IBM OS/390 User Manual

Direct Access Method (or Basic Direct Access Method) -- used for
disk devices. Still in some use, but often replaced by VSAM functions
in many VSE shops. Generally requires complex application handling
for processing, and may be dependent upon physical device
characteristics. Not supported on Fixed-Block Architecture disks.

In OS/390, BDAM is the functional equivalent.

VSE Librarian should be considered an access method, as it meets all
the criteria specified above. The current VSE Librarian has been
available since VSE/SP Version 2, in 1984. The previous
implementation will not be discussed.

In OS/390, Partitioned Data Sets (PDS, PDS-E) provide the equivalent
functions, together with associated utilities.

In the VSE/ESA system, programs define their intent to use an access method
and specify needed parameters through the APIs provided through a set of
Define The File macros. These include:

Define The File CarD

Define The File CoNsole

Define The File Direct Access

Define The File Device Independence

Define The File Document Reader

Define The File Diskette Unit

Define The File Indexed Sequential access

Define The File Magnetic ink character Reader

Define The File Magnetic Tape

Define The File Optical Reader

Define The File for PHysical I/O

Define The File for PRinter

Define The File for Sequential Disk

In addition to these, additional macros are available for definition of VSAM,
Librarian, and some other access method objects or files, including
telecommunication terminals or lines.

In the OS/390 environment, most of these DTFs are replaced by an analogous
control block definition, the Data Control Block, or DCB. The DCB is not device
specific, as the VSE DTFs are, so there is more flexibility for using a single
OS/390 program to read data, for example, from the SYSIN stream, from tape or
from disk. Only the JCL would be changed to specify the device characteristics
at run time to switch from one input or output device type to another.

The VSE application program contains the DTF macro expansion, and at linkage
edit or execution time, an operating system component module (referred to as a

″

Logic Module

″) 

will be connected to the DTF and used by the application

program to handle the functions needed for GET, PUT, or other imperative macro
commands.

VSE to OS/390 Migration Workbook