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IBM System/360 System Programmer's Guide

IBM System/360
137 pages
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devices
supported
by
the
operating
system,
the
data
extent
block
is
produced
during
execution
of
the
OPEN
macro-instruction
for
the
data
control
block.
The
system
places
the
address
of
the
data
extent
block
into
the
data
control
block.
CHANNEL
PROGRAM
EXECUTION
This
section
explains
how
the
system
uses
your
channel
program
and
control
blocks
after
the
EXCP
macro-instruction
has
been
issued.
INITIATION
OF
CHANNEL
PROGRAM
By
issuing
the
EXCP
macro-instruction,
you
request
the
execution
of
the
channel
program
specified
in
the
input/output
block.
The
input/output
supervisor
checks
the
request
for
validity
by
ensuring
that
the
required
control
blocks
contain
the
correct
information.
If
they
do
not,
abnormal
termination
procedures
are
initiated.
A
program
check
occurs
if
the
control
blocks
are
not
on
correct
boundaries.
The
input/output
supervisor
obtains
"the
address
of
the
data
control
block
from
the
input/output
block
and
the
address
of
the
data
extent
block
from
the
data
control
block.
From
the
data
extent
block,
the
system
obtains
the
address
of
the
unit
control
block
(UCB)
for
the
desired
I/O
device.
To
protect
and
facilitate
reference
to
the
addresses
of
the
lOB,
DEB,
and
UCB,
the
input/output
supervisor
places
these
addresses,
along
with
other
information
about
the
channel
program,
into
an
area
called
a
request
element.
The
request
element
is
used
by
the
input/output
supervisor
for
forming
queues
to
keep
track
of
I/O
requests.
A
channel
program's
request
element
is
"available"
if
the
information
it
contains
is
no
longer
to
be
used
by
the
input/output
supervisor
and
if
it
is
ready
to
receive
information
about
another
request.
When a
request
element
is
"made
available",
it
is
removed
from
all
request
queues
and
placed
on
a
queue
of
available
request
elements.
You
are
not
concerned
with
the
contents
of
the
request
element
unless
you
have
provided
appendage
routines,
as
explained
in
the
section
"Appendages."
After
completing
the
request
element
for
the
channel
program,
the
input/output
supervisor
deterITines
whether
a
channel
and
the
requested
I/O
device
are
ready
for
the
channel
prograrr·.
If
they
are
not
ready,
the
request
element
is
placed
into
the
appropriate
queue,
and
control
is
returned
to
the
problem
program.
The
channel
program
is
subsequently
executed
when
the
channel
and
device
are
ready.
To
initiate
execution
of
the
channel
program,
the
system
obtains
its
address
from
the
input/output
block,
places
this
address
into
the
channel
address
word
(CAW),
and
issues
a
start
input/output
(SIO)
instruction.
Before
issuing
the
SIO
instruction
for
direct-access
devices,
the
system
issues
the
initial
seek,
which
is
overlapped
with
other
operations.
You
specify
the
seek
address
in
the
input/output
block.
When
the
seek
has
completed,
the
system
constructs
a command
chain
to
reissue
the
seek,
set
the
file
mask
specified
in
the
data
extent
block,
and
pass
control
to
your
channel
program.
(When
using
the
operating
system,
you
cannot
issue
the
initial
seek
cr
set
the
file
mask
yourself.
)
Before
issuing
SIO
for
magnetic
tape
devices,
the
system
constructs
a
command
chain
to
set
the
mode
specified
in
the
data
extent
block
and
pass
control
to
your
channel
prograIT.
(When
using
the
operating
system,
you
cannot
set
the
mode
yourself.)
78

Table of Contents

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IBM System/360 Specifications

General IconGeneral
BrandIBM
ModelSystem/360
CategoryDesktop
LanguageEnglish

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