libretro/wiki/architecture.md

Architecture

Principles

Every part of the toolchain follows from a few rules.

• The emulator's source code is the authority. A profile records what the code opens, under which names, what it checks and whether it runs without the file. Documentation, .info files and platform lists are starting points to verify. When they disagree with the code, the code wins.
• Every statement rests on a fact anyone can check. A field, a deletion or a wording names what settles it: a file:line at a pinned revision, a hash, a command and its output. Where no fact settles a question, the answer is "not verified" until one is found. A sentence on this site once said Batocera matches md5 prefixes. Its script compares the full string (batocera-systems:1088).
• The collection serves the user. A pack carries what the user has to obtain to run their emulators. When something forces the user or a contributor to work around the toolchain, the toolchain is changed rather than the workaround documented.
• Profiles document the emulator, not this repository. A profile can be taken into another project as is. No field refers to this repository's folders, file names or database, nor to one platform's conventions. The toolchain resolves a profile against the collection.
• Other projects are peers. A finding about another project says what changes, never who erred, and comes with its evidence. Our data can be wrong too, and an issue that shows it is as welcome as a finding sent upstream.
• An inconsistency found is fixed now. A fix comes with a test that fails without it, so the same mistake cannot return unnoticed.
• Collect everything a working setup needs. BIOS, firmware, keys, fonts, boot and DSP ROMs, databases, data files. Age and licence are not criteria. A file the collection lacks is an acquisition target, not an end state.

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 and pyyaml, jsonschema for schema checks)
  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     rewrites each platform's
  emulator profiles       scraped platform      own file, updated

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 for the full pipeline reference.

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 a declared hash is settled by the platform's verification mode (discrepancy detection).

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 record what each platform declares. A list kept by hand can lag behind an emulator release. 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.

A platform YAML is regenerated by its scraper and never edited by hand: an edit would be lost at the next scrape, and the file would stop describing the platform. Where the emulator source states something else, the profile and _shared.yml carry it. Another name for the same file is an aliases: entry in the profile, never a second entry in a platform YAML or in _shared.yml. A name a platform's own launcher reads, such as the 22 names Batocera's DuckStation generator tries per region, describes the platform and lives in its launchers: records (see launchers).

Some entries in a platform YAML are not the platform's own: files a _shared.yml group injects (shared_group), copies the RomM scraper mirrors to a twin system (mirror_of) and additions the libretro scraper makes (curated). native_layer() removes them wherever the question is what the platform itself declares: the export, the truth diff and the site's comparison tables. Counted as the platform's, they were written into System.dat as if libretro had declared them.

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

The inverted model

Packs are built today from what each platform declares, completed by what the cores need. The direction the project works toward is the inverse: generate each platform's file from what the emulators' code needs, file by file, and offer it to that platform's maintainers as a source they can check and adopt. The updated platform files page publishes these files, every change traceable to code, presented as suggestions. Keeping a BIOS list in step with every emulator release is a large amount of work, and the files are written to make it lighter.

An entry a platform declares is never removed from its file on the strength of a missing match. When this was measured, an entry the generated data did not pair was still read in 1189 of 2776 cases, by an alias, a hash, a directory entry or a core outside the resolved set. A removal needs positive proof: every core of the system profiled, each with a reference showing the closed set of names it opens. Until then the entry is listed as a removal candidate with the cores examined (see export_native.py).

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. Two platforms share a pack when they declare the same files, run the same emulators and see file names the same way. Lakka declares RetroArch's files and cores but runs on Linux, where bios.col and BIOS.col are two files that different cores open, while RetroArch's pack folds them for Windows and macOS. So Lakka gets its own pack. RetroPie inherits RetroArch's systems but ships its own package list, standalone emulators included, and is case-sensitive too, so it gets a separate pack and a separate install manifest. 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.

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.

Collection rules

How files enter bios/, and why.

• Variants keep every layer served. A different dump under an existing name goes to .variants/. The primary carries the name the emulator's source uses, and a dump a platform pins under an older name stays a variant, so each layer receives the bytes it checks. A destination that already exists is something a platform depends on: a segaboot.gcm that Batocera requires was once overwritten by the dump RetroDECK requires, and is now kept beside it.
• One content, one file. A byte-identical copy under another name is removed, and the name it carried becomes a profile alias. Trees addressed by path keep their equal files, since equality is part of their layout: those folders are NODEDUP_DIRS in scripts/dedup.py.
• Archives the project builds are TorrentZip. build_torrentzip() makes an archive's bytes depend on its contents alone, so anyone can rebuild it. An upstream romset whose MD5 a platform pins stays verbatim in .variants/. A member renamed by hand in an existing archive gives bytes that match no distributed romset and that nobody can regenerate.
• A pack carries what the user must obtain. A file the emulator installs itself (bundled: true) is not kept once nothing else needs it, and step 4d of the pipeline fails on such a copy. Its sha1 and size are written into the profile first, so the entry keeps the file's identity.
• A platform's older revision needs the core's own claim first. When a platform pins an older revision of a data file, collecting it makes the platform's hash the only content proof at that destination, and a core profile without a hash then receives the old file. The profile first declares the hash of the file at its own source_commit (find_pinned_revision.py finds the revision behind a pinned md5).
• A transformation of a held dump is collected by reproducing it. A byte swap, a permutation, a concatenation or a published patch is replayed from its source and checked against the expected hash, and the profile states what the file derives from.
• A dump says what it is. The version string inside a ROM names its model, whatever folder it arrived in. Dumps once filed for one handheld announced another model in their own strings.
• Game data lives under the tree its engine reads. Engines open generic names in their own folders: pak0.pak is Quake II's under baseq2/ and Half-Life's under valve/. Each game's files are stored under that tail so they resolve by path, and tests/test_game_data_homonyms.py fails when one game's entry reaches another game's file by name.
• A file a platform pins by hash is collected into bios/. A data directory cache that happens to hold it is not enough, because the installer cannot fetch from the cache.
• Collected means committed. A file indexed by the database but never committed gives the installer a 404, so git ls-files --others --exclude-standard bios is empty before manifests are regenerated.
• MAME and FBNeo archives are named the way the emulator opens them. A MAME device's archive takes the short name of its DEFINE_DEVICE_TYPE, not the ROM_START label. An FBNeo archive is named after its RomDesc, once the driver's GetZipName function strips the system prefix (msx_msx opens msx.zip). BIOS root sets change between MAME versions and are listed from each version's source. A distributed non-merged archive also holds the ROMs of every device the machine pulls in, which is what a -listxml DAT describes and why romset recipes come from DATs.

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, and deploy-site.yml holds the Pages deploy identity. No workflow builds or publishes a release.

• 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
Trees addressed by path (RPG Maker, ScummVM, tzdata, c64-emu and the rest of NODEDUP_DIRS)	excluded from dedup to keep files that are equal by design
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 turns a file entry into a local path, and verify.py and generate_pack.py both call it. Every declared hash is tried before a path or a name, so a path never wins over a hash. The steps, in order, are listed under file resolution chain.

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.

Which emulator profiles a platform resolves follows its cores field, as adding a platform describes.

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

python -m unittest discover -s tests -t . -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 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.

License

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