2021-08-05 13:20:38 +00:00
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; Routines for handling tilemaps
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;
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; This module contains higher-level functions than the low-level tile rendering routines. The
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; goal here is to take a rectangular tilemap data structure and efficiently render it into
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2021-08-06 19:42:18 +00:00
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; code buffer. Especially important is to only draw new tiles as they come into view.
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2021-08-05 13:20:38 +00:00
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;
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; Also, we maintain a tilemap cache to track the current state of the tiles rendered into
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; the code field so if, by chance, a tile that comes into view is the same as a tile that
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; has already been drawn, then there is no reason to update it. This happen quite often
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; in actual games since the primary background is often large empty areas, or runs
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; of repeating tiles.
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2021-08-06 19:42:18 +00:00
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; _UpdateBG0TileMap
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;
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; Fill in dirty tiles into the BG0 buffer.
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;
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; A = $FFFF re-render the entire playfield. Otherwise only render difference from the old
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; coordinates.
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2021-08-05 13:20:38 +00:00
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_UpdateBG0TileMap
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:Left equ tmp0
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:Right equ tmp1
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:Top equ tmp2
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:Bottom equ tmp3
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:Width equ tmp4 ; Used in DrawRectBG0
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:Height equ tmp5
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:MulA equ tmp6 ; Scratch space for multiplication
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:MulB equ tmp7
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:Offset equ tmp8 ; Address offset into the tilemap
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:Span equ tmp9
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:GlobalTileIdxX equ tmp10
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:GlobalTileIdxY equ tmp11
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:BlkX equ tmp12
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:BlkY equ tmp13
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2021-08-06 19:42:18 +00:00
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:Refresh equ tmp14
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cmp #$FFFF
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lda #0
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rol
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sta :Refresh ; 1 if A = $FFFF, 0 otherwise
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2021-08-05 13:20:38 +00:00
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lda StartY ; calculate the tile index of the current location
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and #$FFF8
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lsr
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lsr
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2021-08-06 19:42:18 +00:00
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lsr
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2021-08-05 13:20:38 +00:00
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sta BG0TileOriginY
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lda OldStartY
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and #$FFF8
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lsr
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lsr
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2021-08-06 19:42:18 +00:00
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lsr
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sta OldBG0TileOriginY
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lda StartX
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and #$FFFC
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lsr
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lsr
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sta BG0TileOriginX
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lda OldStartX
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2021-08-06 19:42:18 +00:00
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and #$FFFC
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2021-08-05 13:20:38 +00:00
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lsr
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lsr
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2021-08-06 19:42:18 +00:00
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sta OldBG0TileOriginX
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2021-08-05 13:20:38 +00:00
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; Figure out the two rectangular regions that need to be updated. We check for changes in Y-direction
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; first because it's a bit more efficient to redraw tiles in long horizontal strips, because we do not
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; have to skip to different banks.
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;
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; +---------------------------+----------+ <-- Top
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; | | New |
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; | Old Area | (drawn) |
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; | | (second) |
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; | | |
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; +---------------------------+==========|
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; | New Area (drawn first) |
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; | |
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; +--------------------------------------+ <-- Bottom
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; ^ ^
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; | |
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; +--- Left Right --+
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stz :Left ; prepare to do the entire screen
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lda ScreenTileWidth ; and then whack off the parts
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sta :Right ; that are not needed
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stz :Top
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lda ScreenTileHeight
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sta :Bottom
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2021-08-06 19:42:18 +00:00
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; If we are supposed to refresh the while field, just do that and return
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lda :Refresh
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beq :NoRefresh
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jmp :DrawRectBG0 ; Let the DrawRectBG0 RTS take care of the return for us
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:NoRefresh
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lda BG0TileOriginY
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cmp OldBG0TileOriginY
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beq :NoYUpdate ; if equal, don't change Y
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sec
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sbc OldBG0TileOriginY ; find the difference; D = Y_new - Y_old
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bpl :DoBottom ; if we scrolled up, fill in the bottom row(s)
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eor #$FFFF ; if we scrolled down, Y_new < Y_old and we need
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sta :Bottom ; to fill in the top row(s) from 0 to Y_new - Y_old - 1
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bra :DoYUpdate
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:DoBottom
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eor #$FFFF ; same explanation as above, except we are filling in from
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inc a ; Bottom - (Y_new - Y_old) to Bottom
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clc
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adc ScreenTileHeight
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sta :Top
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:DoYUpdate
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jsr :DrawRectBG0 ; Fill in the rectangle.
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; We performed an update in the Y-direction, so now change the bounds so
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; an update in the X-direction will not draw too many rows
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;
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; +---------------------------+----------+
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; | | |
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; | | New |
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; | | |
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; | Old Area | (drawn) |
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; | | (second) |
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; | | |
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; +---------------------------+==========| <-- Top
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; |//////////////////////////////////////|
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; |// New Area (drawn first) ////////////|
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; |//////////////////////////////////////|
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; +--------------------------------------+ <-- Bottom
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; ^ ^
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; +--- Left Right --+
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lda :Top
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beq :drewTop
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dec a ; already did Y to HEIGHT, so only need to draw from
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sta :Bottom ; 0 to (Y-1) for any horizontal updates
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stz :Top
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bra :NoYUpdate
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:drewTop
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lda :Bottom ; opposite, did 0 to Y
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inc a ; so do Y+1 to HEIGHT
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sta :Top
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lda ScreenTileHeight
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sta :Bottom
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; +---------------------------+----------+ <-- Top
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; | | |
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; | | New |
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; | | |
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; | Old Area | (drawn) |
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; | | (second) |
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; | | | <-- Bottom
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; +---------------------------+==========|
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; |//////////////////////////////////////|
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; |// New Area (drawn first) ////////////|
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; |//////////////////////////////////////|
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; +--------------------------------------+
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; ^ ^
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; +--- Left Right --+
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; The Top an Bottom are set the the correct values to draw in whatever potential range of tiles
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; need to be draws if there was any horizontal displacement
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:NoYUpdate
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lda BG0TileOriginX ; Did the first column of the tile map change from before?
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cmp OldBG0TileOriginX ; Did it change from before?
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beq :NoXUpdate ; no, so we can ignore this
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sec
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sbc BG0TileOriginX ; find the difference
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bpl :DoRightSide ; did we move in a pos or neg?
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; Handle the two sides in an analagous way as the vertical code
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eor #$FFFF
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sta :Right
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bra :DoXUpdate
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:DoRightSide
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eor #$FFFF
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inc
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clc
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adc ScreenTileWidth
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sta :Left
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:DoXUpdate
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jsr :DrawRectBG0 ; Fill in the rectangle.
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:NoXUpdate
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rts
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; This is a private subroutine that draws in tiles into the code fields using the
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; data from the tilemap and the local :Top, :Left, :Bottom and :Right parameters.
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:DrawRectBG0
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lda :Bottom
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sec
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sbc :Top
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inc
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sta :Height
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lda :Right
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sec
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sbc :Left
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inc
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sta :Width
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; Compute the offset into the tile array of the top-left corner
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lda :Left
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clc
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adc BG0TileOriginX
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sta :GlobalTileIdxX
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lda :Top
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clc
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adc BG0TileOriginY ; This is the global verical index
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sta :GlobalTileIdxY
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ldx TileMapWidth
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jsr :MulAX
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clc
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adc :GlobalTileIdxX
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asl ; Double for word sizes
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sta :Offset ; Stash the pointer offset in Y
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lda TileMapWidth
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sec
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sbc :Width
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asl ; This is the number of bytes to move the Offset to advance from the end of
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sta :Span ; one line to the beginning of the next
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; Now we need to figure out the code field tile coordinate of corner of
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; play field. That is, becuase the screen is scrolling, the location of
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; tile (0, 0) could be anywhere within the code field
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lda StartYMod208 ; This is the code field line that is at the top of the screen
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and #$FFF8 ; Clamp to the nearest block
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lsr
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lsr
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lsr ; Could optimize because the Tile code shifts back....
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clc
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adc :Top
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sta :BlkY ; This is the Y-block we start drawing from
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lda StartXMod164 ; Dx the same thing for X, except only need to clamp by 4
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and #$FFFC
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lsr
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lsr
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clc
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adc :Left
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sta :BlkX
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; Call the copy tile routine to blit the tile data into the playfield
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; A = Tile ID (0 - 1023)
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; X = Tile column (0 - 40)
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; Y = Tile row (0 - 25)
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pei :BlkX ; cache the starting X-block index to restore later
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pei :Width ; cache the Width value to restore later
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:yloop
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:xloop
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ldy :Offset ; Set up the arguments and call the tile blitter
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lda [TileMapPtr],y
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iny ; pre-increment the address. A bit faster than two "INC DP" instructions
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iny
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sty :Offset
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ldx :BlkX
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ldy :BlkY
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jsr CopyTile
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2021-08-06 20:57:03 +00:00
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lda :BlkX
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inc
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cmp #42 ; If we go past the physical block index, wrap around
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bcc *+5
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lda #0
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sta :BlkX
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2021-08-05 13:20:38 +00:00
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dec :Width ; Decrement out count
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bne :xloop
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lda :Offset ; Move to the next line of the Tile Map
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clc
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adc :Span
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sta :Offset
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lda 3,s ; Reset the BlkX
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sta :BlkX
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lda 1,s ; Reset the width
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sta :Width
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2021-08-06 20:57:03 +00:00
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lda :BlkY ; The y lookup has a double0length array, may not need the bounds check
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inc
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cmp #27
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bcc *+5
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lda #0
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sta :BlkY
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2021-08-05 13:20:38 +00:00
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dec :Height ; Have we done all of the rows?
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bne :yloop
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pla ; Pop off cached values
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pla
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rts
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; Quick multiplication of the accumulator and x-register
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; A = A * X
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:MulAX
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stx :MulA
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cmp :MulA ; Put the smaller value in MulA (less shifts on average)
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bcc :swap
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sta :MulB
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bra :entry
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:swap stx :MulB
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sta :MulA
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:entry
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lda #0
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; Start shifting and adding. We actually do an extra
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; shift if MulA is zero, but a zero value does not
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; change the result and it allows us to eliminate a
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; branch on the inner loop
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:loop
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lsr :MulA ; shift out the LSB
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bcc :skip ; zero is no multiply
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clc
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adc :MulB
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:skip
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asl :MulB ; double the multplicand
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ldx :MulA
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bne :loop
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rts
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2021-08-06 19:42:18 +00:00
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2021-08-06 20:57:03 +00:00
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