MS-DOS/PC DOS since version 5.0, and later operating systems, assigns drive letters according to the following algorithm: ( February 2011) ( Learn how and when to remove this template message) Unsourced material may be challenged and removed. Please help improve this section by adding citations to reliable sources. Operating systems that use drive letter assignment In file systems lacking such naming mechanisms, drive letter assignment proved a useful, simple organizing principle. Increasing microcomputer storage capacities led to their introduction, eventually followed by long filenames. 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.

The important capability of hierarchical directories within each drive letter was initially absent from these systems.

Through their designated position as DOS successor, the concept of drive letters was also inherited by OS/2 and the Microsoft Windows family.

Originally, drive letters always represented physical volumes, but support for logical volumes eventually appeared.

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 DOS.

(This was the era of 8-inch floppy disks, 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. 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 eight characters) and a filetype (three characters) for instance A:README.TXT.

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.

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. A full file reference ( pathname in today's parlance) consists of a filename, a filetype, and a disk letter called a filemode (e.g.

CP/CMS uses drive letters to identify minidisks attached to a user session.

The concept evolved through several steps: The concept of drive letters, as used today, presumably owes its origins to IBM's 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.

5 ASSIGN, JOIN and SUBST in DOS and Windows.2 Operating systems that use drive letter assignment.

Disk 0 partition 1