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feat(memory): support ARM unused-memory open-bus reads

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Snapshot mapped PC+8 bytes for ARM data reads in the documented unused ranges. Preserve strict host/fetch/Thumb/DMA diagnostics, missing-ROM errors, existing write rules, and nominal timing. Add original regressions and document the bounded model.

The unmodified pinned jsmolka ARM ROM now reaches its verified checkpoint with r12=0 in debug and release: 1344 steps and 11257 nominal cycles. No ROM, manifest, or assertion changes.

Validated on Darwin arm64: 842 workspace tests in debug and release; 773 core/demo tests separately; Python adapter tests; native ROM windows; all ten demo window smoke tests; fmt, clippy, and rustdoc.
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0xrsydn 2026-10-05 20:25:18 +07:00
commit a9493d4cc6
11 changed files with 418 additions and 26 deletions
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docs/references.md
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@ -50,7 +50,9 @@ Hardware references used for the core:
- [NanoBoyAdvance keypad implementation](https://github.com/nba-emu/NanoBoyAdvance/blob/master/src/nba/src/hw/keypad/keypad.cc), compared for control access widths; its AND/retrigger behavior differs from the chosen polling model.
- [mGBA keypad timing issue](https://github.com/mgba-emu/mgba/issues/2490), for frontend frame-based input timing limits.
- [Tonc hardware interrupts](https://gbadev.net/tonc/interrupts.html), for keypad source enable and IF/IE bit 12.
- [GBATEK memory mirrors and video byte writes](https://problemkaputt.de/gbatek-gba-unpredictable-things.htm).
- [GBATEK memory mirrors, video byte writes, and unused-memory reads](https://problemkaputt.de/gbatek-gba-unpredictable-things.htm), for unused address ranges, ARM PC+8 open bus, byte lanes, and the distinct Thumb region/history rules.
- [nocash GBA open-bus discussion](https://www.ngemu.com/threads/gba-open-bus.170809/), for the distinction between ARM prefetch and region-dependent Thumb values.
- [mGBA memory implementation](https://github.com/mgba-emu/mgba/blob/master/src/gba/memory.c), reviewed for `GBALoadBad`, ARM prefetch sourcing, byte/halfword lane selection, and separate DMA/history behavior.
- [GBATEK GBA memory map and bus widths](https://problemkaputt.de/gbatek-gba-memory-map.htm).
- [GBATEK GBA DMA transfers](https://problemkaputt.de/gbatek-gba-dma-transfers.htm).
- [mGBA DMA implementation](https://github.com/mgba-emu/mgba/blob/master/src/gba/dma.c), used to cross-check masks, repeat behavior, and Game Pak source increments.