Skip to navigation

⌂ ∞ ▽ Revs on the BBC Micro

Drawing the track: StopDrawingEdge

       Name: StopDrawingEdge                                         [Show more]
       Type: Subroutine
   Category: Drawing the track
    Summary: Stop drawing the current segment's verge edge

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * DrawVergeByteLeft calls StopDrawingEdge
             * DrawVergeByteRight calls StopDrawingEdge




 This routine stops drawing the current segment's edge when called from
 DrawVergeByteLeft or DrawVergeByteRight.



.StopDrawingEdge

 TSX                    \ These instructions remove two bytes from the top of
 INX                    \ the stack so the RTS below returns an extra level up
 INX                    \ the call chain
 TXS                    \
                        \ We jump to this routine from the DrawVergeByteLeft and
                        \ DrawVergeByteRight routines. These are only called
                        \ from the DrawShallowToLeft, DrawShallowToRight,
                        \ DrawSteepToLeft or DrawSteepToRight routines, which in
                        \ turn are only called from DrawSegmentEdge, so this
                        \ returns us to DrawSegmentEdge to stop drawing the
                        \ current segment's verge edge and move on to the next
                        \ segment

 LDA updateBackground   \ If updateBackground is non-zero, then we just drew a
 BNE UpdateBackground   \ verge on the start of the left edge of the screen, so
                        \ call UpdateBackground to update the background colour
                        \ for this track line, returning from the subroutine
                        \ using a tail call

 RTS                    \ Return from the subroutine