197
CHAPTER 6 INTERRUPT AND TEST FUNCTIONS
(2) Multiple interrupt processing by changing the interrupt status flags
Changing the interrupt status flags with the program causes multiple interrupts to be enabled. That is,
when the interrupt processing program changes both IST1 and IST0 to 0 (status 0), multiple interrupt
processing is enabled.
This method is used when two or more interrupts are to be enabled at a time or when the processing of
three or more interrupts is to be performed.
When changing IST1 and IST0, interrupts must be disabled beforehand with a DI instruction.
Figure 6-9. Multiple Interrupt Processing by Changing the Interrupt Status Flags
Low- or high-order 
interrupt occurrence
Normal processing 
(status 0) Single interrupt Dual interrupts
Interrupt is enabled.
Low- or high-order 
interrupt occurrence
Interrupt is 
disabled.
Modification 
of IST
Interrupt is 
enabled.
Status 1
Status 0
Status 0 Status 1
IPS setting
Interrupt is disabled.