Editing Drive letter assignment (section)

==Origin==
The concept of drive letters, as used today, presumably{{citation needed|date=August 2016}} owes its origins to [[IBM]]'s [[VM (operating system)|VM]] family of operating systems, dating back to [[CP/CMS]] in 1967 (and its research predecessor [[CP-40]]), by way of [[Digital Research]]'s (DRI) [[CP/M]]. The concept evolved through several steps:

* CP/CMS uses drive letters to identify ''[[minidisk (CMS)|minidisk]]s'' attached to a user session.<ref name="ducharme"/> A full file reference (''pathname'' in today's parlance) consists of a ''filename'', a ''filetype'', and a disk letter called a ''filemode'' (e.g. '''A''' or '''B'''). Minidisks can correspond to physical disk drives, but more typically refer to logical drives, which are mapped automatically onto shared devices by the operating system as sets of ''virtual cylinders''.
* CP/CMS inspired numerous other operating systems, including the CP/M microcomputer operating system, which uses a drive letter to specify a physical storage device. Early versions of CP/M (and other microcomputer operating systems) implemented a [[flat file system]] on each disk drive, where a complete file reference consists of a ''drive letter'', a colon, a ''filename'' (up to [[8.3 filename|eight characters]]), a dot, and a ''[[Filename extension|filetype]]'' (three characters); for instance <code>A:README.TXT</code>. (This was the era of [[8-inch floppy disk]]s, where such small namespaces did not impose practical constraints.)  This usage was influenced by the device prefixes used in [[Digital Equipment Corporation]]'s (DEC) [[TOPS-10]] operating system.<ref name="johnson"/>
* The drive letter syntax chosen for CP/M was inherited by [[Microsoft]] for its operating system [[MS-DOS]] by way of [[Seattle Computer Products]]' (SCP) [[86-DOS]], and thus also by IBM's OEM version [[PC&nbsp;DOS]]. Originally, drive letters always represented physical volumes, but support for [[logical volume]]s eventually appeared.
* Through their designated position as DOS successor, the concept of drive letters was also inherited by [[OS/2]] and the [[Microsoft Windows]] family.

The important capability of [[file directory|hierarchical directories]] within each drive letter was initially absent from these systems. This was a major feature of [[UNIX]] and other similar operating systems, where hard disk drives held thousands (rather than tens or hundreds) of files. Increasing microcomputer storage capacities led to their introduction, eventually followed by [[long filename]]s. In file systems lacking such naming mechanisms, drive letter assignment proved a useful, simple organizing principle.