Multiple Interrupt Servicing - NEC UPD789426 Series User Manual

Figure 14-13. Interrupt Request Acknowledgment Timing (Example: MOV A, r)
Clock
CPU
Interrupt
If the interrupt request has generated an interrupt request flag (XXIF) by the time the instruction clocks under
execution, n clocks (n = 4 to 10), are n − 1, interrupt request acknowledgment processing will start following the
completion of the instruction under execution. Figure 14-13 shows an example using the 8-bit data transfer instruction
MOV A, r. Because this instruction is executed in 4 clocks, if an interrupt request is generated between the start of
execution and the 3rd clock, interrupt request acknowledgment processing will take place following the completion of
MOV A, r.
(When Interrupt Request Flag Is Generated in Final Clock Under Execution)
Clock
CPU NOP
Interrupt
If the interrupt request flag (XXIF) is generated in the final clock of the instruction, interrupt request
acknowledgment processing will begin after execution of the next instruction is complete.
Figure 14-14 shows an example whereby an interrupt request was generated in the 2nd clock of NOP (a 2-clock
instruction). In this case, the interrupt request will be processed after execution of MOV A, r, which follows NOP, is
complete.
Caution When interrupt request flag registers 0 and 1 (IF0 and IF1), or interrupt mask flag registers 0 and
1 (MK0 and MK1) are being accessed, interrupt requests will be held pending.

14.4.3 Multiple interrupt servicing

Multiple interrupts, in which another interrupt request is acknowledged while an interrupt request being serviced,
can be serviced using the priority order. If multiple interrupts are generated at the same time, they are serviced in the
order according to the priority assigned to each interrupt request in advance (refer to Table 14-1).
CHAPTER 14 INTERRUPT FUNCTIONS
MOV A, r
Figure 14-14. Interrupt Request Acknowledgment Timing
MOV A, r
User's Manual U15075EJ2V1UD
8 clocks
Saving PSW and PC, and
Interrupt servicing program
jump to interrupt servicing
8 clocks
Saving PSW and PC, and
jump to interrupt servicing
Interrupt servicing
program
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