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FAULTS
•
The fault record is copied into the area allocated for it in the new stack frame, beginning at
NFP-1. (See
Figure
8-4.)
•
The processor gets the IP for the first instruction of the fault handling procedure from the
system procedure table (using the index provided in the fault table entry).
•
The processor stores the fault return code (001
fault is not a trace, parallel or override fault, it copies the state of the system procedure table
trace control flag (byte 12, bit 0) into the PC register trace enable bit. If the fault is a trace,
parallel or override fault, the trace enable bit is cleared.
On a return from the fault handling procedure, the processor performs the action described in
section 7.1.3.2, "Return Operation" (pg. 7-7)
•
The fault record arithmetic controls field is copied into the AC register.
•
If the processor is in supervisor mode prior to the return from the fault handling procedure
(which it should be), the fault record process controls field is copied into the PC register. The
mode is then switched back to user, if it was in user mode before the call.
•
The processor switches back to the stack it was using when the fault occurred. (If the
processor was in user mode when the fault occurred, this operation causes a switch from the
supervisor stack to the user stack.)
•
If the trace-fault-pending flag and trace enable bits are set in the PC field of the fault record,
the trace fault on the instruction at the origin of the supervisor fault call is handled at this time.
The user should note that PC register restoration causes any changes to the process controls done
by the fault handling procedure to be lost.
8.8.4
Faults and Interrupts
If an interrupt occurs during an instruction that will fault, an instruction that has already faulted, or
fault handling procedure selection, the processor handles the interrupt in the following way:
1.
Completes the selection of the fault handling procedure.
2.
Creates the fault record.
3.
Services the interrupt just prior to executing the first instruction of the fault handling
procedure.
4.
Handles the fault upon return from the interrupt.
Handling the interrupt before the fault reduces interrupt latency.
8-18
) in the PFP register return type field. If the
2
with the addition of the following:
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