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March 2002 Release
503
Virtex-II Pro™ Platform FPGA Documentation
1-800-255-7778
Interrupt Reference
R
Critical-Input Interrupt (0x0100)
Interrupt Classification
•
Critical—return using the
rfci
instruction.
•
Asynchronous.
•
Precise.
Description
A critical-input exception is caused by an external device (usually the external-interrupt
controller) asserting the critical-interrupt input signal to the processor.
This exception is persistent. To prevent repeated interrupts from occurring, the interrupt
handler must clear the exception status in the appropriate device control register (DCR)
associated with the external-interrupt controller before returning, and before re-enabling
critical interrupts.
This interrupt is enabled using the critical-interrupt enable bit (CE) in the MSR. When
MSR[CE]
=
1, the processor recognizes exceptions caused by asserting the critical-interrupt
input signal and forces a critical-input interrupt to occur. When MSR[CE]
=
0, the processor
does not recognize the critical-interrupt input signal and critical-input interrupts cannot
occur.
All maskable interrupts, except those caused by machine-check exceptions, are disabled
when a critical-input interrupt occurs. The critical-input interrupt handler should not re-
enable MSR[CE] until it has cleared the exception and saved SRR2 and SRR3. Saving these
registers avoids potential corruption of the interrupt handler should a watchdog-timer
interrupt or another critical-input interrupt occur.
In some PowerPC implementations, this exception-vector offset corresponds to a system-
reset interrupt.
Affected Registers
Register
Value After Interrupt
SRR0
Not used.
SRR1
SRR2
Loaded with the effective address of the next-sequential instruction to be
executed at the point the interrupt occurs.
SRR3
Loaded with a copy of the MSR at the point the interrupt occurs.
ESR
Not used.
DEAR
MSR
[AP, APE, WE, CE, EE, PR, FP, FE0, DWE, DE, FE1, IR, DR]
←
0.
[ME]
←
Unchanged.
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