Screenshot

A 32-band red gradient rendered entirely by an indirect HDMA channel on the naked backdrop — no BG layers, no tiles, no assets, zero per-frame CPU. The HDMA table carries [count][pointer] entries; the PPU fetches each band's 4-byte payload ([CGADD][CGADD][color_lo][color_hi]) through the pointer and rewrites CGRAM color 0 every 7 scanlines.

C port of "SNES Red Space Indirect HDMA Demo" by krom (Peter Lemon), github.com/PeterLemon/SNES (PPU/HDMA/RedSpaceIndirectHDMA), on the snes/hdma.h API — the permanent consumer of hdmaSetupIndirect() (the register plain hdmaSetup never programs, $43x7, is the whole point of this mode).

The companion gradient_colors draws the same class of effect with a direct table (data inline); here the payload lives in a separate WRAM array the CPU can edit freely — the layout used for dynamic per-scanline palettes (the HiColor building block).

Fidelity to the original (measured, not assumed)

Register-level — krom's writes vs this ROM's generated code:

Register	krom (ASM)	this example (via the C API)
$4300 DMAP0	%01000011 (indirect \| 2REG_2X)	hdmaSetupIndirect ORs HDMA_INDIRECT into HDMA_MODE_2REG_2X
$4301 BBAD0	$21 (CGADD)	HDMA_DEST_CGADD (0x21)
$4302-3 A1T0	table address (static)	itable far pointer, set once
$4304 A1B0	$00	table pointer's bank byte ($00 WRAM)
$4307 DASB0	$00	dataBank param = (u8)((u32)(void*)band_data >> 16)
$420C HDMAEN	%1 once	hdmaEnable(1 << HDMA_CHANNEL_0) once
$212C TM	0 (backdrop only)	setMainScreen(0)

Behavioral — central-column color profile of both ROMs in luna:

Measure	krom	this example
Red ramp	255 → 0	255 → 0
Green / blue	0 / 0	0 / 0
Bands	32 steps, 7-line median width	32 steps, 7-line median width
Max per-line profile difference	—	0 (pixel-exact)

SNES Concepts

Indirect HDMA (DMAP bit 6): the table holds pointers; $43x7 selects the data bank — hdmaSetupIndirect() programs both
Non-repeat count: count=7 fetches the payload once and holds 7 lines (a repeat count $87 would fetch fresh data every line — HiColor)
HDMA_MODE_2REG_2X to CGADD streams $2121,$2121,$2122,$2122: each block rewrites CGRAM color 0, i.e. the backdrop
Bank of a C array for $43x7: (u8)((u32)(void *)arr >> 16)

How to Build

make

Modules Used

console, dma, hdma