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⌂ ∞ ▽ Revs on the BBC Micro

Screen buffer: DrawTrackView (Part 1 of 4)

       Name: DrawTrackView (Part 1 of 4)                             [Show more]
       Type: Subroutine
   Category: Screen buffer
    Summary: Draw the top part of the track view using the data in the dash
             data blocks
  Deep dive: Drawing around the dashboard
             Drawing the track view
             The track verges

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * MainDrivingLoop (Part 1 of 5) calls DrawTrackView
             * MainDrivingLoop (Part 2 of 5) calls DrawTrackView




.DrawTrackView

 LDA #0                 \ Set (Q P) = &6700 and (S R) = &6800, ready to pass to
 STA P                  \ the DrawTrackLine routine (so the track view gets
 STA R                  \ drawn at the correct place on screen, from &6701
 LDX #&67               \ onwards, as the first line is drawn at (Q P) + 1)
 STX Q
 INX
 STX S

 LDX #79                \ Set X = 79, to point to the first byte to draw from
                        \ each dash data block (i.e. the byte at the end of the
                        \ data block, at offset 79)

 JSR DrawTrackLine      \ Draw one-pixel high lines that correspond to dash data
                        \ offsets 79 to 44, returning from the subroutine after
                        \ drawing the line specified by offset X = 44 (so this
                        \ draws all the lines from the top of the track view,
                        \ down to the line just above the top of the dashboard)

 JMP view1              \ Jump to part 2 to draw the rest of the track view from
                        \ offsets 43 to 3, modifying the code so it draws the
                        \ rest of the lines around the shape of the dashboard