libretro/wiki/architecture.md

---
level: reference
---

# 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.

The site search splits each page at its headings and sends a result to the
heading above the match. Every file card is therefore a level 4 heading with
the card's anchor, and every platform system a heading of its own: a hash
pasted in the search opens that card or that system, not the top of the page.
Level 4 stays out of the table of contents (`toc_depth: 3`). The lines that
list other pages (platforms, packs, source citations, collection paths) sit
in a `data-search-exclude` block, since the browser downloads and indexes the
whole search index before it shows a first result. File names, hashes, the
paths an emulator reads and the notes stay searchable.

## 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](tools.md)
for the full pipeline reference.

<!-- image: pipeline -->

## 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](verification-modes.md#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](adding-a-platform.md#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.

<!-- image: data-layers -->

## 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](../exports.md) 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](tools.md#export_nativepy)).

## 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](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.

## 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](tools.md#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 |
|------|---------|
| MD5 shorter than 32 characters | completed by the Batocera scraper when one MD5 in the collection starts with it, matched by prefix 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](verification-modes.md#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](adding-a-platform.md#required-fields) 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 |

```bash
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](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 on any input of the site build, and manual | validate contracts, generate site, build with MkDocs, validate rendered HTML, deploy to Pages |
| Validation | `validate.yml` | pull request and push to main on the collection, profiles, schemas, scripts, tests, the installer and every input of the site build | schema check and full test suite on both, BIOS hash validation and auto-label on pull requests |

Each workflow's `paths:` list is the exact set of files that starts it.
`tests/test_workflow_paths.py` holds the Deploy Site list to the scripts the build
imports, and the Validation list to every input of the site build, so a change that
would fail the deploy fails the pull request first. A change to `wiki/`, to
`install.sh` or to any other path in that list runs the full suite.

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.