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fix(video): read BG maps and tiles through the 128 KiB VRAM mirror window

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0xrsydn 2026-10-07 10:06:48 +07:00
commit d7de5f7121
6 changed files with 60 additions and 50 deletions
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docs/hardware/video.md
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@ -66,7 +66,8 @@ Mode 0 supports all four text backgrounds, with these features:
- Uncovered pixels use palette entry zero. Green swap runs after composition.
Mode 0 supports mosaic independently on each background. Rendering errors leave the output buffer unchanged.
Background tile or map fetches outside the first 64 KiB return diagnostics.
Background tile or map fetches use the 128 KiB VRAM window; offsets `0x18000..0x1ffff`
mirror `0x10000..0x17fff`. Addresses at or beyond `0x20000` return diagnostics.
This is not a complete picture processing unit (PPU).
Affine background snapshots support these configurations:
@ -94,7 +95,7 @@ Map sizes are 128×128, 256×256, 512×512, or 1024×1024 pixels.
BGCNT bit 13 selects wrapping or transparent edges in Modes 1 and 2. All bitmap modes ignore this bit.
Palette index zero is transparent in tiled backgrounds and Mode 4. Mode 3/5 black bitmap pixels remain opaque.
Background priorities, sprite composition, and green swap apply after sampling.
Map fetches beyond the supported 64 KiB background area still return diagnostics.
Map fetches use the same 128 KiB VRAM mirror. Addresses at or beyond `0x20000` return diagnostics.
**Timing limit:** debug snapshots use one set of programmed registers for the whole image.
Row capture instead samples from separate internal X/Y origins.