Provenance, Derivation, and License¶

This document is the honest record of where RustyNES's code comes from. It exists because earlier versions of this file, of NOTICE, and of the in-source comments got the provenance wrong — they described code that was ported from other emulators as "oracle cross-checks" and licensed the whole project under a permissive MIT/Apache license it was not entitled to use. A NESdev community review (thanks to Fiskbit and the NESdev staff) was correct on the substance, and this document, the relicense to GPLv3, and the attribution below are the correction.

The short version:

RustyNES incorporates and is derived from code from GPL-licensed emulators, principally Mesen2 (GPL-3.0-or-later) and, for several mappers and the FDS drive model, puNES / FCEUX / Nestopia (GPL-2.0-or-later). This is not oracle use; it is derivation. The original source comments said so ("Faithful port of Mesen2's ProcessSpriteEvaluation", "Ported bit-for-bit from puNES JV001.c", etc.) before a v2.2.5 edit reworded them.
RustyNES is therefore a derivative work and is licensed GPL-3.0-or-later. The earlier "MIT OR Apache-2.0" dual license and the "no GPL code is incorporated" claim were incorrect and are withdrawn.
Credit is given below and in NOTICE, per subsystem, to the projects the code was derived from.
Some parts of RustyNES are genuinely original — the crate topology, the determinism contract, the CI accuracy-honesty gates, the measure-first performance record. Those claims are kept, but they never justified calling the whole project "not a port," and they do not exempt the derived code from the GPL.

A note on AI assistance. RustyNES is heavily AI-assisted software. That does not change any of the above: code an LLM emits by reproducing GPL source is still GPL-derived, and the human directing the tool is responsible for what lands in the tree. "Laundering others' code through an AI" — the reviewer's phrase — is exactly the failure mode this document exists to correct, not excuse.

Authoritative companions: NOTICE (the legal attribution file), docs/adr/0036-relicense-gplv3-derivative-work.md (the decision record for this relicense), CHANGELOG.md, and tests/roms/LICENSES.md (test-ROM provenance).

1. What is derived from GPL-licensed emulators¶

The table below is the honest derivation record, rebuilt from the in-source comments as they stood before the v2.2.5 rewording (recoverable from the git history of that change) and cross-checked against the upstream sources at the time (the local reference-emulator clone, since removed from the repo and the agent's reach per the reference firewall — see docs/ai-emulator-provenance-guardrails.md; the citations name each upstream project + file so the record stands without the local clone). Each row is code in RustyNES that was ported, adapted, or closely modeled from the named GPL emulator — not merely behavior observed and reimplemented from documentation. "Source license" is the license the upstream file carries; because every upstream here is GPL-2.0-or-later or GPL-3.0-or-later, all of it is compatible with distributing the combined work under GPL-3.0-or-later.

RustyNES file	Derived from	Upstream source	Upstream license
`crates/rustynes-cpu/src/cpu.rs`	Mesen2	`SyaSxaAxa` unstable-store opcodes, `Core/NES/NesCpu.h`	GPL-3.0-or-later
`crates/rustynes-ppu/src/ppu.rs`	Mesen2	`ProcessSpriteEvaluation` (`NesPpu.cpp:1015-1141`), `ReadSpriteRam`, the OAM-data-bus / sprite-evaluation read paths	GPL-3.0-or-later
`crates/rustynes-ppu/src/palette_gen.rs`	Bisqwit; ares	Bisqwit NES palette method; ares `fc/ppu/color.cpp` integration	Bisqwit (see §6); ares BSD-2/Apache-2.0
`crates/rustynes-apu/src/blip.rs`	blip_buf (Blargg)	band-limited synthesis (`blip_buf`)	LGPL-2.1-or-later
`crates/rustynes-apu/src/opll.rs`	emu2413 (upstream MIT; Mesen2 vendors it)	`emu2413.{h,cpp}`	MIT
`crates/rustynes-frontend/src/ntsc_bisqwit.rs`	Bisqwit; Mesen2	Bisqwit `nes_ntsc`-style composite model as implemented by Mesen2's `BisqwitNtscFilter`; numeric tables ported verbatim	GPL-3.0-or-later (Mesen2)
`crates/rustynes-gfx-shaders/src/crt_stack.rs`, `src/lib.rs`	CRT-Royale, crt-guest-advanced, Sony Megatron	single-pass WGSL reimplementations of those shaders (see §6)	GPL-2.0-or-later / permissive
`crates/rustynes-mappers/src/m016_bandai_fcg.rs`	Mesen2	`Eeprom24C01` / `Eeprom24C02`, `Core/NES/Mappers/Bandai/`	GPL-3.0-or-later
`crates/rustynes-mappers/src/m035_jy_asic.rs`	Mesen2	`JyCompany` register decode, `InvertPrgBits`	GPL-3.0-or-later
`crates/rustynes-mappers/src/m069_sunsoft_fme7.rs`	Mesen2 / Nestopia	Sunsoft 5B audio + FME-7	GPL-3.0-or-later / GPL-2.0-or-later
`crates/rustynes-mappers/src/m176_bmc_fk23c.rs`	Mesen2	`Waixing/Fk23C.h`, `Mmc3Variants/MMC3_Coolboy.h`	GPL-3.0-or-later
`crates/rustynes-mappers/src/m268_bmc_coolboy.rs`	Mesen2 / FCEUX	`Mmc3Variants/MMC3_Coolboy.h` banking	GPL-3.0-or-later / GPL-2.0-or-later
`crates/rustynes-mappers/src/m513_sachen_9602.rs`	Mesen2	`Sachen/Sachen9602.h`, `Txc/TxcChip.h`	GPL-3.0-or-later
`crates/rustynes-mappers/src/mmc3_clones.rs`	Mesen2	`Waixing/Mapper253.h`, `Sachen/Sachen8259.h`, `InvertPrgBits`, MMC3 variants	GPL-3.0-or-later
`crates/rustynes-mappers/src/multicart_discrete.rs`	Mesen2	`Ntdec/Mapper221.h`, `Txc/Bmc11160.h`	GPL-3.0-or-later
`crates/rustynes-mappers/src/ntdec.rs`	Mesen2	NTDEC boards, `Txc/Bmc11160.h`	GPL-3.0-or-later
`crates/rustynes-mappers/src/sachen_discrete.rs`	Mesen2	`Sachen/Sachen8259.h`, `Txc/TxcChip.h`	GPL-3.0-or-later
`crates/rustynes-mappers/src/kaiser.rs`	Mesen2	Kaiser boards, `Waixing/Mapper253.h`	GPL-3.0-or-later
`crates/rustynes-mappers/src/fds.rs`	puNES	`fds.c` per-CRC drive-timing table	GPL-2.0-or-later
`crates/rustynes-mappers/src/lib.rs` (mapper 147 / JV001, UNIF dispatch)	puNES; FCEUX	`JV001.c` / `mapper_147.c` (ported bit-for-bit); UNIF board handling	GPL-2.0-or-later
`crates/rustynes-mappers/src/unif.rs`	Mesen2; FCEUX	`UnifLoader.cpp` + `unif.cpp` board-name tables	GPL-3.0-or-later / GPL-2.0-or-later
`crates/rustynes-frontend/src/debugger/source_map.rs`	Mesen2	`DbgImporter` / `NesDbgImporter`	GPL-3.0-or-later
`crates/rustynes-test-harness/src/bin/pgo_trainer.rs`	Mesen2	`PGOHelper` corpus-sweep harness	GPL-3.0-or-later

This list is maintained as the derivation is audited further; if additional GPL-derived code is found, it is added here and in NOTICE rather than reworded away. Beyond the files above, the reviewer specifically noted that bugs, constants, variable names, and code ordering can carry provenance even without a comment — where that is true of any code in this tree, it is GPL-derived and covered by the GPL-3.0-or-later license of the whole.

2. License: GPL-3.0-or-later, because RustyNES is a derivative work¶

RustyNES is licensed GPL-3.0-or-later (LICENSE). This is not a preference; it is a requirement that follows from §1. Incorporating GPL-3.0 (Mesen2) and GPL-2.0-or-later (puNES/FCEUX/Nestopia, all granting "or any later version") code makes the combined work a derivative that can only be distributed under the GPL. GPL-3.0-or-later is the correct expression: the GPL-2.0-or-later material upgrades to v3, and Mesen2/higan are GPL-3.0-or-later.

The earlier MIT OR Apache-2.0 dual license was wrong for this codebase and is withdrawn. The LICENSE-MIT and LICENSE-APACHE files are removed. Source released under the old license in prior tagged releases remains under whatever terms accompanied it at the time — that history cannot be retroactively changed — but the current tree, and every release from v2.2.9 onward, is GPL-3.0-or-later.

Permissively-licensed components that RustyNES genuinely incorporates (emu2413/MIT, TriCNES/MIT, rcheevos/MIT, blip_buf/LGPL-2.1-or-later, bundled fonts) keep their own licenses; each is GPL-compatible and is attributed in NOTICE. Combining them under the project's GPL-3.0-or-later umbrella is what those licenses permit.

The cargo-deny license gate (deny.toml) allows GPL-3.0-or-later for the project's own crates alongside the permissive licenses of the dependency graph.

3. The reference emulators still consulted as oracles¶

Separately from the derived code in §1, RustyNES also does use emulators as behavioral oracles — running them to observe documented hardware behavior when a test ROM is ambiguous, without deriving code. The distinction is real, but the earlier documents abused it by filing genuine ports under this heading. The honest position is: some use was oracle-only, and some was derivation (§1), and this project previously mislabeled the second as the first.

Reference emulator	License	Documented use
Mesen2 / MesenCE	GPL-3.0-or-later	Derivation (§1) and oracle
puNES	GPL-2.0-or-later	Derivation (§1) and oracle
FCEUX	GPL-2.0-or-later	Derivation (§1) and oracle
Nestopia UE	GPL-2.0-or-later	Derivation (§1, FME-7/5B) and oracle
GeraNES	GPL-3.0-only	Reference / cross-check: source consulted to confirm publicly-documented behavior; no code copied (see note)
higan	GPL-3.0-or-later	Scheduler-structure reference / oracle
ares	BSD-2-Clause / Apache-2.0	Palette-integration reference (§1) / oracle
TriCNES	MIT	Incorporated (§5) and timing-calibration reference (§4)

GeraNES specifically (honest note). Some in-source comments previously cited GeraNES source files, functions, and even quoted a line (e.g. a mapper bus-conflict data &= readPrg(addr) in Mapper093.h) — which means GeraNES's source was consulted as a cross-reference during development, not purely black-box observation. Every behavior so cross-referenced is independently documented: the nesdev wiki for the mapper bus-conflict / address-decode masks (the affected comments already cite INES_Mapper_089/093), and RetroArch's public #pragma parameter specification for the shader-parameter UI. The RustyNES implementations are independent Rust (value & self.read_prg(addr), not the C++ line). On the evidence below, the maintainers' assessment is that only unprotectable, publicly-documented behavior was relied on rather than GeraNES's copyrightable expression, and therefore that no GPL-3.0-only code is incorporated and the project's GPL-3.0-or-later grant is unconstrained. That is a position supported by the record, not a certification — see the caveat at the end of this section. The comments have been reworded to state the relationship accurately and to drop the now-removed source-path citations (the local reference-emulator tree was deleted from disk; see the reference firewall in AGENTS.md).

This was verified two-sided at the maintainer's direction: the GeraNES source (gracioni/GeraNES) for Mapper089.h / Mapper093.h was compared against RustyNES's m089_sunsoft2.rs / m093_sunsoft3r.rs and against the nesdev wiki (INES_Mapper_089 [CPPP MCCC], INES_Mapper_093 [.PPP ...E], both marked "BUS CONFLICTS"). The operations that coincide with GeraNES — the bus-conflict written & rom_byte mask and the bit-field extractions ((v >> 4) & 0x07, etc.) — are exactly the nesdev-documented register layouts, i.e. the single correct expression of the documented hardware (merger doctrine). No arbitrary, non-hardware-dictated choice coincides (identifiers, decomposition, and idiom are independent Rust), which is the signature distinguishing documented-fact convergence from copying.

Status, not a verdict. The paragraphs above record what was examined and what was found; they are deliberately not a clearance. This project's provenance rules (docs/ai-emulator-provenance-guardrails.md) forbid self-certifying license cleanliness, and for good reason: the original provenance failure was caught by an outside NESdev reviewer, not by the project's own tooling or by the AI assistance that produced the code. Two specific residual uncertainties are worth naming. First, whether a given coincidence is "the single correct expression of a documented fact" is ultimately a legal judgement about merger and scenes-a-faire, not something a code comparison settles. Second, the review covered the files the in-source comments pointed at; it does not prove the absence of consultation elsewhere. This assessment is therefore offered for human and expert review, and a qualified reviewer who disagrees should be treated as authoritative over it. The remedy in that case is the same one v2.2.9 already applied once — relicense and attribute, never quietly reword.

Because the license of the derived-from GPL code governs regardless of how any one file was used, the whole project is GPL-3.0-or-later; the oracle/derivation distinction affects attribution, not the license.

4. What is genuinely RustyNES's own¶

These claims are true and are kept — but they describe original architecture and method built around the incorporated code, not a clean-room emulator. Owning the derivation in §1 does not require pretending the surrounding system is not real work; it requires not overstating it into a "not a port" claim, which is what the earlier document did.

The crate topology and ownership model. The strictly one-directional rustynes-{cpu,ppu,apu,mappers,core} graph, the Bus-owns-all-mutable-state design, and the narrow per-chip trait boundaries are RustyNES's own structure (docs/architecture.md).
The determinism contract and the #![no_std] core. Same seed + ROM + input ⇒ bit-identical framebuffer and audio, enforced by the thumbv7em-none-eabihf no-default-features cross-compile in CI. This is a design discipline, not code taken from any emulator.
The one-clock, every-cycle-bus-access timebase (ADR 0029). The single-cycle counter and split-around-access start_cycle/end_cycle PPU catch-up are RustyNES's implementation. It is conceptually similar to Mesen2's cycle-stepped approach (and, given §1, some of the surrounding NES code is Mesen2-derived), but the scheduler substrate itself is original design.
Machine-checked accuracy honesty (ADR 0011). The Core/Curated/BestEffort mapper tiering, the snapshot_schema_audit field-vs-schema gate, and the build-fails-not-the-reader honesty posture are the project's own contribution.
Measure-first performance with published rejections. docs/performance.md records optimizations that were measured and rejected with their numbers — an unusual discipline that is genuinely the project's own.
The 2-cycle-ALE octal-latch PPU model, verified correct (ADR 0030). The physical octal-latch model was an independent modeling choice; v2.2.6 disclosed that its timing had been calibrated to TriCNES (MIT) rather than derived independently, and flagged a suspected TriCNES-specific Rad Racer hybrid-address artifact for a v2.3.0 rework. v2.3.0 "Datum II" investigated it and found the suspected artifact does not reproduce in the shipped build: the model passes the authoritative in-repo AccuracyCoin "Hybrid Addresses" / "ALE + Read" tests (141/141 — disabling the delayed-CopyV drops exactly that test to 140/141), and Rad Racer's road/horizon renders cleanly. The artifact lived in the pre-v2.0.3 "+1 coarse-X" approximation, which the promoted 2-cycle-ALE / delayed-CopyV model superseded. The current behavior is therefore documentation/oracle-derived — it matches the NESdev-documented delayed-CopyV-during-render timing and is pinned by the MIT AccuracyCoin ROM plus an exact-141/141 CI gate. TriCNES (MIT) is retained as the original cross-reference; this is an attribution/fidelity matter, not a GPL one.

5. Incorporated permissive components¶

Genuinely incorporated, each GPL-compatible and attributed in NOTICE:

Component	License	Copyright	Where
emu2413 v1.5.9	MIT	2020 Mitsutaka Okazaki	`crates/rustynes-apu/src/opll.rs` (Rust port; VRC7 audio, ADR 0006)
TriCNES (commit 9199870)	MIT	2025 Chris Siebert	`crates/rustynes-{ppu,cpu,core}` (ported models) + vendored golden oracle
rcheevos v12.3.0	MIT	2018 RetroAchievements.org	`crates/rustynes-cheevos/vendor/rcheevos/` (optional `retroachievements` feature)
blip_buf	LGPL-2.1-or-later	Shay Green (Blargg)	`crates/rustynes-apu/src/blip.rs` (band-limited synthesis; GPLv3-compatible)
Font Awesome Free / bundled fonts	their own licenses (OFL-1.1 etc.)	respective authors	`crates/rustynes-frontend/assets/fonts/`

MIT, ISC, BSD, and LGPL-2.1-or-later are all compatible with GPL-3.0-or-later, so incorporating them into the GPL project is permitted; their own notices are preserved in NOTICE.

6. Video shaders and NTSC-decode filters¶

The CRT shader stack (crates/rustynes-gfx-shaders/) and the NTSC-decode filters (ntsc_bisqwit.rs, ntsc_lmp88959.rs) reproduce the look of community shaders — CRT-Royale (TroggleMonkey, GPL-2.0-or-later), crt-guest-advanced (guest.r), Sony Megatron (MajorPainInTheCactus), Bisqwit's NES composite model, and EMMIR's NTSC-CRT. These were reviewed at the source level and reimplemented as single fullscreen WGSL passes on RustyNES's own uniform/pipeline conventions.

Two honest points here, corrected from the earlier document:

The Bisqwit NTSC filter's numeric tables were ported verbatim (the original comment said so). That is derivation, listed in §1. The two-level composite signal shape is documented at the NESdev wiki, but the specific coefficient tables came from Bisqwit's C as carried by Mesen2, so the GPL applies.
The CRT shaders are single-pass reimplementations rather than translations of the upstream multi-pass sources, and copyright does not protect a visual look. But since the whole project is now GPL-3.0-or-later anyway, and CRT-Royale is itself GPL-2.0-or-later, this is moot for licensing — they are credited as influences in NOTICE and the project's license covers them regardless.

All of these features are optional and default-off and do not affect the deterministic emulation core or its AccuracyCoin results.

7. Test ROMs¶

Every ROM committed under tests/roms/ is a public-domain work released for validating NES emulators, catalogued per-author in tests/roms/LICENSES.md (blargg's suites, kevtris/AccuracyCoin material, and others). No commercial Nintendo software is bundled; users test commercial dumps they own from the gitignored tests/roms/external/. The AccuracyCoin battery is MIT-licensed (Chris Siebert / 100thCoin).

8. The correction, owned¶

For the record, because the reviewer was right that scrubbing the comments looked like sweeping this under the rug:

The in-source comments originally, and correctly, described this code as ports of Mesen2 / puNES / FCEUX (with file, function, and line-number references).
v2.2.5 "Colophon" reworded those comments to call the same code "oracle cross-checks" and asserted "No GPL-licensed emulator source is incorporated." That assertion was false, and the rewording obscured the provenance rather than clarifying it.
v2.2.9 corrects this the right way: it (a) relicenses the project to GPL-3.0-or-later, (b) states plainly that other emulators' code was incorporated during implementation, (c) credits every derived-from source here and in NOTICE, and (d) marks each derived source file with an accurate SPDX-License-Identifier: GPL-3.0-or-later header and a specific provenance note (naming its upstream file/function) that points back to the §1 table. The old scattered, imprecise per-line "port of" comments are not restored verbatim — the SPDX + provenance headers plus this audited table are their accurate, discoverable replacement.

Responsibility for what the AI tooling put into this codebase, and for the earlier mislabeling, rests with the project. This document is the correction of record.

A full forensic reconstruction of how and why this happened — the timeline, the models involved, the root causes, and what is and is not recoverable from the logs — is in provenance-failure-postmortem.md.