Drive letter assignment is the process of assigning drive letters to primary and logical partitions (drive volumes) in the root namespace; this usage is found in Microsoft operating systems. Unlike the concept of UNIX mount points, where the user can create directories of arbitrary name and content in a root namespace, drive letter assignment constrains the highest-level namespace to single letters. 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). In computer engineering, hard disk drive partitioning is the creation of logical divisions upon a hard disk that allows one to apply operating system-specific logical formatting. ... The Microsoft Corporation, (NASDAQ: MSFT, HKSE: 4338) is a multinational computer technology corporation with global annual revenue of US$44. ... An operating system (OS) is a computer program that manages the hardware and software resources of a computer. ... A mount point is a term used to describe where the computer puts the files in a file system on Unix-like systems. ... For library and office filing systems, see Library classification. ...

Origin

The concept of drive letters, as used today, probably owes its origins to IBM's VM family of operating systems, dating back to CP/CMS in 1967 (and its research predecessor CP-40). The concept evolved through several steps: Big Blue redirects here. ... VM may stand for: Virtual memory Virtual machine VM, IBMs virtual machine operating system VM nerve agent, a chemical weapon agent Voice mail VM Corporation, a manufacturer of audio equipment Voynich manuscript This page expands and disambiguates a two-letter combination which might be an abbreviation, an English word... VM/CMS (Virtual Machine/Conversational Monitor System, originally called CP/CMS when it first appeared) is an IBM system used on System/360, System/370, System/390 and zSeries IBM mainframes. ... 1967 (MCMLXVII) was a common year starting on Sunday of the Gregorian calendar (the link is to a full 1967 calendar). ... [Note: This and several related articles are still evolving. ...

• 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. 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. This was vastly easier to use than other mainframe file reference mechanisms, e.g. JCL.

• CP/CMS inspired numerous other operating systems, including the CP/M microcomputer operating system – which used a drive letter prefix (e.g. "A:") to specify a physical storage device. This usage was similar to the device prefixes used in the RSX-11 and VMS operating systems. 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 followed by a filename (eight characters) and a filetype (three characters): A:readme.txt. (This was the era of 8-inch floppy disks, where such small namespaces did not impose practical constraints.)

• The drive letter syntax chosen for CP/M was also adopted by Microsoft for its ubiquitous microcomputer operating systems MS-DOS and, later, Microsoft Windows. 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 robust 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. VM/CMS (Virtual Machine/Conversational Monitor System, originally called CP/CMS when it first appeared) is an IBM system used on System/360, System/370, System/390 and zSeries IBM mainframes. ... Job Control Language (JCL) is a scripting language used on IBM mainframe operating systems to instruct the Job Entry Subsystem (that is, JES2 or JES3) on how to run a batch program or start a subsystem. ... VM/CMS (Virtual Machine/Conversational Monitor System, originally called CP/CMS when it first appeared) is an IBM system used on System/360, System/370, System/390 and zSeries IBM mainframes. ... CP/M is an operating system originally created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc. ... RSX-11: A family of real-time operating systems mainly for PDP-11 computers created by Digital Equipment Corporation (DEC), common in the late 1970s and early 1980s, designed for and much used in process control, but also popular for program development. ... OpenVMS V7. ... CP/M is an operating system originally created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc. ... A floppy disk is a data storage device that is composed of a disk of thin, flexible (floppy) magnetic storage medium encased in a square or rectangular plastic shell. ... CP/M is an operating system originally created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc. ... The Microsoft Corporation, (NASDAQ: MSFT, HKSE: 4338) is a multinational computer technology corporation with global annual revenue of US$44. ... Microsofts disk operating system, MS-DOS, was Microsofts implementation of DOS, which was the first popular operating system for the IBM PC, and until recently, was widely used on the PC compatible platform. ... Microsoft Windows is the name of several families of proprietary operating systems by Microsoft. ... LV - Logical Volume is the part of the abstraction used within the LVM. It can be seen as a partition of a regular disk, and is created out of a Volume group (VG) ... In computing, a directory, catalog or folder,[1] is an entity in a file system which contains a group of files and/or other directories. ... Unix (officially trademarked as UNIX) is a computer operating system originally developed in the 1960s and 1970s by a group of AT&T employees at Bell Labs including Ken Thompson, Dennis Ritchie, and Douglas McIlroy. ... Long filename is the name given to the longer and therefore more descriptive file names on the FAT filesystem, which was previously restricted to eight characters and a three-character extension (referred to as 8. ...

JOIN and SUBST

Drive letters are not the only way of accessing different volumes. MS-DOS offers a JOIN command that allows access to an assigned volume through an arbitrary directory, similar to the Unix mount command. It also offers a SUBST command which allows the assignment of a drive letter to a directory. One or both of these commands are removed in later systems like OS/2 or Windows NT, but starting with Windows 2000 both are again supported: the SUBST command exists as before, while JOIN's functionality is subsumed in LINKD (part of the Windows Resource Kit). The JUNCTION command (available from the third-party sysinternals) provides similar functionality. Also Windows 2000 and later supports mount points. Microsofts disk operating system, MS-DOS, was Microsofts implementation of DOS, which was the first popular operating system for the IBM PC, and until recently, was widely used on the PC compatible platform. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. ... Windows 2000 (also referred to as Win2K) is a preemptible, interruptible, graphical and business-oriented operating system that is designed to work with either uniprocessor or symmetric multi-processor 32-bit Intel x86 computers. ...

Operating systems that use drive letter assignment

• CP/M
• DOS
• Microsoft Windows
• OS/2
• Atari ST

CP/M is an operating system originally created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc. ... ‹ The template below has been proposed for deletion. ... Microsoft Windows is the name of several families of proprietary operating systems by Microsoft. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... The Atari 520ST Atari 1040STF with SC1224 color monitor The Atari ST is a home/personal computer that was commercially popular from 1985 to the early 1990s. ...

Order of assignment

Except for CP/M and early versions of MS-DOS, each of these operating systems assigns drive letters according to the following algorithm:

1. Assign the drive letter 'A' to the boot floppy, and 'B' to the secondary floppy.
2. Assign a drive letter, beginning with 'C' to the first active primary partition recognised upon the first physical hard disk.
3. Assign subsequent drive letters to the first primary partition upon each successive physical hard disk drive, if present within the system.
4. Assign subsequent drive letters to every recognised logical partition, beginning with the first hard drive and proceeding through successive physical hard disk drives, if present within the system.
5. Assign subsequent drive letters to any RAM Disk.
6. Assign subsequent drive letters to any additional floppy, CD/DVD drives.

MS-DOS versions 3 and earlier assign letters to all of the floppy drives before considering hard drives, so a system with four floppy drives would call the first hard drive 'E'. In general, a partition is a splitting into parts. ... Typical hard drives of the mid-1990s. ... A RAM-Disk, Ramdisk or Ramdrive is a virtual solid state disk that uses a segment of active computer memory, RAM, as secondary storage, a role typically filled by hard drives. ...

The order can depend on whether a given disk is managed by a boot-time driver or by a dynamically loaded driver. For example, if the second or third hard disk is of SCSI type and on MS-DOS requires drivers loaded through the CONFIG.SYS file (e.g. the controller card does not offer on-board BIOS or using this BIOS is not practical), then the first SCSI primary partition will appear after all the IDE partitions on MS-DOS. Therefore MS-DOS and, for example, OS/2 could have different drive letters, as OS/2 loads the SCSI driver earlier. A solution was not to use primary partitions on such hard disks. CONFIG.SYS is the primary configuration file for the MS-DOS and OS/2 operating systems. ...

In Windows NT, Windows 2000, Windows XP and OS/2, the operating system uses the aforementioned algorithm to automatically assign letters to floppy disk drives, CD-ROM drives, DVD drives, the boot disk, and other recognised volumes that are not otherwise created by an administrator within the operating system. Volumes that are created within the operating system are manually specified, and some of the automatic drive letters can be changed. Unrecognised volumes are not assigned letters, and are usually left untouched by the operating system. Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. ... Windows 2000 (also referred to as Win2K) is a preemptible, interruptible, graphical and business-oriented operating system that is designed to work with either uniprocessor or symmetric multi-processor 32-bit Intel x86 computers. ... Windows XP is a line of operating systems developed by Microsoft for use on general-purpose computer systems, including home and business desktops, notebook computers, and media centers. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... A floppy disk is a data storage device that is composed of a disk of thin, flexible (floppy) magnetic storage medium encased in a square or rectangular plastic shell. ... The CD-ROM (an abbreviation for Compact Disc Read-Only Memory (ROM)) is a non-volatile optical data storage medium using the same physical format as audio compact discs, readable by a computer with a CD-ROM drive. ... DVD (commonly Digital Versatile Disc, previously Digital Video Disc) is an optical disc storage media format that can be used for data storage, including movies with high video and sound quality. ... A boot disk is a removable media, normally read-only, that can boot an operating system or utility. ... A system administrator, or sysadmin, is a person employed to maintain, and operate a computer system or network for a company or other organization. ...

A common problem that occurs with the drive letter assignment is that the letter assigned to a network drive can interfere with the letter of a local volume (like a newly installed CD/DVD drive or a USB stick). For example, if the last local drive has the letter D: and we have assigned to a network drive the letter E:, then when we connect a USB mass storage device it will also be assigned the letter E: causing to lose connectivity with either the network share or the USB device. To overcome this problem we have to manually assign drive letters or to install a 3rd party software as the USB Drive Letter Manager.

Common assignments

Applying the algorithms discussed above on a fairly modern Windows based system typically results in the following drive letter assignments:

• A: — Floppy drive (3.5-inch is the modern standard).
• B: — Unused, reserved for floppy drive; historically also for a second floppy drive, usually 5.25-inch. Also used for RAM-drives, in case of live CDs.
• C: — Main partition.
• D: to Z: — Other disk partitions are labeled here. (Win98 update really likes to put any CD-ROM drive as D: even putting it above a Primary Partition IDE device)
• D: to Z: — CD, DVD and shared drives begin lettering after the last used hard drive partition designation.
• F: — First Network Drive if using Novell NetWare

The C: drive usually contains all of the operating system files required for operation of the computer. On many modern personal computers only one hard drive is included in the design so it is designated C:. On such a computer, all of a user's personal files are often stored in directories on this drive as well. Keep in mind, that these drives can, however, be different. A floppy disk is a data storage device that is composed of a disk of thin, flexible (floppy) magnetic storage medium encased in a square or rectangular plastic shell. ... CD redirects here. ... DVD (commonly Digital Versatile Disc, previously Digital Video Disc) is an optical disc storage media format that can be used for data storage, including movies with high video and sound quality. ... NetWare is a network operating system developed by Novell, Inc. ... An operating system (OS) is a computer program that manages the hardware and software resources of a computer. ... In computing, a directory, catalog, or folder, is an entity in a file system which can contain a group of files and/or other directories. ...

When there was not a second physical floppy drive, the B: drive was used as a virtual floppy drive marker for the A: drive, whereby the user would be prompted to switch floppies every time a read or write was required to whichever was not most recently used of A: or B:. This allowed for much of the functionality of two floppy drives on a computer that had only one (albeit usually resulting in lots of swapping).

Network drives are often assigned letters towards the end of the alphabet. This is often done to differentiate them from local drives. Local drives typically use letters towards the beginning of the alphabet, by using letters towards the end it reduces the risk of an assignment conflict. This is especially true when the assignment is done automatically across a network (usually by a logon script).

It is not possible to have more than 26 drives. If access to more filesystems than this is required volume mount points must be used.^[1]

References

1. ^ Microsoft TechNet Retrieved on 1 December 2006