101 lines
7 KiB
Markdown
101 lines
7 KiB
Markdown
# Instruction fetches across memory-region boundaries
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## Finding
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The shared sampler already selected bus width from the new fetch address.
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However, `Memory::begin_cpu_access` rejected every Thumb sample whose upper address byte differed from the executing PC.
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It also selected the IWRAM latch from the executing PC rather than the actual fetch address.
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This prevented mapped boundary fetches from supplying an otherwise known bus value.
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The boundary extension removes that blanket restriction.
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Thumb unused-memory reads now use the bus observation at the newly fetched address P+4.
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ARM continues to use its fetched PC+8 word.
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This is a bounded entry-snapshot model, not a complete general-bus or cycle-accurate implementation.
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## Evidence and limits
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A boundary-focused web search revisited [nocash's open-bus discussion](https://www.ngemu.com/threads/gba-open-bus.170809/).
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It describes repeated Thumb halfwords, full BIOS/OAM words, and history-dependent IWRAM lanes.
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The search did not identify a dedicated physical-hardware boundary result for this implementation.
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Those formulas alone do not establish every boundary or timing case.
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The stronger implementation evidence is address-based bus dispatch:
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- [ares bus at 6f6786e0](https://github.com/ares-emulator/ares/blob/6f6786e04f0822a3475463df284f313ab8518d51/ares/gba/cpu/bus.cpp) selects the bus using the actual access address.
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It assigns the full result to `mdr` before selecting the addressed CPU halfword/byte.
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It does not reject a mapped fetch because the executing instruction occupies another region.
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- [ares memory helpers](https://github.com/ares-emulator/ares/blob/6f6786e04f0822a3475463df284f313ab8518d51/ares/gba/cpu/memory.cpp) return repeated halfwords from 16-bit regions and the complete local latch from IWRAM.
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- [ares OAM helper](https://github.com/ares-emulator/ares/blob/6f6786e04f0822a3475463df284f313ab8518d51/ares/gba/ppu/memory.cpp) returns the aligned word before CPU halfword selection.
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- [jgenesis bus at fab6e2cc](https://github.com/jsgroth/jgenesis/blob/fab6e2ccc60e492dd68b7f1e927b0829a6d80195/backend/gba-core/src/bus.rs) independently updates general open bus from access width and the addressed memory helper.
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Its `iwram_update_open_bus` drives the complete local IWRAM word after a partial-lane update.
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That revision marks OAM behavior as a TODO, so it is not independent confirmation of the full OAM rule.
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- [NanoBoyAdvance's loop/refill helpers](https://github.com/nba-emu/NanoBoyAdvance/blob/55b5cf0ae3d929582ac5bfd486558173502b8354/src/nba/src/arm/arm7tdmi.hh) place the new fetch before execution and refill target pairs in order.
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Its [open-bus helper](https://github.com/nba-emu/NanoBoyAdvance/blob/55b5cf0ae3d929582ac5bfd486558173502b8354/src/nba/src/bus/bus.cc) uses opcode-based approximations with a BIOS/OAM TODO.
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We do not treat that helper as an exact boundary oracle.
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These are source comparisons, not physical-hardware measurements or external-emulator differential runs.
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No external code, BIOS image, or test ROM was imported. All regression programs are original.
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## Supported behavior
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The instruction buffer and bus history consume the same captured new fetch.
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The consumer does not reread memory or replay observations from retained instructions.
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| Lookahead transition | Thumb unused-memory snapshot |
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| --- | --- |
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| Palette to VRAM | Newly fetched halfword repeated in both lanes |
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| VRAM to OAM | Full aligned OAM word |
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| OAM to mapped ROM | Newly fetched ROM halfword repeated in both lanes |
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| Mapped ROM page or wait-window boundary | Halfword from the normal mapped cartridge offset, repeated |
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| EWRAM to IWRAM | Persistent IWRAM word after the new fetch drives its addressed halfword |
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The existing ROM mapping applies across all three wait-state windows.
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A 16-bit fetch needs only its two mapped bytes, not an adjacent full word.
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This includes OAM-to-ROM crossings, despite the executing instruction still residing on a 32-bit bus.
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IWRAM does not acquire guessed lanes from adjacent memory.
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At P=`0x02fffffc`, only the low half has been fetched from IWRAM; the high half needs prior observed history.
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Without that history, an unused-memory load remains diagnostic.
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At P=`0x02fffffe`, a cold fill or target-pair refill can capture IWRAM's low half before the high lookahead fetch.
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The resulting complete local word is then available.
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A preceding CPU data access or successful DMA can also establish the required lanes.
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A refill can span two regions. Each actual IWRAM target sample updates only its addressed lanes.
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Later host writes do not replace those captured lanes; the target instruction's new lookahead updates its own lane normally.
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Diagnostic failures discard staged fetch/history changes and preserve the previous instruction buffer.
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A retry takes a new lookahead sample from current memory.
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## Preserved diagnostics and timing
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- Missing ROM bytes and unmapped instruction reads remain strict errors or deferred fetch diagnostics.
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- Unsupported I/O fetch observations do not borrow known IWRAM lanes or another region's bus value.
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- Unknown IWRAM lanes do not become zero or fabricated history.
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- An ordinary instruction or mapped data load does not require a complete unused-memory snapshot.
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- A branch can discard an unavailable sequential path without failing early.
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- Host reads, instruction fetches, and DMA do not inherit CPU unused-memory fallback.
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- Protected BIOS history remains separate and image-derived.
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- All data reads within an instruction still use one entry snapshot.
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The later [fetch-address timing extension](fetch-address-timing.md) charges non-refill code at the actual lookahead address.
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Refill summaries remain destination-based; the instruction-local access kinds and cold-start costs are still nominal.
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Per-access device updates, video-bus contention, general DMA handoff, and Game Pak prefetch remain incomplete.
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No boundary timing accuracy is claimed.
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## Original regression coverage
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Tests cover both Thumb alignments and cold/retained pipelines at mapped video/ROM boundaries.
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Load matrices check byte, halfword, word, signed and unaligned results without flag changes.
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Other tests cover block/stack writeback, CPU/DMA-seeded IWRAM lanes, unknown lanes, split refills, and failed-step retry.
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ROM tests span mapped page and wait-window boundaries with distinct wait settings.
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Short-ROM tests require exactly the fetched halfword.
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Unsupported I/O, missing ROM, discarded branch paths, strict host reads, and unchanged ARM snapshots remain covered.
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The mapped-crossing regressions failed under the old upper-address-byte guard, then passed with fetch-address selection.
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## Validation result
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Workspace and core/demo tests pass in debug and release on Darwin arm64.
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Formatting, lint checks, rustdoc, preparation tests, native ROM windows, and graphics smoke modes pass.
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Public ARM, Thumb, memory, and BIOS reports match the prior persistent-IWRAM reports exactly, including nominal cycle counts.
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Debug and release reports also match for the same fixture paths.
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These checks are regression results, not physical-hardware boundary or timing conformance tests.
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