March 30, 1979 • EXTENT, which you must use when you are defining a catalog or a VSAM data space; you are prompted to enter the volume information. • MULT, which you must use in order to access a multivolume VSAM file;
you are prompted to enter the extent information. • CAT, which you can use to identify a catalog which contains the entry
for the VSAM file you are defining. • BUFSP, which you can use to specify the size of the buffers VSAM should use during program execution.
ALLOCATING EXTENTS ON OS DISKS AND MINIDISKS When you use access method services to manipulate VSAM files under OS, you do not have to worry about allocating the real cylinders and tracks
to contain the files. When you use CMS AMSERV, however, you are
responsible for indicating which cylinders and tracks should contain
particular VSAM spaces when you use the DEFINE control statement to
define space.
Extents for VSAM data spaces can be defined, in AMSERV files, in
terms of cylinders, tracks, or records. Extent information you supply to CMS when executing AMSERV must always be in terms of tracks. When you
define data spaces or unique clusters, the extent information (number of
cylinders, tracks, or records) in the AMSERV file must match the extents
you supply when you issue the DLBL command to define the file. When you
supply extent information for the master catalog, any extents you enter
in excess of those required for the catalog are claimed by the catalog
and used as data space. CMS does not make secondary space allocation for VSAM data spaces.
If you execute an AMSERV file that specifies a secondary space
allocation, CMS ignores the parameter. When you use the DLBL command to define VSAM data space, you must use
the EXTENT option, which indicates to eMS that you are going to enter
data extents. For eXample, if you enter:
dlbl space b (extent CMS prompts you to enter the extents: DMSDLB331R ENTER EXTENT SPECIFICATIONS: When you enter the extents, you specify the relative track number of the
first track of the extent, followed by the number of tracks. Fer
example, if you are allocating an entire 2314 disk, you would enter: 20 3980 (null line) You can never write on cylinder 0, track 0; and, since VSAM data
spaces must be allocated on cylinder boundaries, you should never
allocate cylinder O. Cylinder 0 is often used for the volume table of
contents (VTOC) as well, so it is always best to begin defining space
with cylinder 1.
The list below shows the DASD devices supported by CMS number of cylinders on each that can be allocated for VSAM the number of tracks per cylinder:
198 IBM VMj370 CMS User's Guide VSAM, the
space, and

Disk 2314/2319 3330 Model 1 3330 Model 11 3340 Model 35 3340 Model 70 3350 200 404 808 348 696
555 20 19
19
12
12 30 You can determine which disk extents on an as disk or minidisk are
available for allocation by using the LISTDS command with the FREE option, which also indicates the relative track numbers, as well as
actual cylinder and head numbers. USING VSAM CATALOGS While you are developing and testing your VSAM programs in CMS, you may find it convenient to create and use your own master catalog, which may be on a CMS minidisk. VSAM catalogs, like any other cluster, can be
shared read-only among several users. You name the VSAM master catalog for your terminal session using the
ddname IJSYSCT for the DLBL command. For example, if your VSAM master
catalog is located on an OS disk you have accessed as a C-disk, you
would enter:
d1bl ijsysct c dsn master catalog (perm You must define the master catalog at the start of every terminal If you are always using the same master catalog, you might
include the DLBL co •• and you need to define it in your PROFILE EXEC: ACCESS 555 C
DLBL IJSYSCT C DSN MASTCAT (PERM You should use the PERM option so that you do not have to reset the
master catalog assignment after clearing previous DLBL definitions. The
command:
dlbl * clear
clears all file definitions except those entered with the PERM option.
The sample DLBL command used in the preceding example is almost
identical with the one you would issue to define a master catalog using AMSERV. The only difference is that you must enter the EXTENT option so
that you can list the data spaces that this master catalog is to
control.
As an example, suppose that you have a 30-cylinder 3330 minidisk
assigned to you to use for testing your VSAM programs under CMS. Assuming that the minidisk is in your directory at address 333, you
should first access it:
access 333 d
D(333) R/i - as Section 10. Using Access Method Services and VSAM 199