EXCEPTION PROCESSING STATE (INTERRUPT PROCESSING)
On all level-sensitive interrupts, the interrupt must be externally released be-
fore interrupts are internally re-enabled. Otherwise, the processor will be in-
terrupted repeatedly until the release of the level-sensitive interrupt occurs.
The edge sensitive NMI is a priority 3 interrupt and cannot be masked. Only RESET and
illegal instruction have higher priority than NMI.
18.104.22.168 Software Interrupt Sources
There are two software interrupt sources — software interrupt (SWI) and illegal instruc-
tion interrupt (III).
SWI is a nonmaskable interrupt (IPL 3), which is serviced immediately following the SWI
instruction execution, usually using a long interrupt service routine. The difference
between an SWI and a JSR instruction is that the SWI sets the interrupt mask to prevent
interrupts below IPL 3 from being serviced. The SWI's ability to mask out lower level
interrupts makes it very useful for setting breakpoints in monitor programs. The JSR
instruction does not affect the interrupt mask.
The III is also a nonmaskable interrupt (IPL 3). It is serviced immediately following the
execution or the attempted execution of an illegal instruction (any undefined operation
code). IIIs are fatal errors. Only a long interrupt routine should be used for the III routine.
RTI or RTS should not be used at the end of the interrupt routine because, during the III
service, the JSR located in the III vector will normally stack the address of the illegal
instruction (see Figure 7-4). Returning from the interrupt routine would cause the proces-
sor to attempt to execute the illegal interrupt again and cause an infinite loop which can
only be broken by cycling power. This long interrupt (see Figure 7-4) can be used as a
diagnostic tool to allow the programmer to examine the stack (MOVE SSH, dest) and
locate the illegal instruction, or the application program can be restarted with the hope
that the failure was a soft error. The illegal instruction is useful for triggering the illegal
interrupt service routine to see if the III routine can recover from illegal instructions.
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