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6.3.5 Coprocessor Unusable exception
• Cause
− Attempting to execute a coprocessor CPz instruction when its corresponding CUz bit in the
Status register is cleared to 0 (coprocessor unusable).
− In user mode, attempting to execute a CP0 instruction when the 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.)
• Exception mask
The Coprocessor Unusable exception is not maskable.
• Applicable instructions
Coprocessor instructions : LWCz, SWCz, MTCz, MFCz, CTCz, CFCz, COPz, BCzT, BCzF,
BCzTL, BCzFL
Coprocessor 0 instructions : MTC0, MFC0, RFE, COP0
• Processing
− The common exception vector (0x8000 0080) is used.
− CpU(11) is set for ExcCode in the Cause register.
− The coprocessor number referred to at the time of the exception is stored in the Cause register
CE (Coprocessor Error) field.
− The EPC register points to the address of the instruction causing the exception. If, however,
that instruction is in the branch delay slot (for execution during a branch), the immediately
preceding branch instruction address is retained in the EPC register and the Cause register BD
bit is set to 1.