Specifications

60 IBM System z Personal Development Tool: Volume 3 Additional Topics
A minidisk is a contiguous range of 3390 cylinders
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that is seen by a z/VM user as a complete
disk volume (typically with a small number of cylinders). There are three common formats for
a minidisk:
򐂰 CMS, which includes all the z/VM control files and user files
򐂰 z/OS, which includes a label, VTOC, and the usual z/OS types of data sets
򐂰 Linux for System z
A minidisk is created by defining it in the z/VM directory. You are responsible for formatting
your own minidisks. For CMS this is done with a format 191 a command, for example. For
z/OS, the ICKDSF utility is used.
A minidisk has a virtual address, such as 190, that is assigned in the z/VM directory. Some
minidisk addresses have predefined or conventional meanings. For example, address 190
contains many CMS modules (and is read-only for most users). Most CMS users have a 191
minidisk, which is their default CMS work disk. There are other well-known minidisk
addresses used by multiple users. Otherwise, a minidisk address is simply a hexadecimal
number (usually three digits) that you select.
To access a minidisk under CMS, an access mode letter must be established. For example, if
you have minidisk 456 defined in your directory entry, you might enter the CMS command acc
456 z to access the minidisk as the z disk. Your 191 disk (which is almost always defined for
a CMS user) is your default “a” disk. For some functions, your minidisks are searched in
order, a, b, c, and so forth. The mode letter (the “disk letter”) is not a fixed value. You might
use acc 456 r the next time you log on and access your 456 minidisk as your “r” disk.
A CMS user identifies a file with three qualifiers: a file name, a file type, and a mode (which is
usually just the drive letter for the minidisk containing the file).
MYDATA TEXT A1
| | |
| | +---File mode (fm) (The notation fn, ft, fm is common)
| +--------File type (ft)
+---------------File name (fn)
The first part of the name (MYDATA) is completely arbitrary and should be meaningful to you.
The second part (TEXT) can be arbitrary, but some applications attach meaning to this part of
a file name. For example, if the second part of the name is EXEC, it is assumed to contain
commands for a command interpreter such as REXX. The third part (A1) has a disk letter (A)
and might have a file mode number (1 in the example here) that can be:
򐂰 0 - This makes the file private. Other users with read-only access to your minidisk will not
see this file. Users with read-write access will see it.
򐂰 1 - This is the default file mode number for read-write files.
򐂰 2 and 5- These function like number 1 and can be used to subset files.
򐂰 3 - Files with this number are automatically erased when they are read.
򐂰 4 - These files are in a simulated MVS format (not used in any of our examples).
򐂰 6 - This indicates the file should be updated in place, with appropriate programming.
For practical purposes, you can omit the mode number and let it default to 1. This is suitable
for almost all purposes.
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Geometry other than 3390 can be used, but 3390s are the most common.