ZiLOG Z80 User Manual page 282

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B generates an interrupt and is acknowledged. The interrupt
enable out, IEO, of B goes Low, blocking any lower priority
devices from interrupting while B is being serviced. Then A gen-
erates an interrupt, suspending service of B. The IEO of A goes
Low, indicating that a higher priority device is being serviced.
The A routine is completed and a
of A, allowing the B routine to continue. A second
on completion of the B routine and the IE0 of B is reset (high)
allowing lower priority devices interrupt access.
is issued resetting the IEO
RETI
RETI
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is issued