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The next ICW expected by the 8259A is ICW2.
The ICW2 format uses bits
3 to 7 (All to A15) to determine part of the interrupt vector.
The
three lower bits, 0 to 2, of ICW2 become address bits 8 to 10 and are
inserted by the 8259A to form the complete interrupt vector.
ICW2 bits
3 to 7 are determined by the programmer.
Initialization Control Word ICW4 is the next to be programmed into the
8259A.
Bit 0 of ICW4 must be set for use with the Am96/4116.
When an
interrupt is acknowledged, two bytes are output to the bus.
The first
byte is ignored and the second byte contains the interrupt vector.
If ICW4 bi t 1 is zero, the CPU must output an End of Interrupt (EOI)
command to the 8259A.
If bi
t
1 is set, then an Automatic EO I (AEO I)
will be generated at the trailing edge of the second (last) lACK pulse.
IC~J4
bits 2 and 3 should be zeros.
OPERATION C'ONTROL
The 8259A uses Operat i on Control Words (OCWs) to set interrupt mask
bits, command various operations, and allow the CPU to read the
Interrupt Request Register (IRR) and Interrupt Service Register (ISR)
to determine status.
Figure 3-10 illustrates the three DeWs,.
OCW1 is
the interrupt mask register.,
When a particular bit is set, the
corresponding interrupt input is masked (inhibited).
Any bits cleared
will enable the corresponding interrupts.
Reading the IRR allows the
programmer to look at which bits are masked or unmasked.
Bit 0 of DCW1
is input 0 and bit 7 represents input 7.
The OCW2 format uses bits 5 through 7 for control of the Rotate and EOI
modes.
Figure 3-10 illustrates the various modes.
Bits 0 to 2 are
used to indicate a particular interrupt level to be controlled.
The
following are descriptions of the OCW2 commands:
Non-Specific EOI - Automatically resets the highest ISR bit of the ones
that are set.
Specific EOI -
Resets only the ISR bit specified by bits LO-L2 of
OC~J2
•
Rotate at EOI Automatically (Mode A) - Rotates priorities after
receiving an EOl or AEOI command.
Figure 3-9 illustrates the rotate
after level 2 interrupt was serviced and an EOI received.
Rotate at EOI
Mode B_ - Allows the programmer to fix the bottom
priority bits LO-L2
in order to fix all other priorities.
if level 3
is fixed as the lowest priority device, then level 4 will be the
highest.
Set Rotate A F/F - Allows for automatic rotation of priorities when the
AEOI mode is programmed.
Clear Rotate A F/F - Disables automatic rotation on AEOI.
3-14
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