Constant CAUSE_UNIMPLEMENTED 

pub const CAUSE_UNIMPLEMENTED: u32 = _; // 131_072u32

The Unimplemented Operation cause bit (FCSR bit 17).

Distinct from Invalid: it means “this processor cannot do this in hardware — trap to software”, not “the operation is mathematically undefined”. The VR4300 uses it for the long-integer conversion restriction below, which is a hardware limitation, not a numerical error.