
139
μ
PD75237
Table 5-3 IST1 and IST0 Interrupt Servicing Statuses
When an interrupt is acknowledged, IST1 and IST0 are saved into the stack memory together with other PSW
and is changed to a status higher by one level. When RET1 instruction is executed, the original IST1 and IST0
values are reset.
5.4
MULTI-INTERRUPT SERVICE CONTROL
The following two methods are available for the
μ
PD75237 to generate multi-interrupts.
(1)
Multi-interruption specifying high interrupt
This is a standard multi-interrupt method of the
μ
PD75237 in which one interrupt source is selected and
multi-interruption (dual interrupt) is enabled.
In other words, the high interrupt specified using the interrupt priority select register (IPS) is enabled when
the status of the operation being executed is 0 or 1. All other interrupts (low interrupts) are only enabled when
the status is 0. (Refer to
Fig. 5-6 and Table 5-3
.)
Fig. 5-6 Multi-Interruption by High Interrupt
Interrupt Disable
IPS Set
Interrupt Enable
Low or High
Interrupt Generated
High
Interrupt
Generated
Normal Processing
(Status 0)
Low or High
Interrupt
Servicing
(Status 1)
High Interrupt
Servicing
(Status 2)
After Interrupt
Acknowledgement
IST1
IST0
0
1
1
0
–
–
Interrupt Acknowledgeable
Interrupt Request
All interrupts acknowledgeable
Only high interrupt acknowledgeable
All interrupts not acknowledgeable
CPU Processing
Contents
Normal program being
processed
Low or high interrupt
being servicing
High interrupt being
servicing
Status of Servicing
being Executed
Status 0
Status 1
Status 2
IST1
IST0
0
0
0
1
1
0
1
1
Setting prohibited