Drive Letter Assignment - Origin


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. 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.
  • The drive letter syntax chosen for CP/M was inherited by Microsoft for its operating system MS-DOS and later, OS/2 and the Microsoft Windows family. Originally, drive letters always represented physical volumes, but support for logical volumes eventually appeared.

Note that the important capability of 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 filenames. In file systems lacking such naming mechanisms, drive letter assignment proved a useful, simple organizing principle.

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