Interrupt Controller (8259A)
8259A I/O PORT ADDRESSES
The 8259A programmable interrupt controller on the ZT 8809A uses
I/O port addresses 20h or 21h. I/O address 20h is used to write ICW1,
OCW2, and OCW3 and to read IRR, ISR, and the interrupt level (IL)
(when the PIC is programmed for poll mode). I/O address 21h is used
to write ICW2-ICW4 and to read IMR.
Slave PICs, if employed, are accessed via the STD bus, and their I/O
addresses are determined by the board manufacturer.
Three OCWs are available for programming various modes and
commands. Unlike ICWs, the OCWs need not be in any sequential
order. Rather, they are issued by the processor as needed within a
program.
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