Z80 Datasheet, PDF (49/330 Pages) Zilog, Inc. – Z80 CPU PERIPHERALS

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Highest Priority Channel
Channel 0
Channel 1
Channel 2
Channel 3
+
HI
IEI
HI
IEO
IEI
IEO ^HI IEI
IEO ^HI IEI
HI
IEO
1. Priority interrupt daisy chain before any interrupt occurs.
+
HI _IEI
IEO ^HI IEI
Under Service
IEO ^HI IEI
IEO
LO _IEI
2. Channel 2 requests an interrupt and is acknowledged.
+
HI _IEI
Under Service
Service Suspended
IEO ^HI IEI
IEO ^LO IEI
IEO ^LO IEI
3. Channel 1 interrupts, suspends servicing of Channel 2.
+
HI _IEI
Service Complete Service Resumed
IEO ^HI IEI
IEO ^HI IEI
IEO ^LO IEI
4. Channel 1 service routine complete, ‘RETI’ issued, Channel 2 serviced resumed.
+
HI _IEI
IEO ^HI IEI
Service Complete
IEO ^HI IEI
IEO ^HI IEI
5. Second ‘RETI’ instruction issued on completion of Channel 2 Service Routine.
LO
IEO
LO
IEO
LO
IEO
HI
IEO
Figure 14. Daisy-Chain Interrupt Servicing
UM008101-0601
Counter/Timer Channels