Interrupt Controller (8259A)
Priority Resolver (PR)
All interrupt requests are latched into the IRR. Those not masked by
the IMR are input to the Priority Resolver (PR) to determine which is
to be serviced. The interrupt request with the highest priority is
transferred from the IRR to the ISR during the interrupt acknowledge
cycle.
The PIC includes several programmable operating modes that define
the rules by which the PR determines the highest priority interrupt
request. These modes range from all inputs having equal priority to
rotating the priorities each time an interrupt is serviced.
Interrupt In-Service Register (ISR)
The 8-bit interrupt In-Service register (ISR) maintains a bit position
for each interrupt request that is currently being serviced. More than
one bit can be set if an interrupt is currently under service and a
second interrupt request is acknowledged. The 8-bit ISR is read to
determine the status of the interrupts currently being serviced. A
logical 1 in a bit position means the interrupt is currently being
serviced.
Control Logic
This functional block directs the operation of the other PIC blocks
based on the programmed mode of operation. The Control Logic also
interfaces with the CPU for interrupt request and acknowledge
signals. An interrupt request is generated to the CPU if a PIC has the
correct priority and is not masked. If the CPU interrupts have been
enabled with the "set interrupt" command, the CPU will respond with
an interrupt acknowledge.
12-9
Содержание ZT 8809A
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