7.5 -Saving and Restoring State

7. Floating-Point Exceptions

7.5 Saving and Restoring State

Sixteen doubleword coprocessor load or store operations save or restore the coprocessor floating-point register state in memory. The remainder of control and status information can be saved or restored through Move To/From Coprocessor Control Register instructions, and saving and restoring the processor registers. Normally, the Control/Status register is saved first and restored last.

When the coprocessor Control/Status register (FCR31) is read, and the coprocessor is executing one or more floating-point instructions, the instruction(s) in progress are either completed or reported as exceptions. The architecture requires that no more than one of these pending instructions can cause an exception. If the pending instruction cannot be completed, this instruction is placed in the Exception register, if present. Information indicating the type of exception is placed in the Control/Status register. When state is restored, state information in the status word indicates that exceptions are pending.

Writing a zero value to the Cause field of Control/Status register clears all pending exceptions, permitting normal processing to restart after the floating-point register state is restored.

The Cause field of the Control/Status register holds the results of only one instruction; the FPU examines source operands before an operation is initiated to determine if this instruction can possibly cause an exception. If an exception is possible, the FPU executes the instruction in stall mode to ensure that no more than one instruction (that might cause an exception) is executed at a time.

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