Intel 386

9-11
INTERRUPT CONTROL UNIT
Figure 9-3. Interrupt Process – Master Request from Non-slave Source
A2427-01
Master receives an interrupt request. (From a non-slave source.)
Master sets the request's pending bit.
Is
request
enabled?
Is
special
mask mode
enabled?

Is master
operating in
special-fully
nested
mode?
(operating in
fully nested
mode)
Is
the
in-service
bit for this
request
set?
Is
request
higher level
than any set
in-service
bits?
Is
request
equal or higher
than any set
in-service
bits?
Master sends request to CPU. CPU initiates interrupt acknowledge cycle.
Master clears request's pending bit, sets its in-service bit, and puts its
interrupt vector number on the bus.
An interrupt return instruction is issued, ending the interrupt process.
Is
master in
AEOI
mode?
Master clears its in-service bit. The
CPU uses its operating mode and the
interrupt vector number to find the
interrupt service routine's address.
CPU begins processing interrupt.
The interrupt service routine sends an EOI command, causing the master
to clear its in-service bit.
End
Yes No No
YesYesNo
Yes No No
YesYesNo
Yes
No
The CPU uses its operating mode and the interrupt vector number to find
the interrupt service routine's address. CPU begins processing interrupt.