IBM System/360 System Programmer's Guide

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

IBM System/360 Specifications

General

Brand	IBM
Model	System/360
Category	Desktop
Language	English