Using XDAP

To issue XDAP, provide the actual track address of the track containing the block to be processed. Also provide either the block identification or the key of the block, and specify which you should use to locate the block. If a block is located by identification, both the key and data portions of the block can be read or updated. If a block is located by key, only the data portion can be processed.

For additional control over I/O operations, you can write appendages that must be entered into the LPA library. Descriptions of these routines and their coding specifications are included under IDAW Requirements for EXCP Requests.

When using the XDAP macro instruction, code a DCB macro instruction in your program to generate a data control block (DCB) for the data set to be read or updated. Also code an OPEN macro instruction that initializes the data control block and produces a data extent block (DEB). The OPEN macro instruction must be executed before any XDAP macro instructions are executed.

When the XDAP macro instruction is assembled, a control block and the following executable code are generated. This control block can be logically divided into three sections:

An event control block (ECB) that is supplied with a completion code each time the direct access channel program is terminated.
An input/output block (IOB) that contains information about the direct access channel program.
A direct access channel program consisting of three or four channel command words (CCWs). The type of channel program generated depends on the parameters of the XDAP macro instruction. When executed, it locates a block by either its actual address or its key and reads, updates, or verifies the block.

When the channel program has terminated, a completion code is placed into the event control block. After issuing XDAP and the direct access program has terminated, regain control by using a WAIT macro instruction to specify the address of the event control block. If volume switching is necessary, issue an EOV macro instruction. Once processing of the data set is completed, issue a CLOSE macro instruction to restore the data control block.

The parameters of the XDAP macro instruction are described in Executing Direct Access Programs.