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

Planning Considerations for I I I 2314/ I I 2319 3330 3340 I 3350
r I , , Maximum number of files , 3500 3400 3400 , 3400 I that can be contained on , I , the disk I I .. I I I Maximum minidisk size I 203 246 348 , 115 , (in cylinders) I (model 35) , 1 , 682 , I , (model 70) , .. +-- I I Number of 800-byte blocks I 150 266 96 I 570 , per cylinder I I .. I , Maximum data extent records ""'-',-'...Iv , 12,848,000 bytes
L
Figure 3. CMS Disk File Statistics
IDENTIFYING DISK FILES CMS commands are provided to list the identifications of files on eMS and non-CMS formatted disks and minidisks. The LISTFILE command. lists
the entries in the master file directory for CMS disks; the LISTDS command lists the entries in the VTOC (volume table of contents) for as
and DOS disks, or for listing data spaces on VSAM volumes. eMS Tape Support
Each CMS machine can support up to four magnetic tape units at virtual
addresses 181, 182, 183, and 184. They may be 2401, 2402, 2403, 2415,
2420, 3410/3411, or 3420 drives, or a mixture of tape drives.
Three tape-handling commands (ASSGN, FILEDEF, and TAPE) allow you to
specify the modeset of the tape: track (7-track or 9-track), density,
and, for 7-track tape only, the tape recording technique (odd or even
parity, converter on or off, and translator on or off) • If you do not specify the modeset for a 7-track tape, CMS issues a
modeset indicating 7-track, 800 bpi (bits per inch), odd parity,
converter on, and translate off. If the tape is 9-track, the density is
assumed to be 1600 bpi (or whatever bpi the tape drive was last set at)
for dual density drives; for single density drives, whatever bpi the
drive is (800, 1600, or 6250 bpi) is assumed.
As an alternative to specifying mode in each command that uses the
tape (for example, FILEDEF), you can issue a CMS TAPE command that sets
the mode for the tape and stays in effect until reissued. You must do
this if one of your programs is to use tapes in other than the default
mode. With one exception, eMS commands permit only unlabeled tapes to be
read or written. However, the eMS TAPPDS command can read standard as
tape labels. Your programs executing under eMS must use unlabeled tapes
or provide code to create and read their own labels as data records. Part 1. Planning for System Generation 27