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Drawing suns: DrawSunRowOfBlocks

[NES version, Bank 7]

Name: DrawSunRowOfBlocks [Show more] Type: Subroutine Category: Drawing suns Summary: Draw a row of character blocks that contain sunlight, silhouetting any existing content against the sun
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * SUN (Part 2 of 2) calls DrawSunRowOfBlocks

This routine fills a row of whole character blocks with sunlight, turning any existing content into a black silhouette on the cyan sun. It effectively fills the character blocks containing the horizontal pixel line (P, Y) to (P+1, Y).
Arguments: P A pixel x-coordinate in the character block from which we start the fill P+1 A pixel x-coordinate in the character block where we finish the fill Y A pixel y-coordinate on the character row to fill
Returns: Y Y is preserved
.DrawSunRowOfBlocks SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure ; the PPU to use nametable 0 and pattern table 0 STY YSAV ; Store Y in YSAV so we can retrieve it below LDA P ; Set SC2(1 0) = (nameBufferHi 0) + yLookup(Y) + P * 8 LSR A ; LSR A ; where yLookup(Y) uses the (yLookupHi yLookupLo) table LSR A ; to convert the pixel y-coordinate in Y into the number CLC ; of the first tile on the row containing the pixel ADC yLookupLo,Y ; STA SC2 ; Adding nameBufferHi and P * 8 therefore sets SC2(1 0) LDA nameBufferHi ; to the address of the entry in the nametable buffer ADC yLookupHi,Y ; that contains the tile number for the tile containing STA SC2+1 ; the pixel at (P, Y) LDA P+1 ; Set Y = (P+1 - P) * 8 - 1 SEC ; SBC P ; So Y is the number of tiles we need to fill in the row LSR A LSR A LSR A TAY DEY .fill1 LDA (SC2),Y ; If the nametable entry for the Y-th tile is non-zero, BNE fill2 ; then there is already something there, so jump to ; fill2 to fill this tile using EOR logic (so the pixels ; that are already there are still visible against the ; sun, as black pixels on the sun's cyan background) LDA #51 ; Otherwise the nametable entry is zero, which is just STA (SC2),Y ; the background, so set this tile to pattern 51 DEY ; Decrement the tile counter in Y BPL fill1 ; Loop back until we have filled the entire row of tiles LDY YSAV ; Retrieve the value of Y we stored above RTS ; Return from the subroutine .fill2 ; If we get here then A contains the pattern number of ; the non-empty tile that we want to fill, so we now ; need to fill that pattern in the pattern buffer while ; keeping the existing content STY T ; Store Y in T so we can retrieve it below LDY pattBufferHiDiv8 ; Set SC(1 0) = (pattBufferHiDiv8 A) * 8 STY SC+1 ; = (pattBufferHiAddr A*8) ASL A ; ROL SC+1 ; This is the address of pattern number A in the current ASL A ; pattern buffer, as each pattern in the buffer consists ROL SC+1 ; of eight bytes ASL A ; ROL SC+1 ; So this is the address of the pattern for the tile STA SC ; that we want to fill, so now to fill it SETUP_PPU_FOR_ICON_BAR ; If the PPU has started drawing the icon bar, configure ; the PPU to use nametable 0 and pattern table 0 LDY #7 ; We now loop through each pixel row within this tile's ; pattern, filling the whole pattern with cyan, but ; EOR'ing with the pattern that is already there so it ; is still visible against the sun, as black pixels on ; the sun's cyan background .fill3 LDA #%11111111 ; Invert the Y-th pixel row by EOR'ing with %11111111 EOR (SC),Y STA (SC),Y DEY ; Decrement Y to point to the pixel line above BPL fill3 ; Loop back until we have filled all 8 pixel lines in ; the pattern LDY T ; Retrieve the value of Y we stored above, so it now ; contains the tile counter from the loop at fill1 DEY ; Decrement the tile counter in Y, as we just filled a ; tile BPL fill1 ; If there are still more tiles to fill on this row, ; loop back to fill1 to continue filling them LDY YSAV ; Retrieve the value of Y we stored above RTS ; Return from the subroutine