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VulcanSphere (talk | contribs) m Adding local short description: "File system of the IBM VM/CMS", overriding Wikidata description "native filesystem of the IBM VM/CMS mainframe operating system" |
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[[CP-67]] and [[VM (operating system)|VM]] allow an installation to divide a disk volume into virtual disks called [[Minidisk (VM)|minidisks]]. A minidisk may be a CMS minidisk, initialized with the CMS file system. Other minidisks might be formatted for use by, e.g., [[OS/360]], but these are not CMS minidisks even if they are assigned to a CMS virtual machine.{{efn|Minidisks have virtual channel and unit addresses (CUU) or subchannel numbers, which appear to the virtual machine to be real disk addresses. At the time IBM developed CP-67 and VM, real hardware devices were conventionally installed at specific addresses, for example address 190 would normally be a disk device '90'x on channel 1, and minidisks are still usually defined to CP following this convention. With VM/XA SP2.1 IBM shipped the bimodal CMS 5.5, which could run in an XA virtual machine, and with z/VM V4 stopped allowing an installation to define an S/370 mode virtual machine; due to these changes, the cuu address form is no longer relevant.}}
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.}}. The user references the minidisks by a letter, part of a field called the ''{{not a typo|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 {{not a typo|filemode}} letter specified (<code>FILENAME FILETYPE *</code>) the disks will be searched in alphabetic order. The second character of the {{not a typo|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".
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| 22 || 2 || FSTRDPNT || Read pointer [item number] (binary)
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| 24 || 2 || FSTFMODE || {{not a typo|Filemode}}
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| 26 || 2 || FSTRECCT || Record [item] count
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