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FAULTS
As indicated in
Figure
8-2, two fault table entry types are allowed: local-call entry and system-call
entry. Each is two words in length. The entry type field (bits 0 and 1 of the entry's first word) and
the value in the entry's second word determine the entry type.
local-call entry
Provides an instruction pointer for the fault handling procedure. The
(type 00
)
processor uses this entry to invoke the specified procedure by means of an
2
implicit local-call operation. The second word of a local procedure entry is
reserved. It must be set to zero when the fault table is created and not
accessed after that.
system-call entry
Provides a procedure number in the system procedure table. This entry must
(type 10
)
have an entry type of 10
2
The processor computes the system procedure number by shifting right the
first word of the fault entry by two bit positions. Using this system procedure
number, the processor invokes the specified fault handling procedure by
means of an implicit call-system operation similar to that performed for the
instruction.
calls
Other entry types (01
and 11
2
To summarize, a fault handling procedure can be invoked through the fault table in any of three
ways: a local call, a system-local call or a system-supervisor call.
8.4
STACK USED IN FAULT HANDLING
The i960 architecture does not define a dedicated fault handling stack. Instead, to handle a fault,
the processor uses either the user, interrupt or supervisor stack, whichever is active when the fault
is generated. There is, however, one exception: if the user stack is active when a fault is generated
and the fault handling procedure is called with an implicit system supervisor call, the processor
switches to the supervisor stack to handle the fault.
8.5
FAULT RECORD
When a fault occurs, the processor records information about the fault in a fault record in memory.
The fault handling procedure uses the information in the fault record to correct or recover from the
fault condition and, if possible, resume program execution. The fault record is stored on the same
stack that the fault handling procedure will use to handle the fault.
8-6
and a value in the second word of 0000 027FH.
2
) are reserved and have unpredictable behavior.
2
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