Screen buffer: DrawFence (Part 1 of 2)

       Name: DrawFence (Part 1 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: This subroutine is called as follows:
             * CheckForCrash calls DrawFence


 Other entry points:

   DrawFence-1          Contains an RTS


.DrawFence

 LDA #0                 \ Set playerMoving = 0 to denote that the player's car
 STA playerMoving       \ is not moving

 STA T                  \ Set T = 0, to use as a counter as we work our way
                        \ through the 40 dash data blocks that contain the track
                        \ view

 STA P                  \ Set (Q P) to point to the first dash data block at
 LDA #HI(dashData)      \ dashData (this works because the low byte of dashData
 STA Q                  \ is zero)

.fenc1

 LDX T                  \ If X = 40 then we have drawn the fence across all the
 CPX #40                \ dash data blocks, so return from the subroutine (as
 BEQ DrawFence-1        \ DrawFence-1 contains an RTS)

 LDA dashDataOffset,X   \ Set U to the dashDataOffset for the current dash data
 STA U                  \ block

 LDY #70                \ Set Y = 70, to use as a loop counter that works
                        \ through all 70 bytes in the dash data block

 JMP fenc2              \ We now jump to part 2 to work our way through the 70
                        \ bytes in the dash data block, each of which represents
                        \ a four-pixel line, with 70 lines stacked one on top of
                        \ the other, in a four-pixel-wide vertical strip