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Rotate Priority .iMode B) Independent of EOI - Same as Rotate at EOI
(Mode B), except that EOI is not required.
Fixing of priorities may be
done at any time.
The third Operation Control Word (OCW3) has two basic functions.
Bits
o
and 1 are used to read either the Interrupt Service Register (ISR) or
the Interrupt Request Register (IRR) to determine the interrupt status.
The second function is to set or reset the Special Mask Mode.
When a
mask bit is set in OCW1, the Special Mask Mode inhibits further
interrupts at that
1
evel and enables interrupts from other
1
evel s,
lower as well as higher, that are not masked.
This permits inhibiting
only the masked bi ts set in OCW1, even if an EO I command was not
received to reset the corresponding ISR bit.
Bit 2 of OCW3 selects either the polling mode (bit 2
=
1) or interrupt
mode (bi t
2
=
0).
When the poll i ng mode is act i ve , a CPU read at
address
FFC8H
is
treated
as
an
interrupt
acknowl edge.
The
corresponding ISR bit is set to indicate which interrupt input needs
service.
The byte read at FFC8H contains two pieces of information;
bit
7
is set if an interrupt request exi sts, and the three
1
east
significant bits
(0
to
2)
are coded
(000
to 111) to indicate the
interrupt
1
evel •
The software has the task of determi ni ng how to
handl e requests.
Before Rotate
PRIORITY STATUS
~~_..l..--_.l.....-_.l.....-_.l....-_.l....--.J
Lowest
Highest
ISR
~~_..l..--_.l.....-_.l.....-_.l....-_.l....--.J
After Rotate
PRIORITY STATUS
~~_...l..-_.l.....-_...L-._.l.-.-_.l.-.--.J
Highest
Lowest
Figure 3-11.
Rotating Priority Mode A (Automatic Rotation)
3-16
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