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Peter Flass (talk | contribs) →Minidisks {{anchor|minidisk}}: bit of rewording for clarification |
Peter Flass (talk | contribs) →Minidisks {{anchor|minidisk}}: explain disk addressing, mostly footnote. |
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==Minidisks {{anchor|minidisk}}==
[[CP-67]] and [[VM|VM (operating system)]] 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), which appear to the virtual machine to be real disk addresses. Real hardware devices are conventionally installed at specific addresses, for example address 190 would normally be a disk device '90'x on channel 1, and minidisks are usually defined to CP following this convention.}}
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 ''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>
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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