Bresenham algorytm

        COLOR_RED = 40
        COLOR_GREEN = 70
        COLOR_UNDEF = 110

include 'macro\proc16.inc'
        org     100h

Main:
        stdcall Screen.SetGraphMode
        push    $A000
        pop     es
        stdcall Draw.Line,0,0,45,100,COLOR_RED
        stdcall Draw.Line,0,0,100,45,COLOR_GREEN
        stdcall Draw.Line,45,100,100,45,COLOR_UNDEF
        mov     ah, 08h
        int     21h
        stdcall Screen.RestoreMode
        ret

;NOTE : (X2 >= X1) && (Y2 >= Y1)
;                  OR
;       (X2 >= X1) && (Y2 <= Y1)

proc Draw.Line uses di, x1,y1,x2,y2,color:BYTE
        ; FirstAddr
        ;
        local BasicAdd dw ?
        local OffsetAdd dw ?
        local IsIncY db ?

        mov     di, [y1]
        imul    di, 320
        add     di, [x1]

        mov     [IsIncY], 1

        mov     ax, [x2]
        sub     ax, [x1]
        mov     bx, [y2]
        sub     bx, [y1]
        jns     @F
        neg     bx
        mov     [IsIncY], 0
        @@:
        cmp     ax, bx
        js @F
        mov     [BasicAdd], 0
        mov     al, [IsIncY]
        test    al, al
        jz .Dec
        mov     [OffsetAdd], 320
        jmp     .FI
        .Dec:
        mov     [OffsetAdd], -320
        jmp     .FI
        @@:
        mov     [OffsetAdd], 1
        ;
        mov     ax, [x1]
        mov     bx, [y1]
        mov     [x1], bx
        mov     [y1], ax
        ;
        mov     ax, [x2]
        mov     bx, [y2]
        mov     [x2], bx
        mov     [y2], ax


        mov     al, [IsIncY]
        test    al, al
        jz .Dec2
        mov     [BasicAdd], 319
        jmp     .FI
        .Dec2:
        mov     [BasicAdd], -321
        jmp     .FI

       ; mov     [BasicAdd], 319
        .FI:
       ; mov     di, [y1]
       ; imul    di, 320
       ; add     di, [x1]

        xor     bx, bx

        push    ax

        mov     ax, [x2]
        sub     ax, [x1]
        jns @F
        neg     ax
        @@:
        mov     cx, ax

        pop     ax

        mov     dx, cx
        mov     al, [color]
.DrawLoop:
        stosb
        add     di, [BasicAdd]
        push    ax

        mov     ax, [y2]
        sub     ax, [y1]
        jns @F
        neg     ax
        @@:
        add     bx, ax

        pop     ax
        js      .EndLoop
        sal     bx, 1
        cmp     bx, dx
        jb      @F
        add     di, [OffsetAdd]
        sar     bx, 1
        sub     bx, dx
        jmp     .EndLoop
        @@:
        sar     bx,1
.EndLoop:
        loop .DrawLoop

        ret
endp

proc Screen.SetGraphMode
     mov        ah, $0F
     int        10h
     mov        [Screen.bOldMode], al
     mov        [Screen.bOldPage], bh

     mov        ax, $0013
     int        10h
     ret
endp

proc Screen.RestoreMode
     movzx      ax, [Screen.bOldMode]
     int        10h

     mov        ah, $05
     mov        al, [Screen.bOldPage]
     int        10h
     ret
endp

Screen.bOldMode         db      ?
Screen.bOldPage         db      ?