VBlank routine

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Shadow variables

A register shadow variable is a RAM variable that will hold (shadow) the intended value of a PPU register. This variable can be read or written to at any time. When it is safe to write to the register, usually during the Vertical Blanking period, the shadow variable will be transferred to its associated register.

Shadow variables offer many advantages over directly writing to the register:

  • Shadow variables can be read or written at any time.
  • Shadow variables allow the code to read the shadowed-state of write-only registers.
  • Write-twice registers can store their state in a 16 bit (word) variable. This allows the main-loop to access the shadow with a 16 bit Accumulator or Index.
  • Reading a shadow variable is significantly faster then calculating the intended value of a register. The slow calculation can be preformed in the main-loop and the write to the PPU register can be preformed during VBlank.

Shadow variables should be allocated in Low-RAM. This allows the VBlank routine to access both the shadow-variable and the register with the addr addressing mode.


Write-only byte register example

The following is an example of a shadow variable for the MOSAIC register. Notice how shadow variable allows the main-loop to read the shadowed register state and how tiny the VBlank code is.


Main-loop code: 

;   mosaicShadow   u8 - shadow of the MOSAIC register
;
;   zpTmpByte      u8 - zeropage temporary byte variable


; Increase mosaic size by 1 (with clamping), leave mosaic enable bits unchanged
;
; DB access Low RAM ($00-$3f, $7e, $80-$bf)
.a8
.i16
    ; extract mosaic enable bits
    lda mosaicShadow
    and #0xf0
    sta zpTmpByte

    ; increase mosaic size bits (with clamping)
    lda mosaicShadow
    inc
    and #0x0f
    bne :+
        ; mosaic size overflowed
        lda #0xf
    :

    ; combine enable and size bits
    ora zpTmpByte
    sta mosaicShadow


VBlank code: 

;   mosaicShadow   u8 - shadow of the MOSAIC register

; In VBlank
; DB access registers ($00-$3f, $80-$bf)
.a8
.i16
    lda mosaicShadow
    sta MOSAIC


Write-twice register example

The following is an example of a shadow variable for the BG1HOFS and BG1VOFS scroll registers. Notice how the main-loop code writes to the shadow variables in 16 bit mode, while the VBlank code is required to read the shadow variables one byte at a time.


Main-loop code: 

;   bg1HOffset    u16 - shadow of the BG1HOFS register
;   bg1VOffset    u16 - shadow of the BG1VOFS register

; DB access Low RAM ($00-$3f, $7e, $80-$bf)
.a8
.i16
    ; Set BG1 scroll offset with a 16 bit Index register
    ldx #0
    stx bg1HOffset

    ldx #.loword(-1)
    stx bg1VOffset


VBlank code: 

;   bg1HOffset    u16 - shadow of the BG1HOFS register
;   bg1VOffset    u16 - shadow of the BG1VOFS register


; In VBlank
; DB access registers ($00-$3f, $80-$bf)
.a8
.i16
    ; Transfer a 16 bit variable to a write twice register
    lda .lobyte(bg1HOffset)
    sta BG1HOFS
    lda .hibyte(bg1HOffset)
    sta BG1HOFS

    lda .lobyte(bg1VOffset)
    sta BG1VOFS
    lda .hibyte(bg1VOffset)
    sta BG1VOFS



Buffers

A common method of updating VRAM, OAM or CGRAM while the screen is active is to use a fixed-size buffer. The main-loop would read and write to the buffer, filling it with data. When the buffer is ready, the main-loop would write to one or more control variables. Later, the VBlank routine will read the control variable to determine if the buffer should be transferred and if so, transfers the buffer to the PPU using DMA.

The buffer can be allocated anywhere in RAM and the control variables should be allocated in Low-RAM.

At minimum, there should be a single control variable that determines if the buffer is to be transferred to the PPU on the next VBlank. This variable is usually a byte flag and can be either a transfer-on-zero byte flag or transfer-on-non-zero byte flag (depending on preference). The transfer flag is reset after the VBlank routine has transferred the buffer to the PPU.

Optionally, a buffer can also have control variables for the buffer's destination address and/or transfer size. If there are multiple control variables, the transfer flag should be written last.

To prevent glitches and screen tearing, the buffer should not be modified after the transfer flag is enabled. The simplest method of achieving this is to only write to the buffer once per frame.


Common use-cases for buffers include:


OAM buffer example

VBlank code: 

;   oamBuffer                   u8[544] - OAM buffer data.
;   oamBufferTransferFlag       u8      - byte flag.  If non-zero, `oamBuffer` is transferred to OAM on the next VBlank.  Must be in Low-RAM.


; In VBlank
; DB access registers ($00-$3f, $80-$bf)
.a8
.i16
    ; Read oamBuffer transfer flag
    lda oamBufferTransferFlag
    beq SkipOamTransfer
        ; transfer flag is set

        ; Reset OAM address
        ldx #0
        stx OAMADD


        ; Transfer oamBuffer to OAM using DMA channel 0
        ldx #DMA_LINEAR | ((OAMDATA & 0xff) << 8)
        stx DMAMODE                                 ; also sets B Bus Address

        ldx #oamBuffer
        stx DMAADDR
        lda #.bankbyte(oamBuffer)
        sta DMAADDRBANK

        ldx #544
        stx DMALEN

        lda #1
        sta MDMAEN


        ; Reset oamBuffer transfer flag
        stz oamBufferTransferFlag

SkipOamTransfer:


Main-loop code: 

    GameLoop:
        ; ...

        ; process players, enemies, particles, etc drawing their sprites into the `oamBuffer`

        ; ...


        ; Notify the VBlank routine that the oamBuffer has changed
        lda #1
        sta oamBufferTransferFlag


        ; Wait until the VBlank routine has been processed
        jsr WaitForVBlank

        jmp GameLoop


Tilemap column buffer example

This following is an example of a buffer with multiple control variables.


VBlank code 

;   columnBuffer                u16[32] - buffer containing 32 tilemap columns (64 bytes in size)
;   columnBufferVramWaddr       u16     - VRAM word address to transfer `columnBuffer` to.  Must be in Low-RAM.
;   columnBufferTransferFlag    u8      - byte flag.  If non-zero, `columnBuffer` is transferred to VRAM on the next VBlank.  Must be in Low-RAM.


; In VBlank
; DB access registers ($00-$3f, $80-$bf)
.a8
.i16
    ; Read columnBufferTransferFlag transfer flag
    lda columnBufferTransferFlag
    beq SkipColumnBufferTransfer
        ; transfer flag is set

        ; VRAM word access, increment by 32
        lda #$81
        sta VMAIN

        ; Set VRAM word address
        ldx columnBufferVramWaddr
        stx VMADD


        ; Transfer column buffer to VRAM using DMA channel 0

        ldx #DMA_01 | ((VMDATA & 0xff) << 8)
        stx DMAMODE                             ; also sets B Bus Address

        ldx #columnBuffer
        stx DMAADDR
        lda #.bankbyte(columnBuffer)
        sta DMAADDRBANK

        ldx #64
        stx DMALEN

        lda #1
        sta MDMAEN


        ; Change VRAM access back to "word access, increment by 1"
        ; This is not required if your VBlank code sets `VMAIN` before every VRAM transfer.
        lda #$80
        sta VMAIN


        ; Reset transfer flag
        stz columnBufferTransferFlag

SkipColumnBufferTransfer:









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