INPUT
/OUTPUT
OPERATIONAL
CONTROL
Programming
Considerations
Prior to
Input/Output
Initiation
Input/Output
Initiation
Channel
Servicing
Servicing
a
Data
Transfer
End
and
Chaining
Servicing
I
nput/
Output
Operation
• All
input/output
operations
are
executed by
the
selected channel
and
are
independent
of
normal processor operation.
Prior
to
initiation
of
an
input/output
operation,
the
program
must
supply
information
concerning
the
operation.
The
program
must
store
information
in
main
memory, such
as
the
type
of
operation
(read, write,
etc.),
the
data
area
address
in
main
memory
at
which to begin
the
operation,
and
the
number
of
bytes to be
transferred
by
the
channel. This
information
is called
the
Channel Com-
mand
Word
(CCW).
After
the
channel command word is
stored
in
main
memory,
the
address
of
this
CCW
must
be stored in a
standard
main
memory
location.
This
standard
location is called
the
Channel Address Word ( CAW)
and
is
main
memory locations 72
through
75.
When
in
70/46
Mode
(T
= 1)
the
address
of
the
Channel Control Block
(CCB)
must
also be stored in a
standard
main
memory
location.
This
standard
location is called
the
Channel Block Address (CBA)
and
is
main
memory locations 76
through
79. This
address
gives
the
software
the
address
of
the
Device
Status
Code.
Once
the
Channel Address Word, Channel Block Address,
and
the
Chan-
nel Command Word
have
been assembled,
the
input/output
operation
can
be initiated.
• All
input/output
operations
are
initiated by executing a
Start
Device
instruction
or
by
manually
pressing
the
LOAD
pushbutton/indicator
on
the
Model 70/97 Console. Execution
of
the
Start
Device
instruction
causes
the
information
contained in
the
Channel Address
Word
(
CAW),
Channel
Block Address (CBA)
and
the Channel Command
Word
(CCW) to be
transferred
to
the
input/output
channel
registers
in
scratch-pad
memory
for
the
specified selector channel.
If
the
specified channel is
the
mUltiplexor
channel,
this
information
is
transferred
to
the
sub channel
registers
in
non-
addressable
main
memory
for
the
specified device. Once
this
has
been
accomplished, the
Start
Device
instruction
terminates
and
the
input/output
operation
has
been initiated. Completion
of
the
input/output
operation
is
under
control
of
the
channel,
and
normal processor
operation
can
proceed.
• When
an
input/output
operation
has
been
initiated
and
the
input/
output
device control electronics is
ready
to
send
or
receive a
data
byte,
the channel asks
the
processor
for
a service request. \Vhen
the
processor
permits
the
service request, a
data
transfer
occurs.
This
service
permits
the
transfer
of
a
data
byte
between
main
memory
and
the
input/output
device
to
occur.
It
also
updates
the
information
in
the
input/output
channel
registers
or
the
sub channel
registers
(multiplexor) to
prepare
for
the
next
data
byte.
•
vVhen
an
input/output
operation
has
been completed,
the
channel
asks
the
processor
for
another
service request.
This
service
request
is
required
so
that
the
channel
can
(1)
tell
the
device control electronics to
set
a
channel
interrupt
condition,
or
(2)
check
the
current
command to see
if
chaining is specified,
and
if
it
is
to
initiate
the
next
command.
38
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