Achieving Device Independence

Device independence is the characteristic of programs that work on any type of device, DASD or tape, for example. Achieving device independence is possible only for a sequential data set because input or output can be on DASD, a magnetic tape drive, a card reader or card punch, a printer, a spooled data set, a TSO/E terminal, or a dummy data set. Other data set organizations (partitioned, direct, and VSAM) are device-dependent because they require the use of DASD.

A dummy data set is a DD statement on which the first parameter is DUMMY or you coded the DDNAME= parameter but there is no DD statement by that name. You can use BSAM or QSAM with a dummy data set. A WRITE or PUT macro has no effect. A GET macro or the CHECK macro for the first READ macro causes your EODAD routine to be called.

Device independence can be useful for the following tasks:

Accepting data from several recording devices, such as a disk volume, magnetic tape, or unit-record equipment. This situation could arise when several types of data acquisition devices are feeding a centralized installation.
Bypassing restrictions imposed by the unavailability of I/O devices (for example, when devices on order have not been installed).
Assembling, testing, and debugging on one system or device type and processing on a different one. For example, an IBM 3380 Direct Access Storage drive can be used as a substitute for a magnetic tape unit.
Testing TSO commands such as REXX execs in the TSO/E background.

To make your program device independent, take the following actions:

Omit all device-dependent macros and parameters from your program. For maximum efficiency it is best to omit the BLKSIZE parameter with a BSAM, BPAM or QSAM DCB. See System-Determined Block Size.
Supply the parameters on your data definition (DD) statement, data class, or during the OPEN exit routine. That is, do not specify any required device-dependent parameters until the program is ready for execution. Your program can learn many of the device characteristics by issuing the DEVTYPE macro.