Interrupt Controller (8259A)
EOI
The EOI bit is used for all End-Of-Interrupt commands
(not an automatic End-Of-Interrupt mode). If EOI is set
to 1, a form of End-Of-Interrupt command is executed
depending on the state of the SL and R bits. If EOI is 0,
an End-Of-Interrupt command is not executed.
SL
The SL bit is used to select a specific level for a given
operation. If SL is set to 1, the L0-L2 bits are enabled.
The operation selected by the EOI and R bits is executed
on the specified interrupt level. If SL is 0, the L0-L2 bits
are disabled.
R
The R bit is used to control all 8259A rotation
operations. If the R bit is set to 1, a form of priority
rotation is executed depending on the state of the SL and
EOI bits. If R is 0, rotation is not executed.
OCW3
OCW3 is used to issue various modes and commands to the 8259A. It
is located at I/O address 20h. Two main categories of operation are
associated with OCW3: interrupt status and interrupt masking. Bit
definition of OCW3 is as follows:
RIS
The RIS bit is used to select the In-Service register (ISR)
or Interrupt Request register (IRR) for the read register
command. If RIS is set to 1, ISR is selected. If RIS is 0,
IRR is selected. The state of the RIS is honored only if
the RR bit is 1.
12-19
Содержание ZT 8809A
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