Screen buffer: DrawFence (Part 2 of 2)

       Name: DrawFence (Part 2 of 2)                                 [Show more]
       Type: Subroutine
   Category: Screen buffer
    Summary: Draw the fence that we crash into when running off the track
    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file


.fenc2

 LDX #3                 \ Set X = 3, to use as an index into the fencePixelsSky
                        \ and fencePixelsGrass tables, so we draw pixel bytes
                        \ repeatedly from these two tables to build up the fence
                        \ effect

.fenc3

 CPY horizonLine        \ If Y < horizonLine, then this pixel row is below the
 BCC fenc4              \ horizon, so jump to fenc4 to draw the fence with green
                        \ grass behind it

 LDA fencePixelsSky,X   \ Otherwise this pixel row is above the horizon, so set
                        \ A to the correct pixel byte for the fence with blue
                        \ sky behind it

 BNE fenc5              \ Jump to fenc5 (this BNE is effectively a JMP as A is
                        \ never zero)

.fenc4

 LDA fencePixelsGrass,X \ Set A to the correct pixel byte for the fence with
                        \ green grass behind it

.fenc5

 STA (P),Y              \ Store A in the dash data block at (Q P), to draw this
                        \ four-pixel part of the fence in the track view

 STA tyreRightEdge,Y    \ Store A in the tyreRightEdge and dashRightEdge
 STA dashRightEdge,Y    \ entries for this row, so the drawing routines can wrap
                        \ the fence correctly around the dashboard and tyres

 DEX                    \ Decrement X to point to the next pixel byte in the
                        \ fence pixel byte tables

 BPL fenc6              \ If we just decremented X to -1, set it back to 3, so
 LDX #3                 \ X goes 3, 2, 1, 0, then 3, 2, 1, 0, and so on

.fenc6

 DEY                    \ Decrement the byte counter in Y to point to the next
                        \ byte in the dash data block

 CPY U                  \ If Y <> U then we have not yet drawn the fence in all
 BNE fenc3              \ the bytes in the dash data block (as U contains the
                        \ dashDataOffset for this block, which contains the
                        \ offset of the last byte that we need to fill), so loop
                        \ back to draw the next byte of the fence

                        \ If we get here then we have drawn fence through the
                        \ whole dash data block, so now we move on to the next
                        \ block by updating the counter in T and pointing (Q P)
                        \ to the next dash data block

 INC T                  \ Increment the loop counter to point to the next dash
                        \ data block

 LDA P                  \ Set (Q P) = (Q P) + &80
 EOR #&80               \
 STA P                  \ starting with the low bytes
                        \
                        \ We can do the addition more efficiently by using EOR
                        \ to flip between &xx00 and &xx80, as the dash data
                        \ blocks always start at these addresses

 BMI fenc7              \ We then increment the high byte, but only if the EOR
 INC Q                  \ set the low byte to &00 rather than &80 (if we just
                        \ set it to the latter, the BMI will skip the INC)

.fenc7

 JMP fenc1              \ Loop back to part 1 to draw the fence in the next dash
                        \ data block

 EQUB 0                 \ This byte appears to be unused