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EXECUTE DIRECT ACCESS PROGRAM (XDAP) MACRO-INSTRUCTION
Execute
Direct
Access
Program
eXDAP)
is
a
roacro-instruction of
System/360 Operating System that you rray use to read, verify, or
update
a
block
on
a direct-access volume.
If you are not using the standard
IBM data access methods, you can, by issuing XDAP, generate the
control
information
and
channel
program necessary for reading or updating the
records of a data set.
You cannot use XDAP to add blocks to a data set, but you can
use
it
to
change the keys of existing blocks.
Any block configuration and any
data set organization can be read or updated.
Although the use of XDAP requires much less main storage
space
than
do
the standard access methods, it does not provide many of the control
program services that are
inc~uded
in the access methods.
For
example,
when
XDAP
is
issued, the system does not block or deblock records and
does not verify block length.
To issue XDAP, you must provide the
actual
device
address
of
the
track
containin:j
the
block
to
be
processed.
Yeu roust also provide
either the block identification or the key of
the
tlock,
and
specify
which of these is to be used to locate the block.
If a block is located
by
identification,
both
the key and data portions of the block may be
read or updated.
If a block is located by key, only
the
data
portion
can be processed.
REQUIREMENTS FOR EXECUTION OF DIRECT-ACCESS PROGRAM
Before
issuing
the
XDAP
macro-instruction,
you
roust issue a DCB
macro-instruction, which produces a data control
block
(DCB)
for
the
data
set
to
be
read
or
ppdated.
You
must
also
issue
an
OPEN
macro-instruction, which initializes the data control block and produces
a data extent block (DEB).
When the XDAP macro-instruction is
issued,
another
control
block,
containing
both
control information and executable code, is generated.
This control block may be logically divided into three sections:
• An event control block (ECB), which is supplied
with
a
completion
code each time the direct access channel program is terminated.
• An
input/output
block
(lOB), which contains information about the
direct access channel program.
• A direct access channel program, which
consists
of
three
channel
command words (CCWs).
The type of channel program generated depends
on specifications in the parameters of the XDAP macro-instruction.
After
this XDAP control block is constructed, the direct-access channel
program is executed.
A block is located by either its actual address or
its key, and is either read or updated.
When the channel program has terminated, a completion code is
placed
into
the event control block.
After issuing XDAP, you should therefore
issue a WAIT macro-instruction specifying the
event
control
block
to
determine
whether
the direct-access program has terminated.
If voluwe
switching is necessary, you must issue an EOV
macro-instruction.
When
processing
of
the
data set has been completed, you must issue a CLOSE
macro-instruction to restore the data control block.
Execute Direct Access Program (XDAP) Macro-Instruction
99
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