CMS file system: Difference between revisions

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Revision as of 21:56, 23 May 2017

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CMS uses [[virtual disk]]s called ''minidisks'' to store files. A minidisk is a contiguous subset of a real disk which functions like a real disk. VM allows minidisks to contain various types of data such as [[OS/360]] or [[DOS/360]] datasets as well as CMS-formatted data.

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".