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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/testing.md
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@ -219,9 +219,12 @@ Status tests cover all modes, flag combinations, interrupt masks, field selectio
Exception tests cover vectors, ARM/Thumb return addresses, nested interrupts, mask priority, and return-state restoration.
User-bank transfers and failed returns are checked for unintended changes to active and hidden registers.
Integration tests execute original vector handlers and check optional BIOS mapping boundaries and write protection.
Original BIOS-access regressions check retained ARM PC+8 data for ARM/Thumb callers, all processor modes, load widths, lanes, and rotation.
Original BIOS-access regressions check retained ARM PC+8 and aligned Thumb PC+4 words for ARM/Thumb callers.
They cover all processor modes, load widths, lanes, rotation, both Thumb code alignments, and distinct halfwords.
They verify direct reads while executing inside BIOS, re-entry, SWI/IRQ returns, host-inspection isolation, and original-boot provenance.
Unknown history, Thumb BIOS execution, missing lookahead, and diagnostic-step retention have explicit checks.
Thumb tests cover sequential snapshots, instruction-state transitions, SWI entry, and RAM-backed POP exits.
Unknown history, last valid fetch words, missing lookahead, and diagnostic-step retention have explicit checks.
General Thumb unused-memory reads remain diagnostics; supporting Thumb BIOS snapshots does not enable general open bus.
Protected loads keep normal nominal costs and advance devices only through machine stepping.
Firmware readback regressions separately verify documented boot, SoftReset, returning SWI, IRQ callback, and IRQ return words.
They execute ARM/Thumb loads across widths and lanes, check caller state, and trace actual exits to their image data.