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5. Exception Processing
5.4.2 Interrupt Priority Level
The interrupt priority order is predetermined. When multiple interrupts occur simultaneously
(overlapped interruptions), the interrupt controller (INTC) determines their relative priorities and
starts the exception processing according to the results.
The priority order of interrupts is expressed as priority levels 0 to 16, with priority 0 the lowest
and priority 16 the highest. The NMI interrupt has priority 16 and cannot be masked, so it is
always accepted. The priority level of user break interrupt and H-UDI is 15. IRQ interrupts and
on-chip peripheral module interrupt priority levels can be set freely using the INTC’s interrupt
priority registers A to J (IPRA to IPRJ) as shown in table 5.8. The priority levels that can be set
are 0 to 15. Level 16 cannot be set. See section 6.3.4, Interrupt Priority Registers A to J (IPRA to
IPRJ), for more information on IPRA to IPRJ.
Table 5.8 Interrupt Priority
Type
NMI
User break
H-UDI
IRQ
On-chip peripheral module
Priority Level
16
15
15
0 to 15
Comment
Fixed priority level. Cannot be masked.
Fixed priority level.
Fixed priority level.
Set with interrupt priority registers A to J (IPRA
to IPRJ).
5.4.3 Interrupt Exception Processing
When an interrupt occurs, the interrupt controller (INTC) ascertains its priority level. NMI is
always accepted, but other interrupts are only accepted if they have a priority level higher than the
priority level set in the interrupt mask bits (I3 to I0) of the status register (SR).
When an interrupt is accepted, exception processing begins. In interrupt exception processing, the
CPU saves SR and the program counter (PC) to the stack. The priority level value of the accepted
interrupt is written to SR bits I3 to I0. For NMI, however, the priority level is 16, but the value set
in I3 to I0 is H'F (level 15). Next, the start address of the exception service routine is fetched from
the exception processing vector table for the accepted interrupt, that address is jumped to and
execution begins. See section 6.6, Operation, for more information on the interrupt exception
processing.
Rev.4.00 Mar. 27, 2008 Page 70 of 882
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