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508
March 2002 Release
1-800-255-7778
Virtex-II Pro™ Platform FPGA Documentation
Chapter 7:
Exceptions and Interrupts
R
Instruction-Storage Interrupt (0x0400)
Interrupt Classification
•
Noncritical—return using the
rfi
instruction.
•
Synchronous.
•
Precise.
Description
Instruction-storage exceptions are associated with the
fetching
of an instruction from
memory. However, an instruction-storage interrupt occurs only if an attempt is made to
execute
the instruction as required by the sequential-execution model. Speculative fetches
that are later discarded do not cause instruction-storage interrupts. An instruction-storage
exception occurs when an instruction fetch fails for any of the following reasons:
•
An instruction is fetched from an address with
no-access-allowed
zone protection (the
corresponding zone-field value is 0b00). No-access-allowed zone protection is
possible only in user mode with instruction virtual-mode enabled (MSR[IR]
=
1).
•
An instruction is fetched from a
non-executable
address. Non-executable addresses can
only be specified when instruction virtual-mode is enabled (MSR[IR]
=
1). Non-
executable addresses have the write-executable bit (TLBLO[EX]) in the corresponding
TLB entry cleared to zero. No zone-protection override can be specified:
-
In user mode, the corresponding zone-field value is
not
equal to 0b11.
-
In privileged mode, the corresponding zone-field value is
not
equal to 0b00 or
0b11.
•
An instruction is fetched from guarded storage (G attribute set to 1) regardless of
privilege. In real mode, guarded storage is specified by the SGR register. In virtual
mode, guarded storage is specified by the TLB entry (TLBLO[G]) used to translate the
address.
Software cannot disable instruction-storage interrupts.
Affected Registers
Register
Value After Interrupt
SRR0
Loaded with the effective address of the instruction that caused the instruction-
storage exception.
SRR1
Loaded with a copy of the MSR at the point the interrupt occurs.
SRR2
Not used.
SRR3
ESR
[DIZ]
←
1 if the exception was caused by a zone-protection violation.
[DIZ]
←
0 if the exception was caused by fetching from a non-executable
address or from guarded storage.
[MCI]
←
Unchanged.
All remaining bits are cleared to 0.
DEAR
Not used.
MSR
[AP, APE, WE, EE, PR, FP, FE0, DWE, FE1, IR, DR]
←
0.
[CE, ME, DE]
←
Unchanged.
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