Tandy 1000
inter
SPECIAL MASK MODE
Technical Reference Manual
8259A18259A-2/8259A-8
POLL COMMAND
Some
applications
may require an
interrupt
service
routine
to
dynamically
alter
the system
priority
struc·
ture
during
its
execution
under
software
control.
For
example.
the
routine
may
wish
to
inhibit
lower
priority
requests
for
a
portion
of
its
execution
but enable
some
of
them for
another
portion.
The
difficulty
here is
that
if
an
Interrupt
Request is
acknowledged and
an
End
of
Interrupt
command
did
not
reset
its
IS
bit
(i.e
.•
while
executing
a service routine).
the 8259A
would
have
inhibited
all
lower
priority
requests
with
no
easy way
for
the
routine
to
enable
them
That is where
the
Special Mask Mode
comes
in. In
the
special Mask Mode, when a mask bit is set
in
OCW1.
it
inhibits
further
interrupts
at that level
and
enables inter-
rupts from
a/l
other
levels (lower as well as higher)
that
are
not
masked.
Thus. any
interrupts
may be selectively enabled by
loading
the
mask register.
The special Mask Mode is set by OCW3 where:
SSMM =
1.
SMM =
1.
and cleared where SSMM =
1.
SMM=O.
LT
.....
IT
O=EDGE
l=lE"El
I
IOGI
I
LS:~~~
I
In
this
mode
the
INT
output
is
not
used
or
the
micropro-
cessor internal
Interrupt
Enable
flip-flop
is reset.
disabling
its
interrupt
input.
Service
to
devices
is
achieved
by
software
using a Poll
command.
The Poll
command
is
issued
by
selling
P=
"1"
in OCW3.
The 8259A
treats
the
next
An
pulse
to
the 8259A (i.e.,
AD
=
0,
C"S
=
0)
as
an
interrupt
acknowledge.
sets
the
appropriate
IS
bit
if
there is a request. and reads the
priority
level.
Interrupt
is frozen from
WFlto
AD
The
word
enabled
onto
the
data
bus
during
m5
is:
07
oe
05
04
03 02
01
DO
~
W2 WI
wol
WO-W2:
Binary
code
of
the highest
priority
level
requesting
service
I:
Equal to a
"1"
if
there is an
interrupt.
This
mode
is
useful
if
there is a
routine
command
com-
mon
to
several
levels
so
that
the
iNn
sequence
is
not
needed (saves ROM space).
Another
application
is
to
use
the
poll
mode
to
expand the
number
of
priority
levels
to
more
than
64.
"""
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OU"I9III(I ICWI
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ACTI"VI
OVllllNG
1iiT'i,
.""lJ
fIOLL
SlOUIJriCfS
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J TIlIIUTH T
....
lE
fOIll
0
lATe",
~-+-~-~-----+-~~:-z:..~
o
II.
QrI_l
HOLD
Figure
9.
Priority
Ceil-Simplified
Logic
Diagram
2-131
AFN-00221E
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