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⌂ ∞ ▽ Elite on the BBC Micro and NES

PPU: ConsiderSendTiles

[NES version, Bank 7]

       Name: ConsiderSendTiles                                       [Show more]
       Type: Subroutine
   Category: PPU
    Summary: If there are enough free cycles, move on to the next stage of
             sending patterns to the PPU

    Context: See this subroutine in context in the source code
 References: This subroutine is called as follows:
             * SendBarNamesToPPU calls ConsiderSendTiles
             * SendBarPatts2ToPPU calls ConsiderSendTiles
             * SendBarPatts2ToPPU calls via RTS1
             * SendBarPattsToPPU calls via RTS1
             * SendBuffersToPPU (Part 2 of 3) calls via RTS1
             * SendNametableToPPU calls via RTS1
             * SendOtherBitplane calls via RTS1
             * SendPatternsToPPU (Part 1 of 6) calls via RTS1
             * SendPatternsToPPU (Part 6 of 6) calls via RTS1




 Other entry points:

   RTS1                 Contains an RTS



.ConsiderSendTiles

 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 RTS1               ; the process of sending tile data to the PPU for this
                        ; bitplane, so return from the subroutine (as RTS1
                        ; contains an RTS)

 SUBTRACT_CYCLES 42     ; Subtract 42 from the cycle count

 BMI next1              ; If the result is negative, jump to next1 to stop
                        ; sending patterns in this VBlank, as we have run out of
                        ; cycles (we will pick up where we left off in the next
                        ; VBlank)

 JMP next2              ; The result is positive, so we have enough cycles to
                        ; keep sending PPU data in this VBlank, so jump to
                        ; SendPatternsToPPU via next2 to move on to the next
                        ; stage of sending patterns to the PPU

.next1

 ADD_CYCLES 65521       ; Add 65521 to the cycle count (i.e. subtract 15)

 JMP RTS1               ; Return from the subroutine (as RTS1 contains an RTS)

.next2

 JMP SendPatternsToPPU  ; Jump to SendPatternsToPPU to move on to the next stage
                        ; of sending patterns to the PPU

.RTS1

 RTS                    ; Return from the subroutine