;;drawing line from 2 cordinates
.proc line
    ;;#                               *      (X_end, Y_end) #
    ;;#                                                     #
    ;;#                         *                           #
    ;;#                    *                                #
    ;;# (X_pos, Y_pos)                                      #
    ;;
    ;;NOTE THAT X_pos <= X_end, Y_pos >= Y_end. Max 45deg!

    ;;Not values but register position in memory
    X_end = $04
    Y_end = $05
    X_pos = $FC
    Y_pos = $FB
    dy_2 = $0607
    dx = $0c
    V = $0809
    D = $0a0b

    ;example values ~~~~~ SHOULD BE PRECOMPILED
    LDA #$90
    STA X_pos
    STA Y_pos

    LDA #$aa
    STA X_end
    LDA #$80
    STA Y_end
    ;;~~~~~~~~~~

    ;;We need to clear this memory
    LDA #$00
    STA <V
    STA <dy_2
    STA $FD ; for pixel_draw

    ;; V = 2*(dx -dy)
     ;; where: dy = Y_pos - Y_end, dx = X_end - X_start
     LDA Y_pos
     SEC
     SBC Y_end
     STA >V
     STA >dy_2;  >dy_2 = dy. Needed for dy_2 (not for V)
     LDA X_end
     SEC
     SBC X_pos
     STA dx
     SEC
     SBC >V
     STA >V; <V = dx - dy
     mult_16 >V, <V; V = 2*(dx -dy)

     ;dy_2 = dy*2
     mult_16 >dy_2, <dy_2

     ;;D = 2*dy - dx + 2*255
     ;;Our D is bigger then wikipedia because D is unsigned.
     LDA >dy_2
     STA >D
     LDA <dy_2
     STA <D
     Add_16 >D, <D, #$00, #$01
     Sub_16 >D, <D, dx, #$00
for_x:
    jsr pixel_draw
    ;;Increment X until X_pos = X_end and Y_pos = Y_end
    INC X_pos
    LDX X_pos
    CPX X_end
    BEQ end

    ;;If D <  %00000001 11111111 <==> >D ==0, then: case_2
    LDA <D; if <D > 0 then: case_1
    CMP #$00
    BEQ case_2
case_1:
    DEC Y_pos
    Sub_16 >D, <D, >V, <V; D = D - V
    JMP for_x
case_2:
    Add_16 >D, <D, >dy_2, <dy_2;D = D + 2*dy
    JMP for_x
end:
.endproc