-Coprocessor Unusable Exception

5.3 Processor Exceptions

Coprocessor Unusable Exception

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, or

CP0 instructions, when the unit has not been marked usable and the process executes in either User or Supervisor mode.

This exception is not maskable.

Processing

The common exception vector is used for this exception, and the CPU code in the Cause register is set. The contents of the Coprocessor Usage Error field of the coprocessor Control register indicate which of the four coprocessors was referenced. The EPC register contains the address of the unusable coprocessor instruction unless it is in a branch delay slot, in which case the EPC register contains the address of the preceding branch instruction.

Coprocessor Unusable exception processing is shown in Figure 5-17.

Servicing

The coprocessor unit to which an attempted reference was made is identified by the Coprocessor Usage Error field, which results in one of the following situations:

If the process is entitled access to the coprocessor, the coprocessor is marked usable and the corresponding user 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 is set in the Cause register, 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 process executing at the time is handed a UNIX SIGILL/ILL_PRIVIN_FAULT (illegal instruction/privileged instruction fault) signal. This error is usually fatal.

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