96 lines
6.6 KiB
Markdown
96 lines
6.6 KiB
Markdown
# Persistent ARM instruction buffering
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## Evidence and scope
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The [ARM7TDMI instruction-pipeline description](https://developer.arm.com/documentation/dvi0027/b/arm7tdmi/instruction-pipeline) identifies fetch, decode, and execute stages.
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The [ARM7TDMI technical reference manual](https://documentation-service.arm.com/static/5e8e1323fd977155116a3129) describes simultaneous execution, decoding, and fetching.
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For ARM state, the executing instruction's visible PC is eight bytes ahead.
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The [branch cycle sequence](https://support.arm.com/documentation/ddi0029/g/instruction-cycle-timings/branch-and-branch-with-link) fetches the destination, then destination plus instruction width.
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[NanoBoyAdvance at 55b5cf0a](https://github.com/nba-emu/NanoBoyAdvance/blob/55b5cf0ae3d929582ac5bfd486558173502b8354/src/nba/src/arm/arm7tdmi.hh) makes the retained-word ordering explicit:
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- `ReloadPipeline32` reads target T, then T+4.
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- `Run` takes the current instruction from its retained first slot.
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- It shifts the second slot forward and reads the new fetch word before executing the current instruction.
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- Taken PC writes reload the target pair instead of retaining the sequential continuation.
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The resulting ARM sequence fetches P+8 before the instruction at P changes memory.
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Consequently, a store to P+4 or P+8 cannot change those already fetched instructions without a refill.
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A later store-visible word, such as P+12, is fetched on a subsequent step.
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DMA writes likewise do not replace words already inside the CPU buffer.
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These are source-backed instruction-buffer rules, not a claim of complete ARM7 bus timing.
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No external source code or test ROM was imported. No physical-hardware or external-emulator differential run was performed.
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## Implementation
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`cpu/pipeline.rs` owns a private two-slot continuation with its expected execution PC and instruction state.
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The later [Thumb extension](thumb-instruction-buffer.md) uses this same structure for halfwords.
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Each slot retains either a mapped word or its strict fetch error.
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`Cpu` owns and clones this state; full CPU equality includes it.
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At a cold or discontinuous ARM entry, fetch the current word and sample P+4/P+8.
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At a matching continuation, use its first word for execution and retain its second word for the next step.
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Sample P+8 before instruction effects. Keep that prospective continuation local until execution succeeds.
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A failed current fetch or instruction preserves the previous buffer exactly.
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Successful non-refill ARM execution commits the next pair.
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A successful refill into ARM instead samples T and T+4 after data transfers and status restoration.
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This includes ARM/Thumb BX, ARM PC writes and loads, taken branches, exception returns, and SWI vectors.
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A refill to the sequential address still discards the previous sequential pair.
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Untaken branches retain the pair. A transition to Thumb now replaces it with a Thumb target pair.
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Machine IRQ entry captures its vector pair immediately after exception entry.
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The standalone exception APIs have no Memory argument, so they invalidate the old pair and leave the vector cold.
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This distinction is explicit rather than retaining unrelated words at a coincident PC.
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`Memory::fetch_instruction` reads aligned mapped instructions without CPU data context, timing traces, or bus-history changes.
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Normal memory mirrors apply. Unmapped, truncated, and misaligned fetches retain their existing errors.
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No BIOS-protected data value or general open-bus value can become a synthetic instruction through this path.
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## Diagnostics and debugger behavior
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An unavailable lookahead word is a deferred fetch diagnostic, not an immediate failure of the current instruction.
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A valid branch can discard an unavailable sequential word. An unavailable target does not fail the branch early.
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Execution reports a buffered error only when that slot becomes current.
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Unsupported instruction encodings remain words until decode; they also do not fail preceding instructions.
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The atomic-error policy preserves the entire old CPU buffer on failure.
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A host repair of a buffered unsupported instruction does not silently change that retained word.
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`Cpu::invalidate_pipeline()` explicitly clears the buffer for debugger repair or attachment to different memory.
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It changes no architectural registers, memory, clocks, or separate BIOS/IWRAM bus history.
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Ordinary CPU/DMA stores and host inspection never invoke it automatically.
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A copied CPU retains its own instruction-buffer state. A copied CPU attached to unrelated memory must be invalidated explicitly.
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Existing successful-instruction tests now distinguish architectural equality from full-state equality.
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Failed-instruction tests continue to assert full-state preservation.
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## Remaining limits
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This implements persistent ARM instruction words, not a complete cycle-by-cycle pipeline.
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Cold fill and target-pair samples do not add nominal cycles or data accesses.
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The existing destination-based refill summaries and one-shot DMA resume timing remain unchanged.
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The current instruction address still selects nominal code cost rather than a separately scheduled fetch address.
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ARM PC+8 open bus and protected BIOS readback remain separate snapshots.
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The [shared fetch sample](shared-fetch-samples.md) supplies the instruction-buffer entry and supported bus snapshots without duplicate reads.
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No unified per-access bus model exists yet.
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The target samples do not establish complete ARM IWRAM or BIOS bus history.
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Instruction-state-changing compare quirks, Game Pak prefetch, and DMA arbitration remain incomplete.
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DMA still runs between whole instructions, not during their data accesses or internal cycles.
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## Original regression coverage
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Original programs test stores to the two prefetched words and the next unfetched word.
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Other tests cover host/DMA writes, taken and untaken branches, PC writes to fallthrough, target-pair sample order, and ARM/Thumb transitions.
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SWI/IRQ entry and status-restoring returns verify refill integration.
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Further tests check deferred fetch errors, discarded paths, failed-instruction rollback, explicit invalidation, CPU cloning, mirrors, and alignment.
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Timed and untimed execution retain identical buffers and nominal costs.
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The self-modifying cases failed under the previous direct-fetch interpreter before implementation.
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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 previous passing reports exactly, including nominal cycle counts.
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These suites remain regression checks, not complete instruction-pipeline or hardware-timing conformance tests.
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