.UnpackToPPU LDY #0 ; We work our way through the packed data at SC(1 0), so ; set an index counter in Y, starting from the first ; data byte at offset zero .upak1 LDA (V),Y ; Set A to the Y-th byte of packed data at V(1 0), which ; is the next byte of data to unpack INY ; Increment Y to point to the next byte of packed data BNE upak2 ; If Y has now wrapped round to zero, increment the INC V+1 ; high byte of V(1 0) to point to the next page, so ; that V(1 0) + Y still points to the next data byte .upak2 CMP #$40 ; If A >= $40, jump to upak10 to send the data in A BCS upak10 ; as it is, and move on to the next byte TAX ; Store the packed data byte in X so we can retrieve it ; below AND #$0F ; If the data byte in A is in the format $x0, jump to BEQ upak9 ; upak9 to send the data in X as it is, and move on ; to the next byte CPX #$3F ; If the data byte in X is $3F, then this indicates we BEQ upak11 ; have reached the end of the packed data, so jump to ; upak11 to return from the subroutine, as we are done TXA ; Set A back to the unpacked data byte, which we stored ; in X above CMP #$20 ; If A >= $20, jump to upak5 to process values of $2x BCS upak5 ; and $3x (as we already processed values above $40) ; If we get here then we know that the data byte in A is ; of the form $0x or $1x (and not $x0) CMP #$10 ; If A >= $10, set the C flag AND #$0F ; Set X to the low nibble of A, so it contains the TAX ; number of zeroes or $FF bytes that we need to send ; when the data byte is $0x or $1x BCS upak4 ; If the data byte in A was >= $10, then we know that A ; is of the form $1x, so jump to upak4 to send the ; number of $FF bytes specified in X ; If we get here then we know that A is of the form ; $0x, so we need to send the number of zero bytes ; specified in X LDA #0 ; Set A as the byte to write to the PPU, so we send ; zeroes .upak3 ; This loop sends byte A to the PPU, X times STA PPU_DATA ; Send the byte in A to the PPU DEX ; Decrement the byte counter in X BNE upak3 ; Loop back to upak3 to send the byte in A again, until ; we have sent it X times JMP upak1 ; Jump back to upak1 to unpack the next byte .upak4 ; If we get here then we know that A is of the form ; $1x, so we need to send the number of $FF bytes ; specified in X LDA #$FF ; Set A as the byte to send to the PPU BNE upak3 ; Jump to the loop at upak3 to send $FF to the PPU, ; X times (this BNE is effectively a JMP as A is never ; zero) .upak5 ; If we get here then we know that the data byte in A is ; of the form $2x or $3x (and not $x0) CMP #$30 ; If A >= $30 then jump to upak6 to process bytes in the BCS upak6 ; for $3x AND #$0F ; Set X to the low nibble of A, so it contains the TAX ; number of times that we need to send the byte ; following the $2x data byte LDA (V),Y ; Set A to the Y-th byte of packed data at V(1 0), which ; is the next byte of data to unpack, i.e. the byte that ; we need to write X times INY ; Increment Y to point to the next byte of packed data BNE upak3 ; If Y has now wrapped round to zero, increment the INC V+1 ; high byte of V(1 0) to point to the next page, so JMP upak3 ; that V(1 0) + Y still points to the next data byte, ; and jump to the loop in upak3 to send the byte in A, ; X times .upak6 ; If we get here then we know that the data byte in A is ; of the form $3x (and not $x0), and we jump here with ; X set to 0 AND #$0F ; Set X to the low nibble of A, so it contains the TAX ; number of unchanged bytes that we need to send ; following the $3x data byte .upak7 LDA (V),Y ; Set A to the Y-th byte of packed data at V(1 0), which ; is the next byte of data to unpack INY ; Increment Y to point to the next byte of packed data BNE upak8 ; If Y has now wrapped round to zero, increment the INC V+1 ; high byte of V(1 0) to point to the next page, so ; that V(1 0) + Y still points to the next data byte .upak8 STA PPU_DATA ; Send the unpacked data in A to the PPU DEX ; Decrement the byte counter in X BNE upak7 ; Loop back to upak7 to send the next byte, until we ; have sent the next X bytes JMP upak1 ; Jump back to upak1 to unpack the next byte .upak9 TXA ; Set A back to the unpacked data byte, which we stored ; in X before jumping here .upak10 STA PPU_DATA ; Send the byte in A to the PPU JMP upak1 ; Jump back to upak1 to unpack the next byte .upak11 RTS ; Return from the subroutineName: UnpackToPPU [Show more] Type: Subroutine Category: Utility routines Summary: Unpack compressed image data and send it to the PPU Deep dive: Image and data compressionContext: See this subroutine in context in the source code References: This subroutine is called as follows: * GetCmdrImage calls UnpackToPPU * GetSystemImage calls UnpackToPPU * SendDashImageToPPU calls UnpackToPPU * SendViewToPPU calls UnpackToPPU * GetSystemImage calls via UnpackToPPU+2
This routine unpacks compressed data and sends it straight to the PPU. The data is typically nametable or pattern data that is unpacked into the PPU's nametable or pattern tables. The algorithm is described in the deep dive on "Image and data compression".
Other entry points: UnpackToPPU+2 Unpack data from offset Y onwards
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Configuration variable PPU_DATA = $2007
The PPU data register, which is used to send data to the PPU, to the address specified in PPU_ADDR
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Label upak1 is local to this routine
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Label upak10 is local to this routine
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Label upak11 is local to this routine
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Label upak2 is local to this routine
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Label upak3 is local to this routine
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Label upak4 is local to this routine
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Label upak5 is local to this routine
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Label upak6 is local to this routine
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Label upak7 is local to this routine
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Label upak8 is local to this routine
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Label upak9 is local to this routine