psxrecomp/docs/BIOS_SELECTION.md

Which BIOS a build uses

A PlayStation game needs a BIOS. PSXRecomp can supply one — OpenBIOS, an MIT-licensed, from-scratch PS1 BIOS from the PCSX-Redux project that we are allowed to redistribute — so a player can be handed a build and a disc image and just play. A player who prefers their own dumped retail BIOS can use that instead.

Both recompiled BIOS backends are linked into every normal build. The OpenBIOS image itself and its MIT notice are staged in bios/ beside the executable; the retail image is never shipped and comes from the player. Which backend runs is decided when the game launches, not when it is built.

The rule

No BIOS chosen → OpenBIOS. A BIOS the player explicitly chose → that BIOS.

That is the whole user-facing contract. A player never has to find a BIOS file to start playing, and never loses the ability to use their own.

A developer can switch this off for a title that is known not to work on OpenBIOS; then a retail BIOS is required and the player is prompted for one.

What counts as "chosen"

A deliberate act by the player:

• --bios <path> on the command line, or
• a BIOS selected in the launcher / settings, which is then remembered.

Setup / Generate first-run discovery. When there is no bios.cfg yet (and no settings pick), install & build looks next to the executable / project for a validated retail dump (SCPH1001.BIN and common filename variants; size + CRC must match the compiled-in retail image). If found, that path is written to bios.cfg and used for Generate & Play. If not found, the build stays on OpenBIOS with no prompt.

Discovery does not run when bios.cfg already exists — including after the player cleared the pick to OpenBIOS. Play-time resolution still never invents a BIOS from random files on disk once a choice has been recorded.

A player who has chosen a BIOS can clear that choice and go back to OpenBIOS; the choice is not permanent.

"A retail BIOS" means one specific image

A build compiles in exactly one retail image (currently SCPH-1001) alongside OpenBIOS. The compiled-in code is generated from that exact image, so a different dump — SCPH-101, SCPH-5552, a bad rip — cannot be substituted: the recompiled code would execute against data it does not match, and crash.

Every build therefore checks the identity of a chosen BIOS before using it, and says which image it expects if the check fails. This is not DRM and not a preference; it is the difference between running and wild-jumping.

The acceptance gate compares file size and CRC32 against each linked backend's recorded identity (bios_backend_for_file in runtime/src/main.cpp). A SHA-256 is also recorded per image (image_sha256 in runtime/include/psx_bios_image.h) and is surfaced for provenance and bug reports, but it is not part of the accept/reject decision.

If a chosen BIOS does not match:

• with OpenBIOS available, the player is told why and can continue on OpenBIOS;
• with OpenBIOS disabled for the title, the choice is rejected and re-prompted.

Developer setting

One optional line in game.toml:

[runtime]
openbios = true    # default. false = this title requires a retail BIOS.

Set openbios = false only for a title with a verified OpenBIOS incompatibility. Per-title compatibility is not implied by the framework supporting OpenBIOS — verify a title before shipping it that way.

This setting is deliberately not overridable by a player's settings.toml, unlike most runtime options. It records a developer's compatibility finding, not a preference. A player re-enabling OpenBIOS on a title known to break under it produces confusing bug reports and no benefit.

There is no third mode. A title either allows OpenBIOS (and still honours an explicit retail choice) or requires retail. "OpenBIOS only, retail forbidden" would take away a working option from players for no reason.

Boot behaviour does not depend on which BIOS runs

Whichever BIOS is active, [runtime] bios_hle (on by default) skips the BIOS boot sequence and goes straight to the game. That is deliberate and enforced: the skip is not a synthesized handoff, it just returns immediately from the BIOS's call into its shell, so all it needs is the per-image address of that shell entry — which every linked BIOS records. A player who switches BIOS gets the same boot, and the flag means one thing everywhere.

The HLE tier's other half — servicing a few kernel calls (the B0 event family) in the runtime instead of the recompiled BIOS — is per-image, because it needs the kernel's own DeliverEvent return address to send a delivered callback back to. Retail SCPH-1001 exports it; OpenBIOS deliberately does not until its event semantics are validated, so OpenBIOS runs those calls on the recompiled kernel and prints one line at startup saying so. That refusal is scoped to kernel calls and never cancels the boot-skip.

Until 2026-07 it did: the boot-skip was derived from the already-refused kernel-call decision, so choosing OpenBIOS silently sat the player in the boot animation while the identical build on retail went straight to the game. The two axes are now resolved in one pure, unit-tested function (psx_bios_hle_plan(), runtime/include/bios_hle_plan.h), and the test asserts the boot decision is byte-identical across the two images.

Save data

Memory cards are unaffected. They are card images; they carry no BIOS state and move freely between BIOSes and between builds.

Savestates are BIOS-specific. The kernel occupies RAM, and OpenBIOS and retail lay that RAM out differently, so a savestate captured under one BIOS is not valid under the other. Savestates therefore record which BIOS was active and refuse to load under a different one. Without that check, switching BIOS would silently corrupt a restored state — the only quiet failure mode in this design, which is why it is enforced rather than warned about.

Slot files are also isolated on disk under the per-game memcard root:

• <memcard_dir>/openbios/state_*_slot*.pst
• <memcard_dir>/scph1001/state_*_slot*.pst

On first configure, any legacy loose <memcard_dir>/state_*_slot*.pst files are moved once by .pst header bios_checksum (bundled OpenBIOS wordsum → openbios/, otherwise → scph1001/). Memory cards stay in <memcard_dir>/; netplay guest sandbox remains <memcard_dir>/netplay/ (unscoped).

Netplay lobby settle

Online and LAN lobbies advertise a per-peer BIOS offer and freeze a single session BIOS at host Start (openbios or scph1001):

• Online: bios_offer on set_ready → host publishes match_caps.session_bios.
• LAN: peers append offer fields on MOTK3 JOIN; host broadcasts them on MOTK4 UPDATE and includes the settled token on MOTK1 START.

Settle rule (same for both):

• OpenBIOS if any seated peer prefers OpenBIOS, or any peer cannot run SCPH-1001 (no linked retail backend and/or no validated dump), or a peer sends no offer (legacy client).
• SCPH-1001 only when every seated peer can run it and nobody selected OpenBIOS.

Every peer applies that session BIOS before boot. Mixed BIOSes are invalid for rollback (kernel RAM layout differs). If the session settles to SCPH-1001 but a peer has no validated dump, that peer aborts the launch rather than silently falling back to OpenBIOS (which would desync immediately).

Session BIOS is ephemeral: it affects only that match’s runtime boot. It does not rewrite bios.cfg, settings.toml, or the launcher Settings BIOS row. Soft-return shows the player’s durable preference again; the next Start re-settles from offers as usual.

Why both are compiled in

The alternative — a build flavour per BIOS — was the original design, and it required a game to state its BIOS in three places that nothing cross-checked (bios_config in game.toml, plus two CMake variables). Setting one and forgetting another produced a build that linked one BIOS's code against another BIOS's generated game code, and it compiled cleanly.

Carrying both costs roughly 20 MB of binary and removes that entire class of mistake: one build configuration, one source of truth, the choice made at runtime where it can be validated.

It is affordable because the game's own recompiled code is identical regardless of which BIOS it was generated against — verified byte-for-byte on Tomba!, Tomba! 2 and Ape Escape. Only the BIOS code is duplicated, and the two images collide on just 22 symbols, which are namespaced per BIOS.

Attribution

OpenBIOS is MIT-licensed. Its notice is vendored at bios/OpenBIOS.LICENSE, with the upstream source pin and build recipe in bios/OpenBIOS.toml and attribution in THIRD_PARTY_ATTRIBUTION.md. Builds that ship it credit the PCSX-Redux authors in the launcher, whether or not the licence compels it.

Native runtime builds automatically stage both bios/openbios.bin and bios/OpenBIOS.LICENSE. Release packaging must copy that directory as a unit; shipping the image without its notice violates the distribution contract.

Retail BIOS images are not redistributable and are never shipped. A player using one supplies their own dump.