HDMA
HDMA also known as H-Blank Direct Memory Access or Horizontal Direct Memory Access is a type of DMA that does consecutive small transfers of 1-4 bytes instead of an entire block transfer at once. It's a hardware feature present on the SNES made mostly for manipulating certain PPU registers while the screen is being rendered for giving different rendering effects per scanline basis. It's executed during the H-blank period, which is when the PPU is preparing to render the next scanline of the screen output and some registers can be modified during the small pause.
It's a feature extensively used in games for creating different effects of all types and combined with several PPU registers it allows for bypassing several limitations of the 4th generation consoles. Similar approaches are done as well in systems other than Super Nintendo, by using other timing tricks such as IRQ.
Although it's used most of the time to update PPU registers, since HDMA is just a variation of the DMA, you can do any kind of transfer between the address bus A and B, including but not limited to APU I/O ports, WRAM registers and even do a backwards transfer (read from PPU and write to RAM).
Registers that can be used with HDMA
| Address | Code | Name | Example Usage |
|---|---|---|---|
| $2100 | INIDISP | Screen Display Register | Darkness effect, Brightness control, F-Blank HDMA |
| $2105 | BGMODE | BG Mode and Character Size Register | Multiple screen modes at same time, Screen split |
| $2106 | MOSAIC | Mosaic Register | Partial censorship, Pseudo-3D projection |
| $210D | BG1HOFS | BG1 Horizontal Scroll Registers | Parallax backgrounds, Horizontal waves (water), Pseudo-3D projection, Shape modelation, Mirror effect, Progress bar, Laser projection |
| $210E | BG1VOFS | BG1 Vertical Scroll Registers | Vertical stretching, Vertical rotation, Vertical waves (fire), Screen split, Dissolving effect |
| $210F | BG2HOFS | BG2 Horizontal Scroll Registers | See $210D |
| $2110 | BG2VOFS | BG2 Vertical Scroll Registers | See $210E |
| $2111 | BG3HOFS | BG3 Horizontal Scroll Registers | See $210D, OAM-like per tile mode 2 manipulation |
| $2112 | BG3VOFS | BG3 Vertical Scroll Registers | See $210E, OAM-like per tile mode 2 manipulation |
| $2113 | BG4HOFS | BG4 Horizontal Scroll Registers | See $210D |
| $2114 | BG4VOFS | BG4 Vertical Scroll Registers | See $210E |
| $211B | M7A | Mode 7 Matrix Registers | Pseudo-3D projection, DSP-1 projection, Linear algebra calculation, Screen looping, Pipeline effect, Optical illusions |
| $211C | M7B | Mode 7 Matrix Registers | See $211B |
| $211D | M7C | Mode 7 Matrix Registers | See $211B |
| $211E | M7D | Mode 7 Matrix Registers | See $211B |
| $211F | M7X | Mode 7 Matrix Registers | See $211B, Screen distortion |
| $2120 | M7Y | Mode 7 Matrix Registers | See $211F |
| $2121 | CGADD | CGRAM Address Register | Palette manipulation, Color gradients |
| $2122 | CGDATA | CGRAM Data Write Register | See $211B |
| $2126 | WH0 | Window Position Registers (WH0) | Screen masking, Single-color screen shape/polygon rendering, Optical illusions, Black holes |
| $2127 | WH1 | Window Position Registers (WH1) | See $2126 |
| $2128 | WH2 | Window Position Registers (WH2) | See $2126 |
| $2129 | WH3 | Window Position Registers (WH3) | See $2126 |
| $212C | TM | Screen Destination Registers | Screen masking, Hires rendering, Transparency effects, Blending effects |
| $212D | TS | Screen Destination Registers | See $212C |
| $2132 | COLDATA | Color Math Registers | Color gradients, transparency effects, brightness control |
Unusual cases
| Address | Code | Name | Example Usage |
|---|---|---|---|
| $2107 | BG1SC | BG Tilemap Address Registers (BG1) | Single layer status bar, Screen split |
| $2108 | BG2SC | BG Tilemap Address Registers (BG2) | See $2107 |
| $2109 | BG3SC | BG Tilemap Address Registers (BG3) | See $2107 |
| $210A | BG3SC | BG Tilemap Address Registers (BG4) | See $2107 |
| $210B | BG12NBA | BG Character Address Registers (BG1&2) | See $2107 |
| $210C | BG34NBA | BG Character Address Registers (BG3&4) | See $2107 |
| $211A | M7SEL | Mode 7 Settings Register | N/A |
| $2123 | W12SEL | Window Mask Settings Registers | Black holes, Screen masking, Partial element screen masking |
| $2124 | W34SEL | Window Mask Settings Registers | See $2123 |
| $2125 | WOBJSEL | Window Mask Settings Registers | See $2123 |
| $212A | WBGLOG | Window Mask Logic registers (BG) | See $2123 |
| $212B | WOBJLOG | Window Mask Logic registers (OBJ) | See $2123 |
| $212E | TMW | Window Mask Destination Registers | Used together $212C, otherwise subscreen masking. |
| $212F | TSW | Window Mask Destination Registers | Used together $212D, otherwise subscreen masking. |
| $2130 | CGWSEL | Color Math Registers | N/A |
| $2131 | CGADSUB | Color Math Registers | N/A |
| $2133 | SETINI | Screen Mode Select Register | Undefined behavior |
| $2134 | MPYL | Multiplication Result Registers | Cumulative multiplications |
| $2135 | MPYM | Multiplication Result Registers | See $2134 |
| $2136 | MPYH | Multiplication Result Registers | See $2134 |
| $2137 | SLHV | Software Latch Register | Debug HDMA execution, Verify HDMA delay, Check for H-blank status |
| $213B | CGDATAREAD | CGRAM Data Read Register | N/A |
| $213C | OPHCT | Scanline Location Registers (Horizontal) | See $2137 |
| $213D | OPVCT | Scanline Location Registers (Vertical) | N/A |
| $213E | STAT77 | PPU Status Register | N/A |
| $213F | STAT78 | PPU Status Register | N/A |
| $2140 | APUIO0 | APU IO Registers | Sample streaming, Software interrupts |
| $2141 | APUIO1 | APU IO Registers | See $2140 |
| $2142 | APUIO2 | APU IO Registers | See $2140 |
| $2143 | APUIO3 | APU IO Registers | See $2140 |
| $2180 | WMDATA | WRAM Data Register | Co-processor polling at HDMA level, WRAM pattern filling, Code manipulation together IRQ. |
| $2181 | WMADDL | WRAM Address Registers | See $2180 |
| $2182 | WMADDM | WRAM Address Registers | See $2180 |
| $2183 | WMADDH | WRAM Address Registers | See $2180 |