NEC uPD98502 User Manual page 154

2.5.4.12 Coprocessor unusable exception
(1) Cause
The Coprocessor Unusable exception occurs when an attempt is made to execute a coprocessor instruction for
either:
a corresponding coprocessor unit that has not been marked usable (Status register bit, CU0 = 0), or
CP0 instructions, when the unit has not been marked usable (Status register bit, CU0 = 0) and the process
executes in User or Supervisor mode.
This exception is not maskable.
(2) Processing
The common exception vector is used for this exception, and the CPU code in the ExcCode field of the Cause
register is set. The CE bit of the Cause register indicates which of the four coprocessors was referenced.
The EPC register contains the address of the instruction that caused the exception. However, if this instruction is
in a branch delay slot, the EPC register contains the address of the preceding jump or branch instruction, and the
BD bit of the Cause register is set to 1.
(3) Servicing
The coprocessor unit to which an attempted reference was made is identified by the CE bit of the Cause register.
One of the following processing is performed by the handler:
If the process is entitled access to the coprocessor, the coprocessor is marked usable and the corresponding
state is restored to the coprocessor.
If the process is entitled access to the coprocessor, but the coprocessor does not exist or has failed,
interpretation of the coprocessor instruction is possible.
If the BD bit in the Cause register is set to 1, the branch instruction must be interpreted; then the coprocessor
instruction can be emulated and execution resumed with the EPC register advanced past the coprocessor
instruction.
If the process is not entitled access to the coprocessor, the kernel reports UNIX SIGILL/ILL_PRIVIN_FAULT
(illegal instruction/privileged instruction fault) signal to the current process, and this exception is fatal.
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