.pbuf1 NOP ; This looks like code that has been removed NOP .pbuf2 SUBTRACT_CYCLES 39 ; Subtract 39 from the cycle count .pbuf3 RTS ; Return from the subroutine .pbuf4 ADD_CYCLES_CLC 126 ; Add 126 to the cycle count JMP pbuf13 ; Jump to pbuf13 in part 2 to consider clearing the ; pattern buffer .ClearPlaneBuffers LDA cycleCount+1 ; If the high byte of cycleCount(1 0) is zero, then the BEQ pbuf3 ; cycle count is 255 or less, so jump to pbuf3 to skip ; the buffer clearing, as we have run out of cycles (we ; will pick up where we left off in the next VBlank) LDA bitplaneFlags,X ; If both bits 4 and 5 of the current bitplane flags are BIT flagsForClearing ; clear, then this means: BEQ pbuf1 ; ; * Bit 4 clear = we've not started sending data yet ; * Bit 5 clear = we have not yet sent all the data ; ; So we are not currently sending tile data to the PPU ; for this bitplane, and we have not already sent the ; data, so we do not need to clear this bitplane as we ; only do so after sending its data to the PPU, which ; we are not currently doing AND #%00001000 ; If bit 3 of the of the current bitplane flags is BEQ pbuf2 ; clear, then this bitplane is configured not to be ; cleared after it has been sent to the PPU, so jump to ; pbuf2 to return from the subroutine without clearing ; the buffers ; If we get here then we are either in the process of ; sending this bitplane's data to the PPU, or we have ; already sent it, and the bitplane is configured to be ; cleared ; ; If we have already sent the data to the PPU, then we ; no longer need it, so we need to clear the buffers so ; they are blank and ready to be drawn for the next ; frame ; ; If we are still in the process of sending this ; bitplane's data to the PPU, then we can clear the ; buffers up to the point where we have sent the data, ; as we don't need to keep any data that we have sent ; ; The following routine clears the buffers from the ; first tile we sent, up to the tile and pattern numbers ; given by sendingNameTile and sendingPattern, which ; will work in both cases, whether or not we have ; finished sending all the data to the PPU SUBTRACT_CYCLES 213 ; Subtract 213 from the cycle count BMI pbuf5 ; If the result is negative, jump to pbuf5 to skip the ; buffer clearing, as we have run out of cycles (we ; will pick up where we left off in the next VBlank) JMP pbuf6 ; The result is positive, so we have enough cycles to ; clear the buffers, so jump to pbuf6 to do just that .pbuf5 ADD_CYCLES 153 ; Add 153 to the cycle count JMP pbuf3 ; Jump to pbuf3 to skip the buffer clearing and return ; from the subroutine .pbuf6 LDA clearingNameTile,X ; Set A to clearingNameTile for this bitplane, which ; contains the number of the first tile we need to ; clear in the nametable buffer, divided by 8 LDY sendingNameTile,X ; Set Y to sendingNameTile for this bitplane, which we ; used in SendNametableToPPU to keep track of the ; current tile number as we sent them to the PPU ; nametable, so this contains the number of the last ; tile, divided by 8, that we sent to the PPU nametable ; for this bitplane ; ; So this contains the number of the last tile we need ; to clear in the nametable buffer, divided by 8 CPY maxNameTileToClear ; If Y >= maxNameTileToClear then set Y to the value of BCC pbuf7 ; maxNameTileToClear, so Y is capped to a maximum value LDY maxNameTileToClear ; of maxNameTileToClear .pbuf7 STY clearBlockSize ; Set clearBlockSize to the number of the last tile we ; need to clear, divided by 8 CMP clearBlockSize ; If A >= clearBlockSize, then the first tile we need to BCS pbuf4 ; clear is after the last tile we need to clear, which ; means there are no nametable tiles to clear, so jump ; to pbuf4 to move on to clearing the pattern buffer in ; part 2 LDY #0 ; Set clearAddress(1 0) = (nameBufferHiAddr 0) + A * 8 STY clearAddress+1 ; = (nameBufferHiAddr 0) + first tile ASL A ; ROL clearAddress+1 ; So clearAddress(1 0) contains the address of the first ASL A ; tile we sent in this bitplane's nametable buffer ROL clearAddress+1 ASL A STA clearAddress LDA clearAddress+1 ROL A ADC nameBufferHiAddr,X STA clearAddress+1 LDA #0 ; Set clearBlockSize(1 0) = (0 clearBlockSize) * 8 ASL clearBlockSize ; + (nameBufferHiAddr 0) ROL A ; = (nameBufferHiAddr 0) + last tile ASL clearBlockSize ; ROL A ; So clearBlockSize(1 0) points to the address of the ASL clearBlockSize ; last tile we sent in this bitplane's nametable buffer ROL A ADC nameBufferHiAddr,X STA clearBlockSize+1 LDA clearBlockSize ; Set clearBlockSize(1 0) SEC ; = clearBlockSize(1 0) - clearAddress(1 0) SBC clearAddress ; STA clearBlockSize ; So clearBlockSize(1 0) contains the number of tiles we LDA clearBlockSize+1 ; already sent from this bitplane's nametable buffer SBC clearAddress+1 ; BCC pbuf8 ; If the subtraction underflows, then there are no tiles STA clearBlockSize+1 ; to send, so jump to pbuf8 to move on to clearing the ; pattern buffer in part 2 ; By this point, clearBlockSize(1 0) contains the number ; of tiles we sent from this bitplane's nametable ; buffer, so it contains the number of nametable entries ; we need to clear ; ; Also, clearAddress(1 0) contains the address of the ; first tile we sent from this bitplane's nametable ; buffer ORA clearBlockSize ; If both the high and low bytes of clearBlockSize(1 0) BEQ pbuf9 ; are zero, then there are no tiles to clear, so jump to ; pbuf9 and on to part 2 to consider clearing the ; pattern buffer JSR ClearMemory ; Call ClearMemory to zero clearBlockSize(1 0) nametable ; entries from address clearAddress(1 0) onwards ; ; If we run out of cycles in the current VBlank, then ; this may not clear the whole block, so it updates ; clearBlockSize(1 0) and clearAddress(1 0) as it clears ; so we can pick it up in the next VBlank LDA clearAddress+1 ; Set (A clearAddress) SEC ; = clearAddress(1 0) - (nameBufferHiAddr 0) SBC nameBufferHiAddr,X LSR A ; Set A to the bottom byte of (A clearAddress) / 8 ROR clearAddress ; LSR A ; This effectively reverses the calculation we did ROR clearAddress ; above, so A contains the number of the next tile LSR A ; we need to clear, as returned by ClearMemory, divided LDA clearAddress ; by 8 ROR A ; ; We only need to take the low byte, as we know the high ; byte will be zero after this many shifts, as that's ; how we built the value of clearAddress(1 0) above CMP clearingNameTile,X ; If A >= clearingNameTile then we did manage to clear BCC pbuf12 ; some nametable entries in ClearMemory, so update the STA clearingNameTile,X ; value of clearingNameTile with the new first tile ; number so the next call to this routine will pick up ; where we left off JMP pbuf13 ; Jump to pbuf13 in part 2 to consider clearing the ; pattern buffer .pbuf8 NOP ; This looks like code that has been removed NOP NOP NOP .pbuf9 ADD_CYCLES_CLC 28 ; Add 28 to the cycle count JMP pbuf13 ; Jump to pbuf13 in part 2 to consider clearing the ; pattern buffer .pbuf10 ADD_CYCLES_CLC 126 ; Add 126 to the cycle count .pbuf11 RTS ; Return from the subroutineName: ClearPlaneBuffers (Part 1 of 2) [Show more] Type: Subroutine Category: Drawing the screen Summary: Clear the nametable and pattern buffers of data that has already been sent to the PPU, starting with the nametable buffer Deep dive: Drawing vector graphics using NES tilesContext: See this subroutine in context in the source code References: This subroutine is called as follows: * ClearBuffers calls ClearPlaneBuffers
Arguments: X The bitplane to clear
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Macro ADD_CYCLES (category: Drawing the screen)
Add a specified number to the cycle count
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Macro ADD_CYCLES_CLC (category: Drawing the screen)
Add a specified number to the cycle count
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Subroutine ClearMemory (category: Utility routines)
Clear a block of memory, split across multiple calls if required
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Macro SUBTRACT_CYCLES (category: Drawing the screen)
Subtract a specified number from the cycle count
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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 clearAddress in workspace ZP
The address of a block of memory to clear, for example when clearing the buffers
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Variable clearBlockSize in workspace ZP
The size of the block of memory to clear, for example when clearing the buffers
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Variable clearingNameTile in workspace ZP
The number of the first tile to clear in nametable buffer 0 when the NMI handler clears tiles, divided by 8
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Variable cycleCount in workspace ZP
Counts the number of CPU cycles left in the current VBlank in the NMI handler
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Variable flagsForClearing (category: Drawing the screen)
A bitplane mask to control how bitplane buffer clearing works in the ClearPlaneBuffers routine
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Variable maxNameTileToClear in workspace ZP
The tile number at which the NMI handler should stop clearing tiles in the nametable buffers during its clearing cycle
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Variable nameBufferHiAddr (category: Drawing the screen)
The high bytes of the addresses of the two nametable buffers
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Label pbuf1 is local to this routine
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Label pbuf12 in subroutine ClearPlaneBuffers (Part 2 of 2)
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Label pbuf13 in subroutine ClearPlaneBuffers (Part 2 of 2)
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Label pbuf2 is local to this routine
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Label pbuf3 is local to this routine
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Label pbuf4 is local to this routine
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Label pbuf5 is local to this routine
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Label pbuf6 is local to this routine
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Label pbuf7 is local to this routine
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Label pbuf8 is local to this routine
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Label pbuf9 is local to this routine
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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