HDMA examples

Examples for using HDMA.

Transfer patterns

The following is a suggested list of PPU registers that can be used in the various HDMA transfer patterns.

One register (DMAP pattern 0)

This pattern reads/writes a single byte to a single address on the B-bus (+0).

This pattern should not be used on write-twice or read-twice registers.

• BG Mode: BGMODE
• Mosaic Settings: MOSAIC
• BG Settings: BG1SC, BG2SC, BG3SC, BG4SC, BG12NBA, BG34NBA
• Mode 7 settings: M7SEL
• CGRAM address: CGADD
  • A future HDMA channel should write to CGDATA on the same scanline as the CGADD write.
• Window settings: W12SEL, W34SEL, WOBJSEL, WBGLOG, WOBJLOG
• Layer enable: TM, TS, TMW, TSW
• Color math: CGWSEL, CGADSUB
• Screen Mode/Video Select: SETINI
• Fixed color data: COLDATA
  • Only a single B/G/R channel can be changed in this pattern (unless multiple channels contain the same value). See COLDATA for more details.

Two registers (DMAP pattern 1)

This pattern reads/writes two bytes to two addresses on the B-bus (+0, +1). It is useful when writing to two adjacent byte registers.

This pattern must not be used on write-twice or read-twice registers.

• Window 1 left and right position: WH0 & WH1
• Window 2 left and right position: WH2 & WH3
• Window mask settings: W12SEL & W34SEL, W34SEL & WOBJSEL
• Window mask logic: WBGLOG & WOBJLOG
• Layer Enable: TM & TS, TMW & TSW
• Color math settings: CGWSEL & CGADSUB

One register, write twice (DMAP pattern 2)

This pattern reads/writes two bytes to a single write-twice or read-twice register on the B-bus (+0, +0).

• BG horizontal scroll offset: BGnHOFS
• BG vertical scroll offset: BGnVOFS
• Mode 7 matrix: M7A, M7B, M7C, M7D, M7X, M7Y
• CGRAM data: CGDATA
  • Note: A previous HDMA channel should set the CGRAM address on the same scanline as a CGDATA write
• Fixed color data: COLDATA
  • Only two of the three B/G/R channels can be changed in this pattern (unless two channels share the same value). See COLDATA for more details.

Two registers, write twice (DMAP pattern 3)

This pattern reads/writes four bytes to two adjacent write-twice or read-twice registers on the B-bus (+0, +0, +1, +1).

• BG scroll offsets: BGnHOFS & BGnVOFS
• 2 adjacent mode 7 matrix values: M7A & M7B, M7C & M7D, M7X & M7Y
• CGRAM address and data: CGADD & CGDATA
  • This transfer will write two values to the CGRAM address, followed by a color word value to CGDATA.
  • The first byte will be ignored by the PPU, the second byte contains the target CGRAM address (palette index), the third and forth bytes contain the color data.

Four registers (DMAP pattern 4)

This pattern reads/writes four bytes to four addresses on the B-bus (+0, +1, +2, +3).

This pattern must not be used on write-twice or read-twice registers.

• All four window positions: WH0 & WH1 & WH2 & WH3

HDMA Tables

HDMA tables are made up of multiple HDMA entries. Each entry starts with a Line-Counter byte, followed by register data. If the Line-Counter byte is 0, the HDMA channel will be deactivated and will remain deactivated until the start of the next frame.

Line-Counter byte:

7  bit  0
---- ----
RLLL LLLL
|||| ||||
|+++-++++- Number of scanlines (count)
+--------- Repeat flag

If byte == 0x00:  End of HDMA table
If byte == 0x80:  count = 128, repeat flag = 0

Non-repeat HDMA entries

A non-repeat HDMA entry (bit-7 of Line-Counter clear or Line-Counter == 0x80) is hdma_transfer_bytes + 1 bytes in size. It consists of the Line-Counter byte, followed by the data to transfer on the next Horizontal Blanking Period. Afterwards, the HDMA controller will wait for count (bits 0-6 of Line-Counter) scanlines before processing the next HDMA entry.

If the Line-Counter byte is 0x80, the HDMA entry is a non-repeating entry that will wait 128 scanlines.

// HDMA Table targeting the `TM` register (One register transfer pattern).
HdmaTable:
    db  32              // 32 scanlines, non-repeat entry
        db  0x13        // TM = BG1, BG2, OBJ

    db  64              // 64 scanlines, non-repeat entry
        db  0x04        // TM = BG3

    db  1               // 1 scanline, non-repeat entry
        db  0x13        // TM = BG1, BG2, OBJ

    db  0               // End HDMA table

Each non-repeat HDMA entry is a maximum of 128 scanlines tall. Any HDMA entry with more then 128 scanlines must be split in two, with each entry containing the same data values.

// HDMA Table for the `BG1HOFS` register (one register, write twice transfer pattern).
HdmaTable:
    // 160 scanlines with BG1HOFS = 0
    // Too many scanlines to fit in a single HDMA entry.
    db  127             // 127 scanlines, non-repeat entry
        dw  0
 
    db  33              // 33 scanlines, non-repeat entry (+127 = 160 scanlines total)
        dw  0           // Same data value as the previous entry
 
 
    // Change BG1HOFS every 16 scanlines
    db  16              // 16 scanlines, non-repeat entry
        dw  -2
 
    db  16              // 16 scanlines, non-repeat entry
        dw  -4
 
    db  16              // 16 scanlines, non-repeat entry
        dw  -6
 
    db  16              // 16 scanlines, non-repeat entry
        dw  -8
 
    db  0               // End HDMA table

Links

• HDMA Examples - by nesdoug
• Grog's Guide to DMA and HDMA on the SNES - superfamicom.org wiki