DMA examples

This page provides examples on how to use DMA registers to do fast copies on the SNES.

These examples use the following defines to make the code clearer:

DMAMODE     = $4300 ; DMAPn DMAPPUREG   = $4301 ; BBADn DMAADDR     = $4302 ; A1TnL DMAADDRHI   = $4303 ; A1TnH DMAADDRBANK = $4304 ; A1Bn DMALEN      = $4305 ; DASnL DMALENHI    = $4306 ; DASnH
 * Registers           Also known as...

DMA_LINEAR  = $00 DMA_01      = $01 DMA_00      = $02 DMA_0011    = $03 DMA_0123    = $04 DMA_0101    = $05 DMA_FORWARD = $00 DMA_CONST   = $08 DMA_BACKWARD = $10 DMA_INDIRECT = $40 DMA_READPPU = $80
 * Configuration for $43n0
 * OR these together to get the desired effect

DMAMODE_PPULOFILL = (<VMDATAL << 8) | DMA_LINEAR | DMA_CONST DMAMODE_PPUHIFILL = (<VMDATAH << 8) | DMA_LINEAR | DMA_CONST DMAMODE_PPUFILL  = (<VMDATAL << 8) | DMA_01     | DMA_CONST DMAMODE_RAMFILL  = (<WMDATA  << 8) | DMA_LINEAR | DMA_CONST DMAMODE_PPULODATA = (<VMDATAL << 8) | DMA_LINEAR | DMA_FORWARD DMAMODE_PPUHIDATA = (<VMDATAH << 8) | DMA_LINEAR | DMA_FORWARD DMAMODE_PPUDATA  = (<VMDATAL << 8) | DMA_01     | DMA_FORWARD DMAMODE_CGDATA   = (<CGDATA  << 8) | DMA_00     | DMA_FORWARD DMAMODE_OAMDATA  = (<OAMDATA << 8) | DMA_00     | DMA_FORWARD
 * These defines are meant for a 16-bit write to $43n0 and $43n1
 * and they set up the channel for several common cases.

Copying OAM
This is a simple DMA example that sends an OAM buffer to the PPU.

.proc CopyOAM php

rep #$20 ; Set A to 16-bit

lda #DMAMODE_OAMDATA sta DMAMODE lda #OAM   ; Copy from OAM buffer in RAM sta DMAADDR lda #544   ; 512 bytes + 32 bytes = 544 sta DMALEN

sep #$20 ; Set A to 8-bit

lda #^OAM  ; Set the bank byte of the source address sta DMAADDRBANK

; Start the DMA lda #1 sta MDMAEN

plp rtl .endproc

DMA as part of a scrolling update
This example demonstrates using DMA to write to video RAM while scrolling a large map, and it might make sense to put something like it in a game's vblank handler. Notice the writes to PPU registers - the DMA unit only handles writing the actual data, so anything else (such as setting the destination address) has to be done normally. This also demonstrates using VMAIN to write downwards through a tilemap.

.a16 ; Does a column need to be updated? lda ColumnUpdateAddress beq :+ stz ColumnUpdateAddress sta VMADDL

; Write to VMDATAL and VMDATAH lda #DMAMODE_PPUDATA sta DMAMODE

; Copy from the buffer lda #.loword(ColumnUpdateBuffer) sta DMAADDR

; A tilemap column is 32 tiles long, and each tile is 2 bytes lda #32*2 sta DMALEN

sep #$20 ; 8-bit accumulator

lda #^ColumnUpdateBuffer sta  to VRAM word address rowBufferVramWaddr + 0
 * rowBuffer[ 64 - 128] to VRAM word address rowBufferVramWaddr + 0x400

The DMA A-Bus address will be incremented during the transfer (unless it is in fixed address mode). This means DMAADDR will be prefilled with the address of the second half of the row buffer after the first DMA transfer.


 * 8 bit A
 * 16 bit Index
 * DB access registers
 * DP = 0
 * rowBuffer            u16[64] - buffer containing a 64 tile tilemap row (128 bytes in size)
 * rowBufferVramWaddr   u16     - the VRAM word address to transfer rowBuffer to
 * rowBufferVramWaddr   u16     - the VRAM word address to transfer rowBuffer to

; VRAM word addressing lda #$80 sta VMAIN

;   ; First DMA transfer. ; Transfer `rowBuffer` bytes 0 - 63 to VRAM word address `rowBufferVramWaddr` ;

; Set VRAM word address ldx rowBufferVramWaddr stx VMADD

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

; Set DMA source address ldx #rowBuffer & 0xffff stx DMAADDR lda #rowBuffer >> 16 sta DMAADDRBANK

; Length of the DMA transfer (32 words, 64 bytes) ldx #64 stx DMALEN

; Start DMA transfer lda #1 sta MDMAEN

;   ; Second DMA transfer. ; Transfer `rowBuffer` bytes 64 - 127 to VRAM word address `rowBufferVramWaddr + 0x400` ;

; Set VRAM word address to `rowBufferVramWaddr + 0x400` lda rowBufferVramWaddr sta VMADDL

lda rowBufferVramWaddr + 1 clc adc #$04 sta VMADDH

; No need to set DMAMODE or DMAPPUREG, it remains unchanged after a DMA transfer.

; After the first transfer, DMAADDR will contain `rowBuffer + 64`. ; There is no need to set DMAADDR or DMAADDRBANK.

; We must write to DMALEN, it will be 0 after a successful DMA transfer. ldx #64 stx DMALEN

; Start DMA transfer lda #1 sta MDMAEN

Updating two tilemap rows in a 64 tile-wide background
Due to the discontiguous nature of the tilemap in a 64 tile-wide background, a transfer of a contiguous 64x2 word grid to VRAM requires 4 DMA transfers:


 * rowBuffer[ 0 -  63] to VRAM word address rowBufferVramWaddr + 0
 * rowBuffer[128 - 191] to VRAM word address rowBufferVramWaddr + 0x020
 * rowBuffer[ 64 - 127] to VRAM word address rowBufferVramWaddr + 0x400
 * rowBuffer[192 - 255] to VRAM word address rowBufferVramWaddr + 0x420</tt>

This transfer can be simplified by using two DMA channels to transfer the first and third quarters of the rowBuffer, one after another, in a single DMA transfer. Afterwords, the second and fourth quarters can be transferred to VRAM.


 * rowBuffer[ 0 -  63]</tt> and rowBuffer[128 - 191]</tt> to VRAM word address rowBufferVramWaddr + 0</tt>
 * rowBuffer[ 64 - 127]</tt> and rowBuffer[192 - 255]</tt> to VRAM word address rowBufferVramWaddr + 0x400</tt>


 * 8 bit A
 * 16 bit Index
 * DB access registers
 * DP = 0
 * rowBuffer            u16[128] - buffer containing a 64x2 tilemap grid (256 bytes in size)
 * rowBufferVramWaddr   u16      - the VRAM word address to transfer rowBuffer to
 * rowBufferVramWaddr   u16      - the VRAM word address to transfer rowBuffer to

; VRAM word addressing lda #$80 sta VMAIN

;   ; First DMA transfer (using DMA channels 0 & 1). ; Transfer `rowBuffer` bytes 0 - 63 and bytes 128 - 191 to VRAM word address `rowBufferVramWaddr` ;

; Set VRAM word address ldx rowBufferVramWaddr stx VMADD

; Word transfer to VMDATA ldx #DMA_01 | ((VMDATAL & 0xff) << 8) stx DMAMODE + $00                      ; also sets B Bus Address stx DMAMODE + $10

; Set DMA source addresses ldx #rowBuffer & 0xffff stx DMAADDR + $00

ldx #(rowBuffer + 128) & 0xffff stx DMAADDR + $10

lda #rowBuffer >> 16 sta DMAADDRBANK + $00 sta DMAADDRBANK + $10

; Transfer size for each DMA channel (32 words, 64 bytes) ldx #64 stx DMALEN + $00 stx DMALEN + $10

; Start DMA transfer for channels 0 & 1 lda #$03 sta MDMAEN

;   ; Second DMA transfer (using DMA channels 0 & 1). ; Transfer `rowBuffer` bytes 64 - 127 and bytes 192 - 255 to VRAM word address `rowBufferVramWaddr + 0x400` ;

; Set VRAM word address to `rowBufferVramWaddr + 0x400` lda rowBufferVramWaddr sta VMADDL

lda rowBufferVramWaddr + 1 clc adc #$04 sta VMADDH

; No need to set DMAMODE or DMAPPUREG, it remains unchanged after a DMA transfer.

; After the first transfer, DMAADDR channel 0 will contain `rowBuffer + 64` ; and DMAADDR channel 1 will contain `rowBuffer + 192` ;   ; There is no need to set DMAADDR or DMAADDRBANK.

; We must write to DMALEN, it will be 0 after a successful DMA transfer. ldx #64 stx DMALEN + $00 stx DMALEN + $10

; Start DMA transfer for channels 0 & 1 lda #$03 sta MDMAEN

Fixed address DMA transfers
The fixed address DMA mode is useful for clearing and filling blocks of memory.

WRAM clear
These are reusable subroutines for clearing out sections of RAM, which a program might want to do at the start of a level, or in Init code. Both routines take a start address in X, and a size in Y.

The source address of a DMA transfer to WMDATA cannot be in Work-RAM, because the SNES cannot handle using DMA to copy from one section of Work-RAM to another. If you intend to fill Work-RAM with a non-zero value, use a source address in Cartridge-RAM or ROM.

Note: This routine cannot be used to clear the entire bank $7E of Work-RAM, as it will override the stack and crash the program.

Note: Due to a hardware bug on early SNES consoles it's not recommended to do this while HDMA is enabled.

.i16 ; 16-bit index registers assumed .proc MemClear php sep #$20  ; 8-bit accumulator stz WMADDH ; Set high bit of WRAM address to zero - meaning the first 64KB of RAM UseHighHalf: stx WMADDL ; WRAM address, bottom 16 bits sty DMALEN

; Configure DMA to write to WMDATA, and keep the source address constant ldx #DMAMODE_RAMFILL ZeroSource: stx DMAMODE

; The zero byte used as the source needs to come from somewhere in ROM ; here it's taken from the second byte of a "STX $4300" ldx #.loword(ZeroSource+1) stx DMAADDR ; Set the bank byte of the source address too lda #^MemClear sta DMAADDRBANK

; Start the DMA lda #1 sta MDMAEN plp rtl .endproc

.proc MemClear7F php sep #$20 lda #1    ; Use the second 64KB of RAM sta WMADDH bra MemClear::UseHighHalf .endproc
 * Clear a section of bank 7F instead

Filling VRAM
A fixed byte DMA transfer can be used to clear a block of VRAM. Unlike the clear Work-RAM routine above, the source of the DMA can be a Work-RAM memory address. This allows us to fill VRAM with a byte value of our choosing.


 * 8 bit A
 * 16 bit Index
 * DB access registers
 * DP = 0
 * Uses a single zeropage byte variable (zpTmpByte)
 * Uses a single zeropage byte variable (zpTmpByte)

.proc ResetVram ldx #0 ldy #0 .endproc
 * Clears all of VRAM (using DMA)
 * fallthrough

.proc ClearVram lda #0 .endproc
 * Clear a block of VRAM (using DMA)
 * IN: X - VRAM word address
 * IN: Y - size (in bytes)
 * IN: Y - size (in bytes)
 * fallthrough

.proc FillVRAM ; Store value to fill in zeropage sta zpTmpByte
 * Fill a block of VRAM with a byte value (using DMA)
 * IN: X - VRAM word address
 * IN: Y - size in bytes (if 0 then 64KiB of VRAM is filled)
 * IN: A - byte value
 * IN: A - byte value

; VRAM word addressing lda #$80 sta VMAIN

; Set VRAM word address stx VMADD

; Length of the DMA transfer sty DMALEN

; Fixed byte transfer to word register VMDATA ldx #DMA_01 | DMA_CONST | ((VMDATA & 0xff) << 8) stx DMAMODE                                        ; also sets B Bus Address

; Set DMA source address ldx #zpTmpByte stx DMAADDR stz DMAADDRBANK            ; zeropage bank is always 0

; Disable HDMA (prevents the model-1 HDMA/DMA crash) stz HDMAEN

; Start DMA transfer lda #1 sta MDMAEN

rts .endproc

Filling VRAM with a word value
Filling VRAM with a word value is a bit more complicated. The fixed address DMA mode only allows for byte fills. Fortunately the VMAIN register provides a method of writing to the low and high bytes of VRAM separately, allowing for a VRAM word fill to be preformed in two DMA transfers.


 * 8 bit A
 * 16 bit Index
 * DB access registers
 * DP = 0
 * Uses a single zeropage word variable (zpTmpWord)
 * Uses a single zeropage word variable (zpTmpWord)

.proc FillVramTilemap ; Store value to fill in zeropage sty zpTmpWord
 * Fills a 32x32 tilemap in VRAM with a given word value (using DMA)
 * IN: X = VRAM word address
 * IN: Y = word value

; Must not modify X, it is still required after the first DMA transfer

; Disable HDMA (prevents the model-1 HDMA/DMA crash) stz HDMAEN

;   ; Transfer the low byte of zpTmpWord to VMDATAL `32*32` times ;

; VRAM byte addressing to VMDATAL stz VMAIN

; Set VRAM word address stx VMADD

; Fixed byte transfer to byte register VMDATAL ldy #DMA_LINEAR | DMA_CONST | ((VMDATAL & 0xff) << 8) sty DMAMODE                                            ; also sets B Bus Address

; Set DMA source address to the low byte of zpTmpWord ldy #zpTmpWord sty DMAADDR stz DMAADDRBANK            ; zeropage bank is always 0

; Length of the DMA transfer ldy #32 * 32 sty DMALEN

; Start DMA transfer lda #1 sta MDMAEN

;   ; Transfer the high byte of zpTmpWord to VMDATAH `32*32` times ;

; VRAM byte addressing to VMDATAH lda #$80 sta VMAIN

; Set VRAM word address stx VMADD

; Change DMA B-Bus address to VMDATAH lda #VMDATAH & 0xff sta DMAPPUREG ; DMAMODE is already set

; Set DMA source address to the high byte of zpTmpWord lda #zpTmpWord + 1 sta DMAADDR ; DMAADDRHI and DMAADDRBANK is already set

; Length of the DMA transfer ldy #32 * 32 sty DMALEN

; Start DMA transfer lda #1 sta MDMAEN

rts .endproc