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The CMS file system is the native file system of IBM's Conversational Monitor System (CMS), a component of VM/370. Until the introduction of the CMS Shared File System with VM/SP it was the only file system for CMS.[1]
Disk organization
CMS uses virtual disks called minidisks to store files. A minidisk is a contiguous subset of a real disk which functions like a real disk. VM allows minidiaks 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 A disk contains user files such as customization data, program sources, and executables. Other drive letters B thuu Z can contain data as defined by the user. The second character of the filemode is a number indicating read, write, and sharing attributes.[2] A CMS minidisk is formatted into 800-byte blocks.
The ACCESS command is used to access a minidisk. For example: ACCESS 191 A would access the virtual disk assigned to this user as unit "191" (virtual channel and unit address) as minidisk "A".
The first two blocks on a minidisk are reserved for IPL. The third block contains the label identifying the minidisk. The fourth block, called the Master File Directory or MFD, which is the directory for the minidisk.
File system structure
CMS uses a flat file system: directory entries for all files are stored in the MFD.
References
- ^ IBM Corporation (1976). IBM Virital Machine Facility/370 (PDF). p. 178-. Retrieved August 3, 2016.
- ^ IBM Corporation. "File Mode Letters and Numbers". IBM Knowledge Center. Retrieved August 3, 2016.
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