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feat(memory): retain aligned Thumb BIOS prefetch words

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Snapshot the full word at (executing PC+4)&~3 for Thumb BIOS execution, using strict image reads and incoming instruction state. Keep ARM PC+8 open bus separate and unchanged. Commit snapshots only after successful steps; preserve diagnostic behavior for unavailable lookahead.

Add original regression coverage for both code alignments, load widths and lanes, signed and unaligned values, processor modes, sequential fetches, ARM/Thumb transitions, SWI, RAM-backed POP exits, BIOS boundaries, failure retention, and nominal device progression. Record source evidence and explicit pipeline/hardware-validation limits.

Darwin arm64 validation: 878 workspace tests and 809 core/demo tests in each of debug and release; 39 Python tests; native ROM window tests in both profiles; ten window/graphics smoke modes; fmt, clippy, and rustdoc. Public ARM, Thumb, memory, and BIOS reports match prior passing reports and match between profiles. No physical-hardware validation claimed.
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0xrsydn 2026-10-06 00:57:49 +07:00
commit cb4670c245
12 changed files with 427 additions and 25 deletions
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docs/references.md
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@ -95,6 +95,15 @@ BIOS protected-read root-cause research:
See [the root-cause analysis and implementation decision](research/bios-readback.md).
Thumb BIOS fetch research:
- [NanoBoyAdvance bus at revision 55b5cf0a](https://github.com/nba-emu/NanoBoyAdvance/blob/55b5cf0ae3d929582ac5bfd486558173502b8354/src/nba/src/bus/bus.cc), for full aligned BIOS latch updates on halfword reads.
- [NanoBoyAdvance ARM7TDMI loop at the same revision](https://github.com/nba-emu/NanoBoyAdvance/blob/55b5cf0ae3d929582ac5bfd486558173502b8354/src/nba/src/arm/arm7tdmi.hh), for Thumb PC+4 fetch order.
- [mGBA CPU loop](https://github.com/mgba-emu/mgba/blob/master/src/arm/arm.c), compared with its region-exit retention path; not evidence of exact full-word Thumb BIOS readback.
- GBATEK's BIOS/OAM Thumb lane table above supports aligned-word sampling, but describes general unused-memory reads.
See [Thumb BIOS prefetch snapshots](research/thumb-bios-prefetch.md) for the inference, implementation limits, and original regressions.
Use [GBATEK](https://problemkaputt.de/gbatek.htm) and ARM7TDMI documentation for further hardware work.
Only use game ROMs that you may lawfully use. Do not commit game ROMs, BIOS files, or game assets.
The ignore file excludes `roms/`, common game ROM extensions, and save files.