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A CMS virtual machine can have up to ten minidisks accessed 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".
A minidisk in early versions of CMS is formatted into 800-byte [[block (data storage)|blocks]]. Later versions of CMS allow minidisks formatted as 1024-, 2048-, or 4096-byte blocks, which increased the limits described here by 2**311132,000 disk blocks and 2<sup>31</sup>-records.<ref>{{cite book|last1=IBM Corporation|title=I BM Virtual Machine Facility/370: CMS User's Guide|date=March 1979|page=83|url=http://bitsavers.trailing-edge.com/pdf/ibm/370/VM_370/Release_6/GC20-1819-2_IBM_Virtual_Machine_Facility_370_CMS_Users_Guide_Rel_6_PLC_1_Mar79.pdf|accessdate=August 3, 2016}}</ref>
The first two blocks on a minidisk are reserved for [[Booting#IPL|IPL]]. The third block contains the [[Volume (computing)#Volume label|label]] identifying the minidisk. The fourth block, called the ''Master File Directory'' or MFD, is the [[Directory (computing)|directory]] header for the minidisk. The MFD also contains a [[bitmap]] called ''QMSK'' indicating the status of each 800-byte block on disk, used for allocation. Following the MFD all record types may be scattered and intermixed on a disk.
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[[Category:IBM mainframe operating systems]]
[[Category:IBM file systems| ]]
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