TMPR4956C datasheet(145/286 Pages) TOSHIBA

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Chapter 8 CPU Exception

8-17

8.17 Coprocessor Unusable Exception

8.17.1

Cause

The Coprocessor Unusable exception occurs when an attempt is made to execute a coprocessor

instruction for either.

•

attempting to execute a coprocessor CPz instruction when its corresponding CUz bit in Status

•

in User or Supervisor mode attempting to execute a CP0 instruction when CU0 bit is cleared to

0. (In Kernel mode, an exception is not raised when a CP0 instruction is issued , regardless of

the CU0 bit setting)

•

an attempt is made to execute a FPU instruction in TX49 without FPU

8.17.2

Processing

The common exception vector is used for this exception, and the CpU code in Cause register is set. The

coprocessor number referred to at the time of the exception is stored in Cause register CE (Coprocessor

Error) field.

If EXL bit of Status register is only set to 0, the following operation is executed. EPC register points to

the address of the instruction causing the exception. If, however, the affected instruction was in the branch

delay slot (for execution during a branch), the immediately preceding branch instruction address is

retained in EPC register and BD bit of Cause register is set to 1.

8.17.3

Servicing

The coprocessor unit to which an attempted reference was mode is identified by the Coprocessor Usage

Error field (CE) of Cause register, 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 an illegal instruction/privileged instruction fault signal. This error is usually fatal.

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