Multiple Interrupt Servicing - NEC 78K0 User Manual

19.4.3 Multiple interrupt servicing

Multiple interrupt servicing occurs when another interrupt request is acknowledged during execution of an interrupt.
Multiple interrupt servicing does not occur unless the interrupt request acknowledgment enabled state is selected
(IE = 1). Also, when an interrupt request is acknowledged, interrupt request acknowledgment becomes disabled (IE =
0). Therefore, to enable multiple interrupt servicing, it is necessary to set (1) the IE flag with the EI instruction during
interrupt servicing to enable interrupt acknowledgment.
Moreover, even if interrupts are enabled, multiple interrupt servicing may not be enabled, this being subject to
interrupt priority control. Two types of priority control are available: default priority control and programmable priority
control. Programmable priority control is used for multiple interrupt servicing.
In the interrupt enabled state, if an interrupt request with a priority equal to or higher than that of the interrupt
currently being serviced is generated, it is acknowledged for multiple interrupt servicing. If an interrupt with a priority
lower than that of the interrupt currently being serviced is generated during interrupt servicing, it is not acknowledged
for multiple interrupt servicing. Interrupt requests that are not enabled because interrupts are in the interrupt disabled
state or because they have a lower priority are held pending. When servicing of the current interrupt ends, the
pending interrupt request is acknowledged following execution of one main processing instruction execution.
Table 19-5 shows relationship between interrupt requests enabled for multiple interrupt servicing and Figure 19-10
shows multiple interrupt servicing examples.
Table 19-5. Relationship Between Interrupt Requests Enabled for Multiple Interrupt Servicing
Multiple Interrupt Request
Interrupt Being Serviced
Maskable interrupt
Software interrupt
Remarks 1.
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CHAPTER 19 INTERRUPT FUNCTIONS
During Interrupt Servicing
IE = 1
ISP = 0
ISP = 1
: Multiple interrupt servicing enabled
2. ×: Multiple interrupt servicing disabled
3. ISP and IE are flags contained in the PSW.
ISP = 0: An interrupt with higher priority is being serviced.
ISP = 1: No interrupt request has been acknowledged, or an interrupt with a lower
priority is being serviced.
IE = 0: Interrupt request acknowledgment is disabled.
IE = 1: Interrupt request acknowledgment is enabled.
4. PR is a flag contained in PR0L, PR0H, PR1L, and PR1H.
PR = 0: Higher priority level
PR = 1: Lower priority level
User's Manual U15947EJ2V0UD
Maskable Interrupt Request
PR = 0 PR = 1
IE = 0 IE = 1
*
× ×
×
×
Software
Interrupt
Request
IE = 0
×
×
×