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⌂ ∞ ▽ Elite on the 6502

PPU: SendBuffersToPPU (Part 3 of 3)

[NES version, Bank 7]

       Name: SendBuffersToPPU (Part 3 of 3)                          [Show more]
       Type: Subroutine
   Category: PPU
    Summary: If we need to send tile nametable and pattern data to the PPU for
             either bitplane, start doing just that
  Deep dive: Drawing vector graphics using NES tiles

    Context: See this subroutine in context in the source code
 References: No direct references to this subroutine in this source file




.sbuf7

                        ; If we get here then we are not currently sending tile
                        ; data to the PPU, so now we check which bitplane is
                        ; configured to be sent, configure the NMI handler to
                        ; send data for that bitplane to the PPU (over multiple
                        ; calls to the NMI handler, if required), and we also
                        ; hide the bitplane we are updating from the screen, so
                        ; we don't corrupt the screen while updating it

 SUBTRACT_CYCLES 298    ; Subtract 298 from the cycle count

 LDA bitplaneFlags      ; Set A to the bitplane flags for bitplane 0

 AND #%10100000         ; This jumps to sbuf8 if any of the following are true
 CMP #%10000000         ; for bitplane 0:
 BNE sbuf8              ;
                        ;   * Bit 5 is set (we have already sent all the data
                        ;     to the PPU for bitplane 0)
                        ;
                        ;   * Bit 7 is clear (do not send data to the PPU for
                        ;     bitplane 0)
                        ;
                        ; If any of these are true, we jump to sbuf8 to consider
                        ; sending bitplane 1 instead

                        ; If we get here then we have not already send all the
                        ; data to the PPU for bitplane 0, and bitplane 0 is
                        ; configured to be sent, so we start sending data for
                        ; bitplane 0 to the PPU

 NOP                    ; This looks like code that has been removed
 NOP
 NOP
 NOP
 NOP

 LDX #0                 ; Set X = 0 and jump to sbuf11 to start sending tile
 JMP sbuf11             ; data to the PPU for bitplane 0

.sbuf8

 LDA bitplaneFlags+1    ; Set A to the bitplane flags for bitplane 1

 AND #%10100000         ; This jumps to sbuf10 if both of the following are true
 CMP #%10000000         ; for bitplane 1:
 BEQ sbuf10             ;
                        ;   * Bit 5 is clear (we have not already sent all the
                        ;     data to the PPU for bitplane 1)
                        ;
                        ;   * Bit 7 is set (send data to the PPU for bitplane 1)
                        ;
                        ; If both of these are true then jump to sbuf10 to start
                        ; sending data for bitplane 1 to the PPU

                        ; If we get here then we don't need to send either
                        ; bitplane to the PPU, so we update the cycle count and
                        ; return from the subroutine

 ADD_CYCLES_CLC 223     ; Add 223 to the cycle count

 RTS                    ; Return from the subroutine

.sbuf9

 ADD_CYCLES_CLC 45      ; Add 45 to the cycle count

 JMP SendTilesToPPU     ; Jump to SendTilesToPPU to set up the variables for
                        ; sending tile data to the PPU, and then send them

.sbuf10

 LDX #1                 ; Set X = 1 so we start sending tile data to the PPU
                        ; for bitplane 1

.sbuf11

                        ; If we get here then we are about to start sending tile
                        ; data to the PPU for bitplane X, so we set nmiBitplane
                        ; to X (so the NMI handler sends data to the PPU for
                        ; that bitplane), and we also set hiddenBitplane to X,
                        ; so that the bitplane we are updating is hidden from
                        ; view (and the other bitplane is shown on-screen)
                        ;
                        ; So this is the part of the code that swaps animation
                        ; frames when drawing the space view

 STX nmiBitplane        ; Set the NMI bitplane to the value in X, which will
                        ; be 0 or 1 depending on the value of the bitplane flags
                        ; we tested above

 LDA enableBitplanes    ; If enableBitplanes = 0 then bitplanes are not enabled
 BEQ sbuf9              ; (we must be on the Start screen), so jump to sbuf9 to
                        ; update the cycle count and skip the following two
                        ; instructions

 STX hiddenBitplane     ; Set the hidden bitplane to be the same as the NMI
                        ; bitplane, so the rest of the NMI handler update the
                        ; hidden bitplane (we only want to update the hidden
                        ; bitplane, to avoid messing up the screen)

 JSR SetPaletteForView  ; Send palette 0 for the current view to the PPU

                        ; Fall through into SendTilesToPPU to set up the
                        ; variables for sending tile data to the PPU, and then
                        ; send them