Multiple Interrupt Servicing - NEC 78K0/KD1 Series User Manual

16.4.3 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 acknowledgement enabled state is selected
(IE = 1). Also, when an interrupt request is acknowledged, interrupt request acknowledgement 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 acknowledgement.
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 at least one main processing instruction execution.
Table 16-5 shows interrupt requests enabled for multiple interrupt servicing and Figure 16-10 shows multiple
interrupt servicing examples.
Table 16-5. Interrupt Request Enabled for Multiple Interrupt Servicing During Interrupt Servicing
Interrupt Being Serviced
Maskable interrupt
Software interrupt
Remarks 1.
CHAPTER 16 INTERRUPT FUNCTIONS
Multiple Interrupt Request
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 acknowledgement is disabled.
IE = 1: Interrupt request acknowledgement is enabled.
4. PR is a flag contained in PR0L, PR0H, and PR1L.
PR = 0: Higher priority level
PR = 1: Lower priority level
Preliminary User's Manual U16315EJ1V0UD
Maskable Interrupt Request
PR = 0
IE = 1 IE = 0 IE = 1
× ×
×
×
PR = 1
IE = 0
×
×
×
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