libretro/wiki/architecture.md

# Architecture - RetroBIOS

## Directory structure

```
bios/                    BIOS and firmware files, organized by Manufacturer/Console/
  Manufacturer/Console/  canonical files (one per unique content)
  .variants/             alternate versions (different hash, same purpose)
emulators/               one YAML profile per core/engine
platforms/               one YAML config per platform (scraped from upstream)
  _shared.yml            shared file groups across platforms
  _registry.yml          platform metadata (logos, scrapers, status, install config)
  _data_dirs.yml         data directory definitions (Dolphin Sys, PPSSPP...)
  targets/               hardware target configs + _overrides.yml
provenance/              dump-catalog snapshots (redump, no-intro, tosec)
scripts/                 all tooling (Python, pyyaml only dependency)
  scraper/               upstream scrapers (libretro, batocera, recalbox...)
  scraper/targets/       hardware target scrapers (retroarch, batocera, emudeck, retropie)
  exporter/              native format exporters (batocera, recalbox, emudeck...)
install/                 JSON install manifests per platform
  targets/               JSON target manifests per platform (cores per architecture)
data/                    cached data directories (not BIOS, fetched at build)
schemas/                 JSON schemas for platform and emulator YAML (checked in CI)
tests/                   test suite with synthetic fixtures
wiki/                    hand-written documentation sources
docs/                    generated MkDocs site (gitignored, rebuilt in CI)
_mame_clones.json        MAME parent/clone set mappings
database.json            file index built from bios/ (SHA1 primary key)
dist/                    generated packs (gitignored)
.cache/                  hash cache and large file downloads (gitignored)
```

`docs/` is never edited by hand: `generate_site.py` rebuilds it from
`platforms/`, `emulators/`, `database.json`, and the `wiki/` sources.
Documentation changes belong in `wiki/` or in the generator.

## Data flow

```
Upstream sources          Scrapers parse       generate_db.py scans
  System.dat (libretro)   + fetch versions     bios/ on disk
  batocera-systems                             builds database.json
  es_bios.xml (recalbox)                       (SHA1 primary key,
  core-info .info files                         indexes: by_md5, by_name,
  FirmwareDatabase.cs                           by_crc32, by_sha256, by_path_suffix)
  bios_db.json.zip (MiSTer)
  MAME/FBNeo source

provenance/*.json        generate_db.py joins  database.json entries carry
  redump, no-intro,       by SHA1, then by      a provenance field naming
  tosec snapshots         MD5 + size            the catalogs that list them

emulators/*.yml          verify.py checks      generate_pack.py resolves
  source-verified         platform-native       files by hash, builds ZIP
  from code               verification          packs per platform

truth.py generates       diff_truth.py         export_native.py
  ground truth from       compares truth vs     exports to native formats
  emulator profiles       scraped platform      (DAT, XML, JSON, Bash)
```

Pipeline runs all steps in sequence: DB, provenance report, data dirs,
MAME/FBNeo hashes, verify, packs, install manifests, target manifests,
consistency check, pack integrity, README, site. See [tools](tools.md)
for the full pipeline reference.

```mermaid
graph LR
    A[generate_db] --> A2[provenance report]
    A2 --> B[refresh_data_dirs]
    B --> C[MAME/FBNeo hashes]
    C --> D[verify --all]
    D --> E[generate_pack --all]
    E --> F[install manifests]
    F --> G[target manifests]
    G --> H[consistency check]
    H --> H2[pack integrity]
    H2 --> I[generate_readme]
    I --> J[generate_site]

    style A fill:#2d333b,stroke:#adbac7,color:#adbac7
    style D fill:#2d333b,stroke:#adbac7,color:#adbac7
    style E fill:#2d333b,stroke:#adbac7,color:#adbac7
    style H2 fill:#2d333b,stroke:#adbac7,color:#adbac7
    style J fill:#2d333b,stroke:#adbac7,color:#adbac7
```

## Three layers of data

| Layer | Source | Role |
|-------|--------|------|
| Platform YAML | Scraped from upstream | What the platform declares it needs |
| `_shared.yml` | Curated | Shared files across platforms, reflects actual behavior |
| Emulator profiles | Source-verified | What the code actually loads. Used for cross-reference and gap detection |

The pack combines platform baseline (layer 1) with core requirements (layer 3).
Neither too much (no files from unused cores) nor too few (no missing files for active cores).
A same-named local file that contradicts an explicit hash is recorded as an
unsafe omission on hash-verifying platforms; existence platforms ship it and
report the divergence, because their code never reads the bytes.

The emulator's source code serves as ground truth for what files are needed,
what names they use, and what validation the emulator performs. Platform YAML
configs are scraped from upstream and are generally accurate, though they can
occasionally have gaps or stale entries. The emulator profiles complement the
platform data by documenting what the code actually loads. When the two disagree,
the profile takes precedence for pack generation: files the code needs are included
even if the platform does not declare them. Files the platform declares but no
profile references are kept as well (flagged during cross-reference), since the
upstream may cover cases not yet profiled.

```mermaid
graph TD
    PY[Platform YAML<br/>scraped from upstream] --> PG[Pack generation]
    EP[Emulator profiles<br/>source-verified] --> PG
    SH[_shared.yml<br/>curated shared files] --> PY
    SH --> EP
    PG --> ZIP[ZIP pack per platform]

    style PY fill:#2d333b,stroke:#adbac7,color:#adbac7
    style EP fill:#2d333b,stroke:#adbac7,color:#adbac7
    style SH fill:#2d333b,stroke:#adbac7,color:#adbac7
    style PG fill:#2d333b,stroke:#adbac7,color:#adbac7
    style ZIP fill:#2d333b,stroke:#adbac7,color:#adbac7
```

## Dump-catalog provenance

Snapshots of the dump-preservation catalogs (Redump, No-Intro, TOSEC) live in
`provenance/` as committed JSON. `generate_db.py` joins them against the
collection by SHA1, falling back to MD5 + size, and writes a `provenance` field
on each matching database entry. The system pages render it as a verified dump
badge.

Provenance is an annotation, never an authority. It answers "does this file
byte-match a catalogued dump", which is a different question from "does the
emulator accept it". When the two disagree, pack contents and verification
follow the emulator source code.

`provenance_report.py` reports the reverse direction: catalog entries absent
from the collection, which become acquisition targets. A DAT counts as covered
when the collection holds at least one of its entries; entries from DATs the
collection does not cover are counted but not listed, since No-Intro tags every
non-game dump as `[BIOS]`, including tens of thousands of digital-distribution
entries that are out of scope here.

## Pack grouping

Platforms that produce identical packs are grouped automatically.
RetroArch and Lakka share the same files and `base_destination` (`system/`),
so they produce one combined pack (`RetroArch_Lakka_BIOS_Pack.zip`).
RetroPie uses `BIOS/` as base path, so it gets a separate pack.
With `--target`, the fingerprint includes target cores so platforms
with different hardware filters get separate packs. Emulator and system packs
also retain system identity, `variant_group` and requested region; same-named
regional files are never collapsed merely because their display label matches.

## Pack reproducibility

A pack is a function of its inputs. Two builds from the same collection and
the same `database.json` produce the same bytes, so a third party can rebuild
a published pack and compare it against the checksum in `SHA256SUMS.txt`.
That list is signed with a release key whose public half is `allowed_signers`
at the repository root, so the comparison does not rest on the release page
being intact: see [verifying a release](release-process.md#verifying-a-release).

Three things make that hold. Generated members (`README.txt`, `manifest.json`)
carry a fixed date rather than the wall clock, and the manifest's `generated`
field is read from `database.json` instead of the build time. MAME and FBNeo
romsets are rebuilt deterministically from their ROMs, so a pack does not
inherit whatever metadata the source archive happened to carry. And every
member is written with the same fixed date: `ZipFile.write` copies the source
file's mtime, which is the checkout time for a file from the collection and
the wall clock for one the build just staged in `tmp/`.

`tests/test_deterministic_zip.py` builds the same fixture twice and compares
the bytes, for both the platform and the emulator pack paths. Its fixture
holds a romset, because without one the comparison never reaches the rebuild
path and passes while the real packs still move.

## Storage tiers

| Tier | Meaning |
|------|---------|
| `embedded` (default) | file is in the `bios/` directory, included in packs |
| `external` | file has a `source_url`, downloaded at pack build time |
| `user_provided` | user must provide the file (instructions included in pack) |

## Verification severity

How missing or mismatched files are reported:

| Mode | required + missing | optional + missing | hash mismatch |
|------|-------------------|-------------------|--------------|
| existence | WARNING | INFO | N/A |
| md5 | CRITICAL | WARNING | UNTESTED |

Files with `hle_fallback: true` are downgraded to INFO when missing
(the emulator has a software fallback).

## Discrepancy detection

When a file passes platform verification (MD5 match) but fails
emulator-level validation (wrong CRC32, wrong size), a DISCREPANCY is reported.
The pack generator searches the repo for a variant that satisfies both.
If none exists, the platform version is kept and the discrepancy stays
reported. A filename or destination never overrides a declared content hash
during resolution; whether a mismatch is packed is then decided by the
platform's own verification mode.

## Security

- install metadata is fetched from the same revision as the installer that
  reads it, and validated before use;
- destinations are normalized and contained below the selected BIOS root;
- downloads are bounded by declared and global size limits, verified with
  SHA256/SHA1, written to unique temporary siblings and atomically replaced;
- ZIP extraction rejects traversal, absolute paths, duplicates, links, special
  files, encryption, excessive expansion and suspicious compression ratios;
- standalone-emulator copies require explicit `--standalone-copies` consent;
- CI writes only what its job needs: `validate.yml` holds `pull-requests: write`
  for the validation comment and labels, `deploy-site.yml` holds the Pages
  deploy identity, and the release job holds `contents: write` but runs only
  when someone dispatches it.

- `safe_extract_zip()` prevents zip-slip path traversal attacks
- `deterministic_zip` rebuilds MAME ZIPs so same ROMs always produce the same hash
- `crypto_verify.py` and `sect233r1.py` verify 3DS RSA-2048 signatures and AES-128-CBC integrity
- ZIP inner ROM verification via `checkInsideZip()` replicates Batocera's behavior
- `md5_composite()` replicates Recalbox's composite ZIP hash

## Edge cases

| Case | Handling |
|------|---------|
| Batocera truncated MD5 (29 chars) | prefix match in resolution |
| `zippedFile` entries | MD5 is of the ROM inside the ZIP, not the ZIP itself |
| Regional variants (same filename) | `by_path_suffix` index disambiguates |
| MAME BIOS ZIPs | `contents` field documents inner structure |
| RPG Maker/ScummVM | excluded from dedup (NODEDUP) to preserve directory structure |
| `strip_components` in data dirs | flattens cache prefix to match expected path |
| case-insensitive dedup | prevents `font.rom` + `FONT.ROM` conflicts on Windows/macOS |
| frozen snapshot cores | `.info` may reflect current version while code is pinned to an old one. Only the frozen source at the pinned tag is reliable (e.g. desmume2015, mame2003) |

### File resolution chain

`resolve_local_file` in `common.py` tries each strategy in order, returning the
first match. Used by both `verify.py` and `generate_pack.py`.

```mermaid
graph TD
    START([resolve_local_file]) --> S0{path_suffix<br/>exact match?}
    S0 -- yes --> EXACT([exact])
    S0 -- no --> S1{SHA1<br/>exact match?}
    S1 -- yes --> EXACT
    S1 -- no --> S1B{SHA256<br/>exact match?}
    S1B -- yes --> EXACT
    S1B -- no --> S1C{CRC32 + size,<br/>no stronger hash?}
    S1C -- yes --> EXACT
    S1C -- no --> S2{MD5 direct<br/>or truncated?}
    S2 -- yes --> MD5([md5_exact])
    S2 -- no --> S3{name + aliases<br/>no MD5?}
    S3 -- yes --> EXACT
    S3 -- no --> S4{name + aliases<br/>md5_composite /<br/>direct MD5?}
    S4 -- match --> EXACT
    S4 -- name only --> HM([hash_mismatch])
    S4 -- no --> S5{zippedFile<br/>inner ROM MD5?}
    S5 -- yes --> ZE([zip_exact])
    S5 -- no --> S6{MAME clone<br/>map lookup?}
    S6 -- yes --> MC([mame_clone])
    S6 -- no --> S7{data_dir<br/>cache scan?}
    S7 -- yes --> DD([data_dir])
    S7 -- no --> S8{agnostic<br/>fallback?}
    S8 -- yes --> AG([agnostic_fallback])
    S8 -- no --> NF([not_found])

    style START fill:#2d333b,stroke:#adbac7,color:#adbac7
    style EXACT fill:#2d333b,stroke:#adbac7,color:#adbac7
    style MD5 fill:#2d333b,stroke:#adbac7,color:#adbac7
    style HM fill:#2d333b,stroke:#adbac7,color:#adbac7
    style ZE fill:#2d333b,stroke:#adbac7,color:#adbac7
    style MC fill:#2d333b,stroke:#adbac7,color:#adbac7
    style DD fill:#2d333b,stroke:#adbac7,color:#adbac7
    style AG fill:#2d333b,stroke:#adbac7,color:#adbac7
    style NF fill:#2d333b,stroke:#adbac7,color:#adbac7
```

## Platform inheritance

Platform configs support `inherits:` to share definitions.
Lakka inherits from RetroArch, RetroPie inherits from RetroArch with `base_destination: BIOS`.
`overrides:` allows child platforms to modify specific systems from the parent.

Core resolution (`resolve_platform_cores`) uses three strategies:

- `cores: all_libretro` - include all profiles with `libretro` in their type
- `cores: [list]` - include only named profiles
- `cores:` absent - fallback to system ID intersection between platform and profiles

## Hardware target filtering

`--target TARGET` filters packs and verification by hardware (e.g. `switch`, `rpi4`, `x86_64`).
Target configs are in `platforms/targets/`. Overrides in `_overrides.yml` add aliases and
adjust core lists per target. `filter_systems_by_target` excludes systems whose cores are
not available on the target. Without `--target`, all systems are included.

## MAME clone map

`_mame_clones.json` at repo root maps MAME clone ROM names to their canonical parent.
When a clone ZIP was deduplicated, `resolve_local_file` uses this map to find the canonical file.

## Install manifests

`generate_pack.py --manifest` produces JSON manifests in `install/` for each platform.
These contain downloadable file lists with SHA256 and SHA1 hashes, explicit
unsafe/unavailable omissions, platform detection config, and standalone copy
instructions. `install/targets/` contains per-architecture core availability.
The cross-platform installer (`install.py`) uses these manifests to auto-detect the
user's platform, filter files by hardware target, and download with SHA256/SHA1,
size, path-containment and atomic-write checks.

## Tests

The suite is discovered dynamically rather than maintained as a hand-counted
subset:

| File | Coverage |
|------|----------|
| `test_e2e.py` | resolution, verification, packs, inheritance, targets, truth and exporters |
| `test_profile_sync.py`, `test_upstream.py` | source anchoring, forge parsing, revisions, cache and guarded writes |
| `test_install.py`, `test_audit_regressions.py` | automatic wrappers, manifest boundaries, pipeline exits, strong hashes and safe archives |
| `test_region.py` | region vocabulary, ranking, per-system grouping and repository conformance |
| `test_site_exports.py`, `test_site_validation.py` | versioned exports, SQLite, metadata, accessibility structure and complete local-link graph |
| parser/provenance modules | MAME, FBNeo, DAT import, merge and provenance joins |
| artifact modules | deterministic ZIPs, locking, large-file cache, pack integrity and portable paths |

```bash
python -m unittest discover tests -v      # full suite
python -m unittest tests.test_e2e -v      # single module
```

`test_e2e.py`, `test_install.py`, `test_provenance.py`, the parser tests,
`test_profile_sync.py` and `test_upstream.py` run on synthetic fixtures with no
network and no real BIOS files. `test_pack_integrity.py`, `test_torrentzip.py` and
`test_no_case_collisions.py` read the working tree and skip when the data they
need is absent. See the [testing guide](testing-guide.md) for the fixture
pattern and how to add a test.

## CI workflows

| Workflow | File | Trigger | Role |
|----------|------|---------|------|
| Deploy Site | `deploy-site.yml` | push to main (platforms, emulators, wiki, scripts) + manual | validate contracts, generate site, build with MkDocs, validate rendered HTML, deploy to Pages |
| Validation | `validate.yml` | pull request and push to main on bios/, platforms/, emulators/, schemas/, scripts/, tests/ | schema check and full test suite on both; BIOS hash validation and auto-label on pull requests |

Releases are not built in CI: the packs are generated and checked on the
maintainer's machine and uploaded with `gh`, see the
[release process](release-process.md).

## License

See `LICENSE` at repo root. Files are provided for personal backup and archival.