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