Screenshot

Port of krom (Peter Lemon)'s WindowMultiHDMA demo (PeterLemon/SNES, PPU/Window/WindowMultiHDMA). One HDMA channel in 4-register mode streams WH0-WH3 ($2126-$2129) — both windows' left AND right edges — per scanline band, cutting a 2×2 grid of portholes into BG1. The green backdrop shows wherever the combined mask disables the layer; the D-pad scrolls the ring artwork behind the fixed portholes. hdmaWindowShape covers a single window; this is the two-window composition the arc audit flagged.

Window algebra (krom's exact setup): W1 and W2 both enabled on BG1 and both inverted, combined with AND — masked where (outside W1) AND (outside W2) = visible inside either window.

The table is krom's exact 14-band data (in main.c); the artwork is original (procedural radial color wheel, tiled 2×2 so gfx4snes dedups it into 16 KB of tiles).

SNES Concepts

HDMA_MODE_4REG: one channel drives 4 consecutive PPU registers/line
Two windows shaped per scanline (W1L/W1R/W2L/W2R in one stream)
Window combination logic: windowSetInvert ×2 + windowSetLogic(AND)
- windowSetMainMask — the full W12SEL/WBGLOG/TMW surface from C
Degenerate window (left=1, right=0) as the "fully masked" band value

Register fidelity vs the original

Register	krom	this port
BGMODE	$0B (Mode 3, priority)	same (setMode(BG_MODE3, 0x08))
W12SEL	$0F (W1+W2 on BG1, inverted)	same (windowEnable+windowSetInvert ×2)
WBGLOG	$01 (BG1 = AND)	same (windowSetLogic(WINDOW_BG1, WINDOW_LOGIC_AND))
TMW	$01	same (windowSetMainMask(WINDOW_BG1))
DMAP0/BBAD0	%100 / $26	same (hdmaSetup(ch0, HDMA_MODE_4REG, HDMA_DEST_WH0, …))
HDMA table	14 bands of 16 lines	krom's exact bytes (see main.c)
Backdrop	green (CGRAM 0)	same (setColor(0, RGB(0,31,0)))

How to Build

cd examples/windows/window_multi_hdma && make

Modules Used

console, dma, background, hdma, window, input