Function stray_interrupt_sources 

pub const fn stray_interrupt_sources(status: u32, mask: u32, cause: u32) -> u32

The sources psx-rt’s handler acknowledges as strays for one entry, given I_STAT, I_MASK and COP0 Cause; zero when it leaves them alone.

It acts only on an interrupt (Cause.ExcCode 0) with no VBlank pending: a pending VBlank means a game’s own handler may have chained here with a source of its own still waiting for it, which must stay in I_STAT. The handler’s assembly makes exactly this decision.