Skip to navigation

⌂ ∞ ▽ Elite on the 6502

Dashboard: DOT

[Apple II version]

       Name: DOT                                                     [Show more]
       Type: Subroutine
   Category: Dashboard
    Summary: Draw a dash on the compass

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * COMPAS calls DOT




 Arguments:

   COMX                 The screen pixel x-coordinate of the dash

   COMY                 The screen pixel y-coordinate of the dash

   COMC                 The thickness of the dash (this is the opposite way
                        around to the other versions, which have a larger
                        compass dot when the item is in front):

                          * 0 = do not draw a dot on the compass

                          * $30 = a double-height dash in white, for when the
                            object in the compass is behind us

                          * $60 = a single-height dash in white, for when the
                            object in the compass is in front of us



.DOT

 LDA COMC               ; If COMC = 0 then jump to COR1 to return from the
 BEQ COR1               ; subroutine without drawing a dot, as COR1 contains
                        ; an RTS

 STA ZZ                 ; Set ZZ = A, so ZZ is $30 or $60, which can be passed
                        ; to the PIXEL routine as a "distance" so the first
                        ; value of $30 draws a large ("close") dot for when the
                        ; compass item is behind, while the second value of $60
                        ; draws a small ("faraway") dot for when it is in front
                        ;
                        ; This is the opposite way around to the other versions,
                        ; which have a larger compass dot when the item is in
                        ; front

 LDA COMY               ; Set A = COMY, the y-coordinate of the dash

 LDX COMX               ; Set X = COMX, the x-coordinate of the dash

 JMP PIXEL              ; Call the PIXEL routine to draw a large or small white
                        ; dot on the compass, returning from the subroutine
                        ; using a tail call