Interrupt
Servicing
Servicing Priority
Input/Output
Operation
•
If
an
input/output
operation
has
been completed
and
chaining
has
not
been specified,
the
input/output
device control electronics causes
the
appropriate
channel
interrupt
flag
to
be
set
in
the
Interrupt
Flag
register.
If
the
Interrupt
Mask
register
for
the
current
processor
state
permits
the
interrupt,
it
is taken.
At
this
time
the
channel
asks
the
processor
for
another
service request. This service
request
is
required
so
that
the
channel
can
transfer
information
concerning
the
status
of
the
device
and
the
channel to
the
input/output
channel
registers
in
scratch-pad memory.
If
the
interrupt
is caused by a device on
the
multiplexor channel,
the
appro-
priate
subchannel
registers
are
transferred
from
non-addressable
main
memory to scratch-pad memory.
Because all
input/output
servicing requires
that
the
channel utilize
main
memory, scratch-pad memory
and
nonaddressable
main
memory
(multiplexor devices), normal processor operation is
held-off
until
the
servicing
has
been completed. Servicing is
time-shared
with
normal
mode processing.
• Because
input/output
operations on all selector channels
and
the
mul-
tiplexor channel proceed simultaneously,
the
processing is stopped
if
servicing is required
and
the
input/output
device is serviced.
After
a
device is serviced, processing resumes,
or
another
device is serviced.
Each
selector channel
and
the
multiplexor channel
has
a
scanning
priority.
If
servicing is required by devices on more
than
one channel,
the
channel
with
the
highest
priority
is serviced first.
The
priority
is
as
follows:
Selector Channel
No.1
Selector Channel No. 2
Selector Channel No. 3
Selector Channel No. 4
Multiplexor Channel
The devices on
the
mUltiplexor have a
priority
depending upon
the
standard
interface
trunk
to which
they
are
connected;
the
lower
the
standard
interface
trunk
in
the
scanning
sequence,
the
higher
the
priority.
Servicing
of
a device connected to
the
multiplexor channel
may
be
temporarily
interrupted
by a selector channel service request.
If
this
occurs,
all selector channels
requiring
service
are
served before multiplexor channel
servicing resumes.
The
optimum
connection
of
device control electronics to selector chan-
nels
and
the
multiplexor channel depends on
the
requirements
of
each
installation. However, a general rule is to connect
the
device control elec-
tronics
which control devices
with
the
highest
data
transfer
requirements
to
the
channels
with
the
highest
priority. The
remaining
device control
electronics
are
connected
in
descending
order
of
data
transfer
require-
ments
to
descending
priority
sequence
of
channels. Buffered devices should
always have lowest
priority.
39
To Next Page
Loading...
Need help?
General
