chore(release): cut v2.4.8 "Palimpsest" — read-modify-write, and a gate that cannot see its own subject - #444
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that cannot see its own subject
A palimpsest is a surface written over an earlier text, where the earlier
writing is still there underneath. That is literally what a 6502
read-modify-write does to a memory location: it writes the old byte back
before it writes the new one, so the address is written twice and the
first write carries what was already there.
The group lands in SystemVerilog in the sibling repository
(RustyNES_MiSTer@0cb628f): ASL, LSR, ROL, ROR, INC, DEC, across the
accumulator form and four memory modes -- 28 opcodes.
opgroup1 147 unchanged
opgroup2 140 unchanged
opgroup3 286 unchanged
opgroup4 179 unchanged
opgroup5 358 NEW
-------------
1110
## The release is named for something rung 1 cannot verify
Two mutations, from a verified-clean baseline:
the dummy write is skipped entirely NOT CAUGHT
the dummy write emits the modified value NOT CAUGHT
Neither changes a register, a flag, the final memory contents or the
cycle count, and CpuBootTrace carries exactly those. An implementation
performing one write instead of two is identical on every field this rung
compares.
It is not harmless on hardware: that middle write reaches mapper
registers and I/O, and writing twice to $2007 is not writing once.
The RTL implements it. The claim rests on the code and on
RustyNES_MiSTer/docs/rung1-6502.md, NOT on a green gate, and this release
says so in the CHANGELOG, the notes, docs/mister.md and VERSION-PLAN
rather than letting a green rung imply coverage it does not have. A rung
that quietly appeared to cover its own namesake would be the exact
failure this programme exists to catch.
The other eight mutations were caught.
## v2.4.9 now carries rung 2's bus half, and that is evidence-based
Folded in beside the undocumented opcodes rather than left to v2.5.0,
because the bus half needs NO new RTL -- and rather than plan that on an
estimate, it was tested.
`Observable` already exists on both sides with a byte-identical wire
encoding (the oracle's checkpoint.rs, the DUT's tb/checkpoint.h), both
selftested; the oracle already emits it as `.obs.bin`; and cpu6502
already exposes its whole bus. Wiring a writer took about forty lines and
immediately produced SEVEN divergences across 793 cycles on a program
rung 1 scores 358/358 -- which is the plan's own justification for rung 2
arriving on first contact: a 6502 can pass nestest with entirely wrong
dummy reads.
Two causes, needing different fixes: the testbench zeroes RAM the oracle
seeds, so every dummy read of unwritten memory diverges; and there is a
genuine per-cycle access difference on the absolute-indexed store's
dummy-read cycle, where instruction boundaries still agree.
Two alignment facts recorded now, both of the family that has bitten this
programme once already: the DUT emits the reset sequence while the
oracle's per-cycle trace starts at cycle 8, so alignment is by cycle
number and never by index; and the oracle's Observable.pc reads zero in
all 793 records, so pc cannot be compared as-is.
Interrupts stay at v2.5.0 -- cpu6502 has no nmi_n/irq_n pins and no BRK,
so nmi_line, both IRQ samples and put_cycle will be inert at v2.4.9, and
the comparison must DECLARE that it skips them rather than silently
comparing four constants against the oracle's real values.
## The release tooling's own fix is confirmed working
v2.4.7 shipped a run-on into six documents at once, because bump_release
inserted `--lead` verbatim and every anchor concatenates it onto the
demoted history. `terminate()` landed in that release; this is its first
real use, and the anchors now read "cannot see its own subject. Built
on", with the full stop present.
The current-tag rule added in v2.4.7 also fired again on this cut:
to-dos/ROADMAP.md:62 calls v2.4.7 the current tag,
but the workspace is at 2.4.8
Second consecutive release it has caught the drift it was built for.
## Gates
cargo fmt --all --check clean
cargo clippy --workspace --all-targets -D warnings 0 errors
cargo test --workspace 2233 passed, 0 suites failed
release_anchor_audit 8 passed
release_state_prose_audit 8 passed
release_notes_render_audit 2 passed
libretro_info_audit 3 passed
No crate under rustynes-{cpu,ppu,apu,mappers,core} changes, so
AccuracyCoin 141/141 (100.00%, RAM decoder) and nestest 0-diff hold BY
CONSTRUCTION rather than needing re-verification.
The libretro .info display_version moves to v2.4.8 in this repository's
copy only. Per the cadence rule an upstream sync happens on vX.Y.0
boundaries, so the next is v2.5.0; a licence change would override that,
and this is not one.
Written from public documentation only -- the NESdev wiki's 6502
instruction, addressing-mode and cycle-time pages. No reference NES core
was opened; none is present in the tree.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014qfTKi2M3swo7qnwvYCkDj
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Important
This repository does not receive automatic reviews because it has fewer than 10 stars. ⚙️ Run configurationConfiguration used: Path: .coderabbit.yaml Review profile: ASSERTIVE Plan: Pro Plus Run ID: 📝 WalkthroughWalkthroughRustyNES advances from v2.4.7 to v2.4.8 “Palimpsest.” The release materials document 6502 read-modify-write coverage, mutation-harness corrections, verification results, known bus-level limits, and planned v2.4.9 work. ChangesRelease update
Estimated code review effort: 2 (Simple) | ~10 minutes Merge Risk: 🟡 Moderate · up to The release documentation currently gives conflicting information about the current version, coverage status, and planned work, which could mislead users and maintainers. Merge should wait for these documentation corrections. 🚥 Pre-merge checks | ✅ 9✅ Passed checks (9 passed)
✨ Finishing Touches 💡 1🛠️ Fix failing CI checks 💡
🧪 Generate unit tests (beta)
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Antigravity review (Gemini via Ultra)Bumps project versions to v2.4.8 "Palimpsest" and updates documentation to reflect the latest testing milestones in the sibling MiSTer repository. Blocking issuesNone found. Suggestions
Nitpicks
Automated first-pass review by Earlier review rounds (newest first)Round reviewed at 2026-08-23 04:05 UTCAntigravity review (Gemini via Ultra)This PR performs a trivial release cut for v2.4.8 "Palimpsest", bumping workspace version numbers and updating project documentation and changelogs. Blocking issuesNone found. SuggestionsNone. Nitpicks
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Actionable comments posted: 4
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Inline comments:
In `@docs/mister.md`:
- Around line 512-514: Update the later “four ROMs” reference in the
documentation to “five ROMs,” keeping the surrounding statement about hand-built
opcode groups unchanged and consistent with the five-ROM status described
elsewhere.
In `@docs/STATUS.md`:
- Line 3: Update the release status entry anchored by “Current release: v2.4.8”
to retain v2.4.8 as current and complete its record with the 1,110-record,
five-ROM coverage, undetectable skipped or modified-value dummy writes, and the
boundary between verified results and planned v2.4.9/v2.5.0 work. Remove
later-release claims, including the v2.5.0 entry and v2.6.0 reference.
In `@ROADMAP.md`:
- Line 5: Update the ROADMAP header’s “Last Updated” value from 2026-07-08 to
2026-08-23, leaving the surrounding historical snapshot content unchanged.
In `@to-dos/ROADMAP.md`:
- Around line 58-62: Update the release-line description to replace “the current
tag” with “the current release,” reflecting that the repository has no version
tags while preserving the surrounding version history.
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.github/release-notes/v2.4.8.mdAGENTS.mdARCHITECTURE.mdCHANGELOG.mdCargo.tomlOVERVIEW.mdREADME.mdROADMAP.mdSECURITY.mdSUPPORT.mdVERSION-PLAN.mdcrates/rustynes-cosim/Cargo.tomlcrates/rustynes-libretro/rustynes_libretro.infodocs/STATUS.mddocs/mister.mdto-dos/ROADMAP.mdto-dos/plans/v2.5.0-fabric-plan.md
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## Applied
**docs/mister.md** still said "the four ROMs are hand-built opcode groups"
eighty lines below a paragraph this release changed to five. Corrected.
Exactly the drift the reviewer's own path instruction names: docs are the
spec here, so a count that contradicts the count above it is a defect.
**ROADMAP.md** carried `Last Updated: 2026-07-08` above a status block
dated 2026-08-23. Corrected.
**docs/STATUS.md** gains rung 1's actual record, which it did not have:
five ROMs, 1110 records, eight mutations caught -- and, stated rather
than implied, what the rung does NOT verify. The double write changes no
register, flag, final memory content or cycle count, so both mutations
against it come back NOT CAUGHT; that is a rung-2 property, scoped to
v2.4.9 with the undocumented opcodes, while the interrupt half stays at
v2.5.0 because it needs pins cpu6502 does not have.
## Refuted, and recorded rather than skipped
The same finding also asked to "remove later-release claims, including
the v2.5.0 release entry and v2.6.0 reference" from docs/STATUS.md.
Those are not release claims. They sit inside a section headed **Engine
lineage**, whose own preamble says it "preserves the engine-lineage
milestone history", and they document the upstream accuracy programme
whose v2.x milestones shipped as RustyNES **v1.0.0** in 2026-06-13.
AGENTS.md carries a standing rule about exactly this confusion -- two
distinct "v2.0"s exist and must not be conflated -- and deleting those
entries would destroy documented history while reintroducing the
ambiguity the rule exists to prevent.
Checked before acting rather than after: the entries are inside the
lineage section, and their dates (2026-06-11) precede RustyNES v1.0.0.
The reviewer's other two findings on this PR were both correct, which is
precisely what makes a third easy to wave through.
## Also refuted: the Antigravity nitpick
It suggested adding a `[2.4.8]` diff link "if the project uses Markdown
link references for versions at the bottom of the file". It does not --
`grep -cE '^\[[0-9]+\.[0-9]+\.[0-9]+\]:' CHANGELOG.md` returns 0. The
condition the nitpick is guarded on is false, so there is nothing to add.
Antigravity reported no blocking issues and no suggestions otherwise.
Gates: all four release audits green (8 / 8 / 2 / 3). No crate under
rustynes-{cpu,ppu,apu,mappers,core} is touched.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014qfTKi2M3swo7qnwvYCkDj
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Cuts v2.4.8 "Palimpsest" — read-modify-write, and a gate that cannot see its own subject.
A palimpsest is a surface written over an earlier text that is still there underneath, which is literally what a 6502 read-modify-write does to a memory location: it writes the old byte back before the new one.
The group lands in SystemVerilog in the sibling repository (
RustyNES_MiSTer@0cb628f) —ASL,LSR,ROL,ROR,INC,DECacross the accumulator form and four memory modes, 28 opcodes. Rung 1 now stands at five ROMs, 1110 records (147 / 140 / 286 / 179 / 358), matching the oracle on all seven CPU fields.The release is named for something rung 1 cannot verify
Neither changes a register, flag, final memory content, or cycle count — and
CpuBootTracecarries exactly those. The RTL implements the double write; the claim rests on the code and the docs, not on a green gate, and this release says so in four places rather than letting a green rung imply coverage it does not have. The other eight mutations were caught.The mutation harness had been measuring against its own mutants
It captured its pristine copy at source time and restored at the start of each run, so re-sourcing promoted the last mutant to baseline. Two were silently live in the tree while later results were measured against them, all reported CAUGHT for free — and the gate's own first PASS had been against a stale binary.
tb/mutate.shcaptures once into a file it refuses to overwrite, requires the baseline to pass first, reports three outcomes, and restores on exit via a trap. Third distinct route to this failure in the project's history.v2.4.9 now carries rung 2's bus half — and that was tested, not estimated
Observablealready exists on both sides with a byte-identical encoding, the oracle already emits.obs.bin, andcpu6502already exposes its whole bus. Wiring a writer took ~40 lines and produced 7 divergences across 793 cycles on a program rung 1 scores 358/358 — the plan's own justification for rung 2, arriving on first contact. Interrupts stay at v2.5.0 (nonmi_n/irq_npins, noBRK).Release tooling
terminate()from v2.4.7 is confirmed working — the anchors read "cannot see its own subject**.** Built on", where last release the full stop was missing across six files. The current-tag rule fired again on this cut, catchingto-dos/ROADMAP.md:62. Second consecutive release it has earned its place.Gates
fmtclean ·clippy --workspace --all-targets -D warnings0 errors · 2233 workspace tests passed, 0 suites failed · all four release audits green (8 / 8 / 2 / 3).No chip crate changes, so AccuracyCoin 141/141 and nestest 0-diff hold by construction.
Written from public documentation only. No reference NES core was opened; none is present in the tree.
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