LDX nmiBitplane ; Set X to the current NMI bitplane (i.e. the bitplane ; for which we are sending data to the PPU in the NMI ; handler) LDA bitplaneFlags,X ; Set A to the bitplane flags for the NMI bitplane AND #%00010000 ; If bit 4 of A is clear, then we are not currently in BEQ sbuf7 ; the process of sending tile data to the PPU for this ; bitplane, so jump to sbuf7 in part 3 to start sending ; tile data ; If we get here then we are already in the process of ; sending tile data to the PPU, split across multiple ; calls to the NMI handler, so before we can consider ; sending data for anything else, we need to finish the ; job that we already started SUBTRACT_CYCLES 56 ; Subtract 56 from the cycle count TXA ; Set Y to the inverse of X, so Y is the opposite EOR #1 ; bitplane to the NMI bitplane TAY LDA bitplaneFlags,Y ; Set A to the bitplane flags for the opposite plane ; to the NMI bitplane AND #%10100000 ; If bitplanes are enabled then enableBitplanes = 1, so ORA enableBitplanes ; this jumps to sbuf2 if any of the following are true CMP #%10000001 ; for the opposite bitplane: BNE sbuf2 ; ; * Bitplanes are disabled ; ; * Bit 5 is set (we have already sent all the data ; to the PPU for the opposite bitplane) ; ; * Bit 7 is clear (do not send data to the PPU for ; the opposite bitplane) ; ; If any of these are true, we jump to SendPatternsToPPU ; via sbuf2 to continue sending tiles to the PPU for the ; current bitplane ; If we get here then the following are true: ; ; * Bitplanes are enabled ; ; * We have not sent all the data for the opposite ; bitplane to the PPU ; ; * The opposite bitplane is configured to be sent to ; the PPU LDA lastPattern,X ; Set A to the number of the last pattern number to send ; for this bitplane BNE sbuf1 ; If it is zero (i.e. we have no free tiles), then set LDA #255 ; A to 255, so we can use A as an upper limit .sbuf1 CMP sendingPattern,X ; If A >= sendingPattern, then the number of the last BEQ sbuf3 ; pattern to send is bigger than the number of the BCS sbuf3 ; pattern which we are currently sending pattern data ; to the PPU for this bitplane, which means there is ; still some pattern data to send before we have ; processed all the patterns, so jump to sbuf3 ; ; The BEQ appears to be superfluous here as BCS will ; catch an equality ; If we get here then we have finished sending pattern ; data to the PPU, so we now move on to the next stage ; by jumping to SendPatternsToPPU after adjusting the ; cycle count SUBTRACT_CYCLES 32 ; Subtract 32 from the cycle count .sbuf2 JMP SendPatternsToPPU ; Jump to SendPatternsToPPU to continue sending tile ; data to the PPU .sbuf3 ; If we get here then the following are true: ; ; * Bitplanes are enabled ; ; * We have not sent all the data for the opposite ; bitplane to the PPU ; ; * The opposite bitplane is configured to be sent to ; the PPU ; ; * We are in the process of sending data for the ; current bitplane to the PPU ; ; * We still have pattern data to send to the PPU for ; this bitplane LDA bitplaneFlags,X ; Set A to the bitplane flags for the NMI bitplane ASL A ; Shift A left by one place, so bit 7 becomes bit 6 of ; the original flags, and so on BPL RTS1 ; If bit 6 of the bitplane flags is clear, then this ; bitplane is only configured to send pattern data and ; not nametable data, and to stop sending the pattern ; data if the other bitplane is ready to be sent ; ; This is the case here as we only jump to sbuf3 if ; the other bitplane is configured to send data to the ; PPU, so we stop sending the pattern data for this ; bitplane by returning from the subroutine (as RTS1 ; contains an RTS) LDY lastNameTile,X ; Set Y to the number of the last tile we need to send ; for this bitplane, divided by 8 AND #%00001000 ; If bit 2 of the bitplane flags is set (as A was BEQ sbuf4 ; shifted left above), set Y = 128 to override the last LDY #128 ; tile number with 128, which means send all tiles (as ; 128 * 8 = 1024 and 1024 is the buffer size) .sbuf4 TYA ; Set A = Y - sendingNameTile SEC ; = lastNameTile - sendingNameTile SBC sendingNameTile,X ; ; So this is the number of tiles for which we still have ; to send nametable entries, as sendingNameTile is the ; number of the tile for which we are currently sending ; nametable entries to the PPU, divided by 8 CMP #48 ; If A < 48, then we have fewer than 48 * 8 = 384 BCC sbuf6 ; nametable entries to send, so jump to sbuf6 to swap ; the hidden and visible bitplanes before sending the ; next batch of tiles SUBTRACT_CYCLES 60 ; Subtract 60 from the cycle count .sbuf5 JMP SendPatternsToPPU ; Jump to SendPatternsToPPU to continue sending tile ; data to the PPU .sbuf6 LDA ppuCtrlCopy ; If ppuCtrlCopy is zero then we are not worried about BEQ sbuf5 ; keeping PPU writes within VBlank, so jump to sbuf5 to ; skip the following bitplane flip and crack on with ; sending data to the PPU SUBTRACT_CYCLES 134 ; Subtract 134 from the cycle count LDA enableBitplanes ; If bitplanes are enabled then enableBitplanes will be EOR hiddenBitplane ; 1, so this flips hiddenBitplane between 0 and 1 when STA hiddenBitplane ; bitplanes are enabled, and does nothing when they ; aren't (so it effectively swaps the hidden and visible ; bitplanes) JSR SetPaletteForView ; Send palette 0 for the current view to the PPU JMP SendPatternsToPPU ; Jump to SendPatternsToPPU to continue sending tile ; data to the PPUName: SendBuffersToPPU (Part 2 of 3) [Show more] Type: Subroutine Category: PPU Summary: If we are already sending tile data to the PPU, pick up where we left off, otherwise jump to part 3 to check for new data to send Deep dive: Drawing vector graphics using NES tilesContext: See this subroutine in context in the source code References: No direct references to this subroutine in this source file
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Entry point RTS1 in subroutine ConsiderSendTiles (category: PPU)
Contains an RTS
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Macro SUBTRACT_CYCLES (category: Drawing the screen)
Subtract a specified number from the cycle count
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Subroutine SendPatternsToPPU (Part 1 of 6) (category: PPU)
Calculate how many patterns we need to send and jump to the most efficient routine for sending them
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Subroutine SetPaletteForView (category: Drawing the screen)
Send palette 0 for the current view to the PPU
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Variable bitplaneFlags in workspace WP
Flags for bitplane 0 that control the sending of data for this bitplane to the PPU during VBlank in the NMI handler
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Variable enableBitplanes in workspace ZP
A flag to control whether two different bitplanes are implemented when drawing the screen, so smooth vector graphics can be shown
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Variable lastNameTile in workspace ZP
The number of the last nametable buffer entry to send to the PPU nametable table in the NMI handler for bitplane 0, divided by 8
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Variable lastPattern in workspace ZP
The number of the last pattern entry to send from pattern buffer 0 to bitplane 0 of the PPU pattern table in the NMI handler
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Variable nmiBitplane in workspace ZP
The number of the bitplane (0 or 1) that is currently being processed in the NMI handler during VBlank
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Variable ppuCtrlCopy in workspace ZP
Contains a copy of PPU_CTRL, so we can check the PPU configuration without having to access the PPU
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Label sbuf1 is local to this routine
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Label sbuf2 is local to this routine
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Label sbuf3 is local to this routine
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Label sbuf4 is local to this routine
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Label sbuf5 is local to this routine
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Label sbuf6 is local to this routine
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Label sbuf7 in subroutine SendBuffersToPPU (Part 3 of 3)
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Variable sendingNameTile in workspace ZP
The number of the most recent tile that was sent to the PPU nametable by the NMI handler for bitplane 0 (or the number of the first tile to send if none have been sent), divided by 8
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Variable sendingPattern in workspace ZP
The number of the most recent pattern that was sent to the PPU pattern table by the NMI handler for bitplane 0 (or the number of the first pattern to send if none have been sent)