Zilog Z08470 - Manual (Page 327)

Z80 CPU Peripherals
User Manual
UM008101-0601 Serial Input/Output
307
Figure 130. Interrupt Acknowledge Cycle Timing
Return From Interrupt Cycle
Normally, the Z80 CPU issues a RETI, RETurn from Interrupt, instruction
at the end of an interrupt service routine. The RETI is a 2-byte Op Code,
ED-4D, which resets the interrupt-underservice latch that terminates the
interrupt just processed. This is accomplished by manipulating the daisy-
chain in the following way.
The normal daisy-chain operation can detect a pending interrupt; however,
it cannot distinguish between an interrupt under service and a pending
unacknowledged interrupt of a higher priority. Whenever ED is decoded,
the daisy-chain is modified by forcing High the IEO of any interrupt not yet
acknowledged.
Thus the daisy-chain identifies the device presently under service as the
only one with an IEI High and an IEO Low. If the next Op Code byte is 4D,
the interrupt-underservice latch is reset. See Figure 131.
T
1
Φ
IORQ
RD
M1
Data
T
2
T
W
T
3
T
4
IEI
Vector
T
W