Editing VM (operating system) (section)

== Minidisks ==
[[File:VM370 CMS 6.0.png|thumb|CMS starting up after the user MAINT (system administrator) has logged in]]
[[File:VM370 Rel 6 editor.png|thumb|The CMS editor on VM/370, editing a COBOL program source file]]

CMS and other operating systems often have DASD requirements much smaller than the sizes of actual volumes. For this reason CP allows an installation to define virtual disks of any size up to the capacity of the device. For [[Count key data|CKD]] volumes, a minidisk must be defined in full cylinders. A minidisk has the same attributes as the underlying real disk, except that it is usually smaller and the beginning of each minidisk is mapped to cylinder or block 0. The minidisk may be{{efn|CMS can use DIAG for I/O on CMS file systems.}} accessed using the same [[channel program]]s as the real disk.

A minidisk that has been initialized with a CMS file system is referred to as a CMS minidisk, although CMS is not the only system that can use them.

It is common practice to define full volume minidisks for use by such guest operating systems as [[z/OS]] instead of using <code>DEDICATE</code> to assign the volume to a specific virtual machine. In addition, "full-pack links" are often defined for every DASD on the system, and are owned by the MAINT userid. These are used for backing up the system using the DASD Dump/Restore program, where the entire contents of a DASD are written to tape (or another DASD) exactly.