Planning Considerations for CMS DL/I IN THE CMS/DOS ENVIRONMENT Batch DL/I application programs can be written and tested in the CMS/DOS environment. This includes all batch application programs written in COBOL, PL/I, or Assembler language.
You can also execute any data base description generation and program
specification block generation. The data base recovery and
reorganization utilities must also be executed in a DOS/VS virtual
machine.
For more information, see the CMS and DL/I Infofmati2n, GH20-1246. eMS Disk and File Management CMS can manage up to ten virtual disks for each user. These disks may
be minidisks or full packs. Moreover, they may be in: • eMS format • as or DOS format • VSAM format When the VM/370 MSS support is installed, and the VM/370 processor is
attached to an MSS, any CMS virtual disk can be located on an MSS 3330V volume. eMS disks are formatted with the CMS FORMAT command; files contained
on these disks are in a format unique to CMS, and cannot be read or
wri tten using other operating systems.-
as and DOS disks or minidisks may be used in CMS. as or DOS programs
executing in CMS may read data sets or files on as or DOS disks, but may
not write or update them: as and DOS minidisks may be formatted with
the IBCDASDI service program, or with an appropriate OS/VS or DOS/VS disk initialization program, if the disk is a full pack. VSAM disks used in CMS are fully compatible with as and DOS VSAM disks. Minidisks for use with VSAK must be formatted with the IBCDASDI
program; full disks must be initialized using the appropriate as/iS or DOS/VS disk initialization program. DISK ACCESS Disks can be accessed in two ways: read-only, where files on that disk
can only be read; and read/write, where files can be read and written.
Both CP and CMS can control read/write access. If a disk is
designated read/write by CP, then the CMS access determines its
read/write status. If a disk is desi9nated read-only by CP, then it can
only be accessed read-only in CMS. Part 1. Planning for System Generation 25

Planning Considerations for CMS To access a disk, you must: • Identify the disk to CP as part of your virtual machine
configuration. This disk is available if it is defined in your V8/310 directory entry, or it can be acquired dynamically with the CP
LINK or DEFINE commands. • Identify the disk to CMS by assigning it a filemode letter. You do
this using the ACCESS command in CMS. While you may have many virtual disks known to CP in your virtual
machine configuration at one time, CMS allows a maximum of ten to be
accessed, with filemode letters A through G, S, Y, and Z. The S-disk (usually at virtual address 190) is the CftS system disk. The A-disk
(usually at virtual address 191) is the user's primary read/write work
disk. Disks may be dynamically accessed and released during a terminal
session.
FILE SHARING CP provides for sharing of disks and minidisks among several users. The
type of access (multiple users read-only or read/write) is controlled by
LINK command operands. Password protection is provided. Since CMS does
not provide any control for multiple writes (such as ENQ, DEQ), it is
not recommended that CftS disks be used with multiple-write access. CMS DISK FILE FORMAT All CMS disks (that is, disks that are to contain CMS files) must be
formatted before being used the first time. The CMS FORMAT command
initializes disks in CMS format and writes a label on the disk. The 10-byte label (written on record 3 of cylinder 0, track 0) consists of
the following: • Four characters: CMS= • Six characters: Desired label (blank-filled
characters; truncated if more than 6 characters)
if less • The remaining bytes of the record are all binary zeros
than 6
The disks are formatted into aOO-byte physical records, called
blocks. Logical records, which may be fixed-length or variable-length,
are imposed on constant physical blocks. Space required for files is
automatically allocated by CMS. As a file grows, its space is expanded,
and it is contracted as its space requirements are reduced.
Files on a CMS disk are identified by means of a file directory,
called the master file directory. The file directory is updated when a
command is issued that changes the status of the file on the disk.
Figure 3 compares the disk devices supported by CftS. For more information about planning CMS minidisk requirements, see
"Estimating VM/370 Storage Requirements" later in this section.
26 IBM VM/370 Planning and System Generation Guide