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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]].
==Disk organization==
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 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'' 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>
Later versions of CMS allowed 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 <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 CMS minidisk is formatted into 800-byte [[block (data storage)|blocks]]. Later versions of CMS allowed 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. Following the MFD all record types may be scattered and intermixed on a disk.▼
▲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.
==File system structure==
CMS uses a [[flat file system]]. The MFD contains an array of disk addresses of blocks containing ''File Status Table'' (FST) (directory) entries
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The FST entry points to the first ''chain link block'' for the file. The first chain link block contains the disk addresses of up to 40 additional chain link blocks, followed by the disk addresses of up to 60 data blocks. The remaining chain link blocks each contain the disk addresses of up to 400 data blocks. this results in a maximum size of 16,060 800-byte blocks, or 12,848,000 bytes, for any CMS file. The maximum number of records in one file is 65,533.
Records are usually called ''items'' in CMS terminology. CMS files can have either ''fixed'' or ''variable'' record format; record types may not be mixed in a file. For fixed-length records the length is defined by FSTLRECL, and the ___location of any fixed-length record can be computed by (item_number-1) * record_length/800. The quotient will be the
==References==
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