %el %al
            add %ch %bh
        .L2:
        cmp %eh %ah
        jgz @.loop
            add %el %ah
            add %cl %bl
        jmp @.loop

    .loop_end:

    pop %e
    pop %d
    pop %c
    pop %b
    pop %a
    ret

start:
    mov %sp FFFFh
    outb 20h 1h
    outb 21h 20h
    out  22h @T1

    mov %al 1h
    int 10h

    mov %ch 60h
    mov %cl 50h

    mov %dh 5h
    mov %dl 3h

    call @drawline

    .loop:
        jmp @.loop

vbi:
    ret

.org 1000h
T1: