[[CP-67]] and [[VM|VM (operating system)]] allow an installation to divide a disk volume into virtual disks called [[Minidisk (VM)|minidisks]]. A CMS minidisk ismay simplybe a CMS minidisk, initialized forwith the CMS file system. AOther minidiskminidisks might be formatted for use by, e.g., [[OS/360]], isbut these are not a CMS minidiskminidisks even if itthey isare assigned to a CMS virtual machine.
A CMS virtual machine can have up to ten minidisks ''accessed'' at one time{{efn|There can be additional minidisks assigned to the CMS virtual machine but not accessed for use by the CMS file system.}} during one session. The user references the minidisks by a letter, part of a field called the ''filemode''. The ''S'' disk contains CMS system files and is read-only; the ''Y'' disk is usually an extension of ''S''. The read/write ''A'' disk contains user files such as customization data, program sources, and executables. Other drive letters ''B'' through ''Z'' can contain data as defined by the user. If a file is opened without a filemode letter specified (<code> FILENAME FILETYPE *</code>) the disks will be searched in alphabetic order. The second character of the filemode is a number indicating read, write, and sharing attributes.<ref name="IBMKB">{{cite web|last1=IBM Corporation|title=File Mode Letters and Numbers|url=https://www.ibm.com/support/knowledgecenter/SSB27U_6.2.0/com.ibm.zvm.v620.dmsb3/letnums.htm|website=IBM Knowledge Center|accessdate=August 3, 2016}}</ref>
The <code>ACCESS</code> command is used to access a minidisk. For example: <code>ACCESS 191 A</code> would access the virtual disk assigned to this user as unit "191" (virtual channel and unit address) as minidisk "A".
