Writes down the contract before implementing it, because one part is a
behaviour change someone would otherwise file as a bug.
The rule: no BIOS chosen -> OpenBIOS; a BIOS the player explicitly chose
-> that BIOS. A developer can require retail for a title with a verified
OpenBIOS incompatibility via [runtime] openbios = false.
"Chosen" means --bios or a launcher/settings pick, and nothing else.
Today resolve_bios_path() also searches directories near the executable,
so a stray SCPH1001.BIN would silently win even though the player never
asked — making behaviour depend on what happens to be on disk. Discovery
stops being a selection mechanism.
Also recorded: why both BIOSes are compiled into one binary (a build
flavour per BIOS needed the same choice restated in three places that
nothing cross-checked, and it compiled cleanly when they disagreed); why
that is affordable (game C is byte-identical across profiles — verified
on Tomba!, Tomba! 2 and Ape Escape — so only the BIOS is duplicated, and
the two collide on just 22 symbols); that the developer setting is not
player-overridable; and that savestates are BIOS-specific and must be
refused across BIOSes, which is the one silent-corruption risk here.
README gets a player-facing version: you probably don't need a BIOS,
here's how to use your own, saves work either way, savestates don't mix.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Matthew Stanley