Interrupt Servicing
(Cont'd)
Block
1
Block 2
Block 3
Input/Output Operation
set. This
interrupt
condition
has
no effect upon
the
input/output
operation
specified
by
the
Channel Command Word. The
standard
device byte
and
the
subchannel
registers
are
not
stored.
Device Request
Interrupt-This
interrupt
occurs
as
a
result
of
a condition
arising
in
an
input/output
device control electronics.
It
may
occur inde-
pendent
of
a processor initiated
input/output
operation. Examples
of
this
type
of
interrupt
are
as
follows:
1.
A remote processor wishes to send
data
via a
Data
Exchange Control.
'The
Data
Exchange Control initiates
the
channel
interrupt.
(This
interrupt
occurs independent of a procesor
initiated
input/output
operation. )
2.
The processor initiates
an
off-line seek to a random access device.
When
the
seek is complete, the random access device control elec-
tronics initiates a channel
interrupt.
(This
interrupt
occurs in
conjunction
with
a processor initiated
input/output
operation.)
When
an
external device request
interrupt
occurs,
the
standard
device
byte
and
the
sub channel
registers
(if
a multiplexor device)
are
stored
in
the
appropriate
input/output
channel registers.
Terminating
Interrupt-This
interrupt
occurs when
an
input/output
operation initiated
by
the
processor
has
terminated. When
this
interrupt
occurs,
the
standard
device byte
and
the
subchannel registers
(if
a multi-
plexor device)
are
stored in
the
appropriate
input/output
channel registers.
This is
the
final servicing of
the
channel
and
device.
At
the
completion
of
this
servicing,
the
channel is free to accept
another
operation. The
contents
of
the
input/output
channel
registers
must
be utilized by
the
program
before
another
operation is initiated. (When
another
operation is
initiated,
the
contents of these
registers
are
altered.) The following infor-
mation is available
in
the
input/output
channel
registers
for
interrogation
by
the
program:
Channel
status
byte
Standard
device byte
Byte count
Address of
next
CCW
Low-order 4
bits
of
the
command code
Device
number
Interrupt
servicing causes
the
following events
to
occur (see figure
11).
• The device control electronics is asked
for
the
address of
the
device
requiring
interrupt
servicing.
• A
test
is made
to
see
if
the
device control electronics is operable.
The
device control electronics
has
50
microseconds to signal the processor
that
it
is operable.
If
it
does not,
the
processor generates a
standard
device byte
of
all zeros. Control is
then
transferred
to Block
4.
•
If
the
device control electronics is operable,
it
sends
the
standard
device
byte
to
the
processor.
79
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