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Exception
An exception is a transfer of control away from a
program’s normal flow of execution
Caused by an event, either internal or external to the
processor
Requires immediate attention
An exception causes the Nios II processor to take
the following steps:
1. Copies the contents of status (ctl0) register to
estatus (ctl1), saving the processor’s pre-exception
status
2. Clears PIE bit of status register (ctl0) to disable external
processor interrupts
3. Write the address of the instruction after (PC+4) the
exception to the ea (r29) register
4. Transfers execution to the address of the exception
handler that determines the cause
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Returning from an Exception
eret is used to resume execution to the pre-
exception address
During eret execution
ctl1 is copied to ctl0
Transfer the execution to the address in ea register
Be careful with return address!
ea contains the address of the instruction after the point
where the exception was generated
pc+4
For software trap and unimplemented instructions, this is
correct address
Hardware interrupt exceptions must resume execution from
the interrupted instruction
The exception handler must subtract 4 from ea
