Drive letter assignment

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Drive letter assignment is the process of assigning alphabetical identifiers to physical or logical disk drives or partitions (drive volumes) in the root filesystem namespace; this usage is now found only in Microsoft operating systems. Unlike the concept of UNIX mount points, where volumes are named and located arbitrarily in a single hierarchical namespace, drive letter assignment allows multiple highest-level namespaces. Drive letter assignment is thus a process of using letters to name the roots of the "forest" representing the file system; each volume holds an independent "tree" (or, for non-hierarchical file systems, an independent list of files).



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 CP/M. The concept evolved through several steps:

  • CP/CMS used drive letters to identify minidisks attached to a user session.[1] A full file reference (pathname in today's parlance) consisted of a filename, a filetype, and a disk letter called a filemode (e.g. A or B). Minidisks could correspond to physical disk drives, but more typically referred to logical drives, which were mapped automatically onto shared devices by the operating system as sets of virtual cylinders of fixed-size blocks.
  • CP/CMS inspired numerous other operating systems, including the CP/M microcomputer operating system, which used 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 consisted of a drive letter, a colon, a filename (up to eight characters) and a filetype (three characters); for instance A:README.TXT. (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 TOPS-10 operating system.[2]

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