Track geometry: GetVergeAndMarkers (Part 3 of 4)

       Name: GetVergeAndMarkers (Part 3 of 4)                        [Show more]
       Type: Subroutine
   Category: Track geometry
    Summary: Process the segment's corner markers
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


 LDA W                  \ If bits 3 and 4 of W are clear, which are these bits
 AND #%00011000         \ in the segment flags:
 BEQ gmar9              \
                        \   * Bit 3 (show right corner markers)
                        \   * Bit 4 (show left corner markers)
                        \
                        \ then we do not show any corner markers for this
                        \ segment, so jump to gmar9 to move on to the verge
                        \ marks in part 4

                        \ If we get here then we have a marker to draw for this
                        \ segment

 LDY markersToDraw      \ Set Y to the number of markers we have to draw

 CPY #3                 \ If Y >= 3, then we already have three markers ready
 BCS gmar9              \ to show, which is the maximum at any one time, so
                        \ jump to gmar9 to skip the following

 LDA segmentListPointer \ Set markerListIndex for marker Y to segmentListPointer
 STA markerListIndex,Y

 LDA W                  \ Set markerData for marker Y to the segment flags for
 STA markerData,Y       \ this marker in W

 AND #1                 \ If bit 0 of W is clear, then this is a straight track
 BEQ gmar8              \ section, so jump to gmar8 to skip the following
                        \ instruction

                        \ This is a curved section, so move the markers closer
                        \ to the track edge by halving the distance that we
                        \ store in xMarker

 LSR U                  \ Set (U T) = (U T) >> 1
 ROR T

.gmar8

 LDA T                  \ Set (xMarkerHi xMarkerLo) for marker Y to (U T), so
 STA xMarkerLo,Y        \ xMarker contains the width of the verge (halved if
 LDA U                  \ this is a corner), which we can use as the x-axis
 STA xMarkerHi,Y        \ distance from the track verge to the marker

 INC markersToDraw      \ Increment markersToDraw, as we have just added a new
                        \ marker to the list