![WinFile 10](https://upload.wikimedia.org/wikipedia/commons/0/03/WinFile_10.0_Screen.PNG)
In
computer data storage
Computer data storage is a technology consisting of computer components and Data storage, recording media that are used to retain digital data (computing), data. It is a core function and fundamental component of computers.
The central pro ...
, drive letter assignment is the process of assigning alphabetical identifiers to
volumes
Volume is a measure of occupied three-dimensional space. It is often quantified numerically using SI derived units (such as the cubic metre and litre) or by various imperial or US customary units (such as the gallon, quart, cubic inch). The defi ...
. 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).
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
CP/CMS (Control Program/Cambridge Monitor System) is a discontinued time-sharing operating system of the late 1960s and early 1970s, known for its excellent performance and advanced features. It had three distinct versions:
* CP-40/CMS, an imp ...
in 1967 (and its research predecessor
CP-40
CP-40 was a research precursor to CP-67, which in turn was part of IBM's then-revolutionary CP 67CMS – a virtual machine/virtual memory time-sharing operating system for the IBM System/360 Model 67, and the parent of IBM's VM family. CP-40 ran ...
), by way of
Digital Research
Digital Research, Inc. (DR or DRI) was a company created by Gary Kildall to market and develop his CP/M operating system and related 8-bit, 16-bit and 32-bit systems like MP/M, Concurrent DOS, FlexOS, Multiuser DOS, DOS Plus, DR DOS and ...
's (DRI)
CP/M
CP/M, originally standing for Control Program/Monitor and later Control Program for Microcomputers, is a mass-market operating system created in 1974 for Intel 8080/ 85-based microcomputers by Gary Kildall of Digital Research, Inc. Initial ...
. The concept evolved through several steps:
* CP/CMS uses drive letters to identify ''
minidisks'' attached to a user session.
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
In computing, file system or filesystem (often abbreviated to fs) is a method and data structure that the operating system uses to control how data is stored and retrieved. Without a file system, data placed in a storage medium would be one larg ...
on each disk drive, where a complete file reference consists of a ''drive letter'', a colon, a ''filename'' (up to
eight characters), a dot, and a ''
filetype
A file format is a standard way that information is encoded for storage in a computer file. It specifies how bits are used to encode information in a digital storage medium. File formats may be either proprietary or free.
Some file formats ...
'' (three characters); for instance
A:README.TXT
. (This was the era of
8-inch floppy disk
A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined wi ...
s, where such small namespaces did not impose practical constraints.) This usage was influenced by the device prefixes used in
Digital Equipment Corporation
Digital Equipment Corporation (DEC ), using the trademark Digital, was a major American company in the computer industry from the 1960s to the 1990s. The company was co-founded by Ken Olsen and Harlan Anderson in 1957. Olsen was president unt ...
's (DEC)
TOPS-10
TOPS-10 System (''Timesharing / Total Operating System-10'') is a discontinued operating system from Digital Equipment Corporation (DEC) for the PDP-10 (or DECsystem-10) mainframe computer family. Launched in 1967, TOPS-10 evolved from the earlier ...
operating system.
* The drive letter syntax chosen for CP/M was inherited by
Microsoft
Microsoft Corporation is an American multinational technology corporation producing computer software, consumer electronics, personal computers, and related services headquartered at the Microsoft Redmond campus located in Redmond, Washing ...
for its operating system
MS-DOS
MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few ope ...
by way of
Seattle Computer Products
Seattle Computer Products (SCP) was a Tukwila, Washington, microcomputer hardware company which was one of the first manufacturers of computer systems based on the 16-bit Intel 8086 processor. SCP began shipping its first S-100 bus 8086 CPU bo ...
' (SCP)
86-DOS
86-DOS (known internally as QDOS, for Quick and Dirty Operating System) is a discontinued operating system developed and marketed by Seattle Computer Products (SCP) for its Intel 8086-based computer kit.
86-DOS shared a few of its commands wit ...
, and thus also by IBM's OEM version
PC DOS. Originally, drive letters always represented physical volumes, but support for
logical volume
In computer storage, logical volume management or LVM provides a method of allocating space on mass-storage devices that is more flexible than conventional partitioning schemes to store volumes. In particular, a volume manager can concatenate, ...
s eventually appeared.
* Through their designated position as DOS successor, the concept of drive letters was also inherited by
OS/2
OS/2 (Operating System/2) is a series of computer operating systems, initially created by Microsoft and IBM under the leadership of IBM software designer Ed Iacobucci. As a result of a feud between the two companies over how to position OS/2 ...
and the
Microsoft Windows
Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...
family.
The important capability of
hierarchical directories within each drive letter was initially absent from these systems. This was a major feature of
UNIX
Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and ot ...
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
Long filename (LFN) support is Microsoft's backward-compatible extension of the 8.3 filename (short filename) naming scheme used in DOS. Long filenames can be more descriptive, including longer filename extensions such as .jpeg, .tiff, .html, a ...
s. In file systems lacking such naming mechanisms, drive letter assignment proved a useful, simple organizing principle.
Operating systems that use drive letter assignment
* CP/M family
**
CP/M
CP/M, originally standing for Control Program/Monitor and later Control Program for Microcomputers, is a mass-market operating system created in 1974 for Intel 8080/ 85-based microcomputers by Gary Kildall of Digital Research, Inc. Initial ...
,
MP/M
MP/M (Multi-Programming Monitor Control Program) is a discontinued multi-user version of the CP/M operating system, created by Digital Research developer Tom Rolander in 1979. It allowed multiple users to connect to a single computer, each us ...
,
Concurrent CP/M
MP/M (Multi-Programming Monitor Control Program) is a discontinued multi-user version of the CP/M operating system, created by Digital Research developer Tom Rolander in 1979. It allowed multiple users to connect to a single computer, each us ...
,
Concurrent DOS
Multiuser DOS is a Real-time operating system, real-time multi-user multi-tasking operating system for IBM Personal Computer, IBM PC-compatible microcomputers.
An evolution of the older Concurrent CP/M-86, Concurrent DOS and Concurrent DOS 386 ...
,
FlexOS
FlexOS is a discontinued modular real-time multiuser computer multitasking, multitasking operating system (RTOS) designed for computer-integrated manufacturing, laboratory, retail and financial markets. Developed by Digital Research's Flexible ...
,
4680 OS
FlexOS is a discontinued modular real-time multiuser multitasking operating system (RTOS) designed for computer-integrated manufacturing, laboratory, retail and financial markets. Developed by Digital Research's Flexible Automation Business U ...
,
4690 OS
4690 Operating System (sometimes shortened to 4690 OS or 4690) is a specially designed point of sale (POS) operating system, originally sold by IBM. In 2012, IBM sold its retail business, including this product, to Toshiba, which assumed support. ...
,
S5-DOS/MT
FlexOS is a discontinued modular real-time multiuser multitasking operating system (RTOS) designed for computer-integrated manufacturing, laboratory, retail and financial markets. Developed by Digital Research's Flexible Automation Business U ...
,
Multiuser DOS
Multiuser DOS is a real-time multi-user multi-tasking operating system for IBM PC-compatible microcomputers.
An evolution of the older Concurrent CP/M-86, Concurrent DOS and Concurrent DOS 386 operating systems, it was originally developed by ...
,
System Manager,
REAL/32
Multiuser DOS is a real-time multi-user multi-tasking operating system for IBM PC-compatible microcomputers.
An evolution of the older Concurrent CP/M-86, Concurrent DOS and Concurrent DOS 386 operating systems, it was originally developed by ...
,
REAL/NG
Multiuser DOS is a real-time multi-user multi-tasking operating system for IBM PC-compatible microcomputers.
An evolution of the older Concurrent CP/M-86, Concurrent DOS and Concurrent DOS 386 operating systems, it was originally developed by ...
,
Personal CP/M,
S5-DOS
CP/M-86 was a version of the CP/M operating system that Digital Research (DR) made for the Intel 8086 and Intel 8088. The system commands are the same as in CP/M-80. Executable files used the relocatable .CMD file format. Digital Research also ...
,
DOS Plus
DOS Plus (erroneously also known as DOS+) was the first operating system developed by Digital Research's OEM Support Group in Newbury, Berkshire, UK, first released in 1985. DOS Plus 1.0 was based on CP/M-86 Plus combined with the PCMODE e ...
*
AMSDOS
AMSDOS is a disk operating system for the 8-bit Amstrad CPC Computer (and various clones). The name is a contraction of Amstrad Disk
Operating System.
AMSDOS first appeared in 1984 on the CPC 464, with added 3 inch disk drive, and then on the ...
* DOS family
**
86-DOS
86-DOS (known internally as QDOS, for Quick and Dirty Operating System) is a discontinued operating system developed and marketed by Seattle Computer Products (SCP) for its Intel 8086-based computer kit.
86-DOS shared a few of its commands wit ...
,
MS-DOS
MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few ope ...
,
PC DOS
**
DR DOS
DR-DOS (written as DR DOS, without a hyphen, in versions up to and including 6.0) is a disk operating system for IBM PC compatibles. Upon its introduction in 1988, it was the first DOS attempting to be compatible with IBM PC DOS and MS-DO ...
,
Novell DOS
DR-DOS (written as DR DOS, without a hyphen, in versions up to and including 6.0) is a disk operating system for IBM PC compatibles. Upon its introduction in 1988, it was the first DOS attempting to be compatible with IBM PC DOS and MS-D ...
,
PalmDOS
DR-DOS (written as DR DOS, without a hyphen, in versions up to and including 6.0) is a disk operating system for IBM PC compatibles. Upon its introduction in 1988, it was the first DOS attempting to be compatible with IBM PC DOS and MS-D ...
,
OpenDOS
DR-DOS (written as DR DOS, without a hyphen, in versions up to and including 6.0) is a disk operating system for IBM PC compatibles. Upon its introduction in 1988, it was the first DOS attempting to be compatible with IBM PC DOS and MS-D ...
**
ROM-DOS
Datalight was a privately held software company specializing in power failsafe and high performance software for preserving data integrity in embedded systems. The company was founded in 1983 by Roy Sherrill, and is headquartered in Bothell, Washi ...
**
PTS-DOS
PTS-DOS (aka PTS/DOS) is a disk operating system, a DOS clone, developed in Russia by PhysTechSoft and Paragon Technology Systems.
History and versions
PhysTechSoft was formed in 1991 in Moscow, Russia by graduates and members of MIPT, infor ...
,
S/DOS
**
FreeDOS
FreeDOS (formerly Free-DOS and PD-DOS) is a free software operating system for IBM PC compatible computers. It intends to provide a complete MS-DOS-compatible environment for running Legacy system, legacy software and supporting embedded system ...
**
PC-MOS/386
PC-MOS/386 is a multi-user, multitasking computer operating system produced by The Software Link (TSL), announced at COMDEX in November 1986 for February 1987 release. PC-MOS/386, a successor to PC-MOS, can run many MS-DOS programs on the host m ...
**
SISNE plus
SISNE plus is a DOS 3.3 compatible clone created by Itautec and Scopus Tecnologia in Brazil prior to the end of the Market Reserve in 1991, which, at that time, forbade the importation of electronic equipment and software for general use. With ...
*
GEMDOS
GEM (for Graphics Environment Manager) is an operating environment released by Digital Research (DRI) in 1985 for use with the DOS operating system on Intel 8088 and Motorola 68000 microprocessors.
GEM is known primarily as the graphical user i ...
,
TOS,
MiNT
MiNT is Now TOS (MiNT) is a free software alternative operating system kernel for the Atari ST system and its successors. It is a multi-tasking alternative to TOS and MagiC. Together with the free system components fVDI device drivers, XaAES g ...
,
MagiC
Magic or Magick most commonly refers to:
* Magic (supernatural), beliefs and actions employed to influence supernatural beings and forces
* Ceremonial magic, encompasses a wide variety of rituals of magic
* Magical thinking, the belief that unrela ...
,
MultiTOS
MiNT is Now TOS (MiNT) is a free software alternative operating system kernel for the Atari ST system and its successors. It is a multi-tasking alternative to TOS and MagiC. Together with the free system components fVDI device drivers, XaAES g ...
,
EmuTOS
EmuTOS is a replacement for TOS (the operating system of the Atari ST and its successors), released as free software. It is mainly intended to be used with Atari emulators and clones, such as Hatari or FireBee. EmuTOS provides support for more ...
*
Atari DOS
Atari DOS is the disk operating system used with the Atari 8-bit family of computers. Operating system extensions loaded into memory were required in order for an Atari computer to manage files stored on a disk drive. These extensions to t ...
family
*
MSX-DOS
MSX-DOS is a discontinued disk operating system developed by Microsoft for the 8-bit home computer standard MSX, and is a cross between MS-DOS 1.25 and CP/M-80 2.
MSX-DOS
MSX-DOS and the extended BASIC with 3½-inch floppy disk support ...
*
ANDOS
ANDOS is a Russian operating system for Electronika BK series computers: BK-0010, BK-0011, and BK-0011M. They were based on the PDP-11 architecture by Digital Equipment Corporation. ANDOS was created in 1990 and released first in 1992. Initially ...
,
CSI-DOS
CSI-DOS is an operating system, created in Samara, for the Soviet Elektronika BK-0011M and Elektronika BK-0011 microcomputers. CSI-DOS did not support the earlier BK-0010. CSI-DOS used its own unique file system and only supported a color graph ...
,
MK-DOS
*
GEOS
*
OS/2
OS/2 (Operating System/2) is a series of computer operating systems, initially created by Microsoft and IBM under the leadership of IBM software designer Ed Iacobucci. As a result of a feud between the two companies over how to position OS/2 ...
(including
eComStation
eComStation or eCS is an operating system based on OS/2 Warp for the 32-bit x86 architecture. It was originally developed by Serenity Systems and Mensys BV under license from IBM. It includes additional applications, and support for new hardwa ...
and
ArcaOS
ArcaOS is an operating system based on OS/2, developed and marketed by Arca Noae, LLC under license from IBM. It was codenamed Blue Lion during its development. It builds on OS/2 Warp 4.52 by adding support for new hardware, fixing defects and l ...
)
* Windows family
**
Windows 9x
Windows 9x is a generic term referring to a series of Microsoft Windows computer operating systems produced from 1995 to 2000, which were based on the Windows 95 kernel and its underlying foundation of MS-DOS, both of which were updated in subs ...
family
**
Windows NT
Windows NT is a proprietary graphical operating system
An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs.
Time-sharing operating systems sc ...
family
**
Xbox system software
The Xbox system software is the operating system developed exclusively for the Xbox home video game console, consoles. Across the four generations of Xbox consoles, the software has been based on a version of Microsoft Windows and incorporating D ...
**
ReactOS
ReactOS is a free and open-source operating system for amd64/i686 personal computers intended to be binary-compatible with computer programs and device drivers made for Windows Server 2003 and later versions of Windows. ReactOS has been noted a ...
*
Symbian OS
Symbian is a discontinued mobile operating system (OS) and computing platform designed for smartphones. It was originally developed as a proprietary software OS for personal digital assistants in 1998 by the Symbian Ltd. consortium. Symbian ...
*
Hobbyist operating system
The development of a hobbyist operating system is one of the more involved and technical options for a computer hobbyist.
The definition of a hobby operating system can sometimes be vague. It can be from the developer's view, where the developers ...
s
**
SymbOS
SYmbiosis Multitasking Based Operating System (SymbOS) is a multitasking operating system for Zilog Z80-based 8-bit computer systems.
Contrary to early 8-bit operating systems it is based on a microkernel, which provides preemptive and prior ...
**
TempleOS
TempleOS (formerly J Operating System, LoseThos, and SparrowOS) is a biblical-themed lightweight operating system (OS) designed to be the Third Temple prophesied in the Bible. It was created by American programmer Terry A. Davis, who developed ...
Order of assignment
MS-DOS/PC DOS since version 5.0, and later operating systems, assigns drive letters according to the following algorithm:
# Assign the drive letter A: to the first
floppy disk drive
A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined w ...
(drive 0), and B: to the second floppy disk drive (drive 1). If only one physical floppy is present, drive B: will be assigned to a phantom floppy drive mapped to the same physical drive and dynamically assigned to either A: or B: for easier floppy file operations. If no physical floppy drive is present, DOS 4.0 will assign both A: and B: to the non-existent drive, whereas DOS 5.0 and higher will invalidate these drive letters. If more than two physical floppy drives are present, DOS versions prior to 5.0 will assign subsequent drive letters, whereas DOS 5.0 and higher will remap these drives to higher drive letters at a later stage; see below.
# Assign a drive letter to the first active primary
partition
Partition may refer to:
Computing Hardware
* Disk partitioning, the division of a hard disk drive
* Memory partition, a subdivision of a computer's memory, usually for use by a single job
Software
* Partition (database), the division of a ...
recognized upon the first physical
hard disk
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...
. DOS 5.0 and higher will ensure that it will become drive C:, so that the boot drive will either have drive A: or C:.
# Assign subsequent drive letters to the first primary partition upon each successive physical hard disk drive (DOS versions prior to 5.0 will probe for only two physical hard disks, whereas DOS 5.0 and higher support eight physical hard disks).
# Assign subsequent drive letters to every recognized logical partition present in the first extended partition, beginning with the first hard drive and proceeding through successive physical hard disk drives.
# DOS 5.0 and higher: Assign drive letters to all remaining primary partitions, beginning with the first hard drive and proceeding through successive physical hard disk drives.
# DOS 5.0 and higher: Assign drive letters to all physical floppy drives beyond the second physical floppy drive.
# Assign subsequent drive letters to any block device drivers loaded in
CONFIG.SYS
CONFIG.SYS is the primary configuration file for the DOS and OS/2 operating systems. It is a special ASCII text file that contains user-accessible setup or configuration directives evaluated by the operating system's DOS BIOS (typically residing ...
via
DEVICE statements, e.g.
RAM disk
Ram, ram, or RAM may refer to:
Animals
* A male sheep
* Ram cichlid, a freshwater tropical fish
People
* Ram (given name)
* Ram (surname)
* Ram (director) (Ramsubramaniam), an Indian Tamil film director
* RAM (musician) (born 1974), Dutch
* Ra ...
s.
# Assign subsequent drive letters to any dynamically loaded drives via CONFIG.SYS
INSTALL statements, in
AUTOEXEC.BAT
AUTOEXEC.BAT is a system file that was originally on DOS-type operating systems. It is a plain-text batch file in the root directory of the boot device. The name of the file is an abbreviation of "automatic execution", which describes its funct ...
or later, i.e. additional
optical disc drive
In computing, an optical disc drive is a disc drive that uses laser light or electromagnetic waves within or near the visible light spectrum as part of the process of reading or writing data to or from optical discs. Some drives can only r ...
s (
MSCDEX
MSCDEX or Microsoft CD-ROM Extensions is a software program produced by Microsoft and included with MS-DOS 6.x and certain versions of Windows to provide CD-ROM support. Earlier versions of MSCDEX since 1986 were installable add-ons for MS ...
etc.),
PCMCIA
The Personal Computer Memory Card International Association (PCMCIA) was a group of computer hardware manufacturers, operating under that name from 1989 to 2009. Starting with the PCMCIA card in 1990 (the name later simplified to ''PC Card''), i ...
/
PC Card
In computing, PC Card is a configuration for computer parallel communication peripheral interface, designed for laptop computers. Originally introduced as PCMCIA, the PC Card standard as well as its successors like CardBus were defined and devel ...
drives, USB or Firewire drives, or
network drive
In computing, a file server (or fileserver) is a computer attached to a network that provides a location for shared disk access, i.e. storage of computer files (such as text, image, sound, video) that can be accessed by the workstations that are ab ...
s.
* Only partitions of recognized
partition type
The partition type (or partition ID) in a partition's entry in the partition table inside a master boot record (MBR) is a byte value intended to specify the file system the partition contains or to flag special access methods used to access thes ...
s are assigned letters. In particular, "hidden partitions" (those with their type ID changed to an unrecognized value, usually by adding
10h
) are not.
MS-DOS/PC DOS versions 4.0 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:. Starting with DOS 5.0, the system ensures that drive C: is always a hard disk, even if the system has more than two physical floppy drives.
While without deliberate remapping, the drive letter assignments are typically fixed until the next reboot, however,
Zenith MS-DOS 3.21 will update the drive letter assignments when resetting a drive. This may cause drive letters to change without reboot if the partitioning of the harddisk was changed.
MS-DOS on the
Apricot PC
The Apricot PC (originally called the ''ACT Apricot'') is a personal computer produced by Apricot Computers, then still known as Applied Computer Techniques or ACT. Released in late 1983, it was ACT's first independently developed microcomputer, f ...
assigns letters to hard drives, starting with A:, before considering floppy drives. A system with two of each drive would call the hard drives A: and B:, and the floppies C: and D:.
On the Japanese
PC-98
The , commonly shortened to PC-98 or , is a lineup of Japanese 16-bit and 32-bit personal computers manufactured by NEC from 1982 to 2000. The platform established NEC's dominance in the Japanese personal computer market, and, by 1999, more ...
, if the system is booted from floppy disk, the dedicated version of MS-DOS assigns letters to all floppy drives before considering hard drives; it does the opposite if it is booted from a hard drive, that is, if the OS was installed on the hard drive, MS-DOS would assign this drive as drive "A:" and a potentially existing floppy as drive "B:". The Japanese version of the
Windows 95
Windows 95 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of operating systems. The first operating system in the 9x family, it is the successor to Windows 3.1x, and was released to manufacturin ...
SETUP program supports a special option
/AT to enforce that Windows will be on drive C:.
Some versions of DOS do not assign the drive letter, beginning with C:, to the first active primary partition recognized upon the first physical hard disk, but on the first primary partition recognized of the first hard disk, even if it is not set active.
If there is more than one extended partition in a partition table, only the logical drives in the first recognized extended partition type are processed.
Some late versions of the DR-DOS IBMBIO.COM provide a preboot config structure, holding bit flags to select (beside others) between various drive letter assignment strategies. These strategies can be preselected by a user or OEM or be changed by a boot loader on the fly when launching DR-DOS. Under these issues, the boot drive can be different from A: or C: as well.
The drive letter 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 DOS, requires drivers loaded through the
CONFIG.SYS
CONFIG.SYS is the primary configuration file for the DOS and OS/2 operating systems. It is a special ASCII text file that contains user-accessible setup or configuration directives evaluated by the operating system's DOS BIOS (typically residing ...
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 DOS. Therefore, 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.
In
Windows NT
Windows NT is a proprietary graphical operating system
An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs.
Time-sharing operating systems sc ...
and
OS/2
OS/2 (Operating System/2) is a series of computer operating systems, initially created by Microsoft and IBM under the leadership of IBM software designer Ed Iacobucci. As a result of a feud between the two companies over how to position OS/2 ...
, the operating system uses the aforementioned algorithm to automatically assign letters to
floppy disk drive
A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined w ...
s,
optical disc drive
In computing, an optical disc drive is a disc drive that uses laser light or electromagnetic waves within or near the visible light spectrum as part of the process of reading or writing data to or from optical discs. Some drives can only r ...
s, the
boot disk
A boot disk is a removable digital data storage medium from which a computer can load and run ( boot) an operating system or utility program. The computer must have a built-in program which will load and execute a program from a boot disk meeting ...
, and other recognized volumes that are not otherwise created by an
administrator
Administrator or admin may refer to:
Job roles Computing and internet
* Database administrator, a person who is responsible for the environmental aspects of a database
* Forum administrator, one who oversees discussions on an Internet forum
* N ...
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. Unrecognized volumes are not assigned letters, and are usually left untouched by the operating system.
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 is drive D: and a network drive would have been assigned as E:, then a newly attached USB mass storage device would also be assigned drive E: causing loss of connectivity with either the network share or the USB device. Users with administrative privileges can assign drive letters manually to overcome this problem.
Another condition that can cause problems on
Windows XP
Windows XP is a major release of Microsoft's Windows NT operating system. It was released to manufacturing on August 24, 2001, and later to retail on October 25, 2001. It is a direct upgrade to its predecessors, Windows 2000 for high-end and ...
is when there are network drives defined, but in an error condition (as they would be on a laptop operating outside the network). Even when the unconnected network drive is not the next available drive letter, Windows XP may be unable to map a drive and this error may also prevent the mounting of the USB device.
Common assignments
Applying the scheme discussed above on a fairly modern Windows-based system typically results in the following drive letter assignments:
*A: —
Floppy disk drive
A floppy disk or floppy diskette (casually referred to as a floppy, or a diskette) is an obsolescent type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined w ...
s, ″ or ″, and possibly other types of disk drives, if present.
*B: — Reserved for a second floppy drive (that was present on many PCs).
*C: — First
hard disk
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with magnet ...
partition.
*D: to Z: — Other disk partitions get labeled here. Windows assigns the next free drive letter to the next drive it encounters while enumerating the disk drives on the system. Drives can be partitioned, thereby creating more drive letters. This applies to MS-DOS, as well as all Windows operating systems. Windows offers other ways to change the drive letters, either through the Disk Management snap-in or
diskpart
In computing, diskpart is a command-line disk partitioning utility included in Windows 2000 and later Microsoft operating systems, replacing its predecessor, fdisk. The command is also available in ReactOS.
Overview
The diskpart is a utility ...
. MS-DOS typically uses parameters on the line loading device drivers inside the
CONFIG.SYS
CONFIG.SYS is the primary configuration file for the DOS and OS/2 operating systems. It is a special ASCII text file that contains user-accessible setup or configuration directives evaluated by the operating system's DOS BIOS (typically residing ...
file.
Case-specific drive letters:
*F: — First network drive if using
Novell NetWare
NetWare is a discontinued computer network operating system developed by Novell, Inc. It initially used cooperative multitasking to run various services on a personal computer, using the IPX network protocol.
The original NetWare product in 19 ...
.
*G: — "Google Drive File Stream" if using
Google Drive
Google Drive is a file storage and synchronization service developed by Google. Launched on April 24, 2012, Google Drive allows users to store files in the cloud (on Google's servers), synchronize files across devices, and share files. In addi ...
.
*H: — "Home" directory on a network server.
*L: — Dynamically assigned
load drive
In computer data storage, drive letter assignment is the process of assigning alphabetical identifiers to volumes. Unlike the concept of UNIX mount points, where volumes are named and located arbitrarily in a single hierarchical namespace, driv ...
under Concurrent DOS, Multiuser DOS, System Manager and REAL/32.
*M: — Drive letter for optionally
memory drive MDISK under Concurrent DOS.
*N:, O:, P: — Assignable
floating drive
In computer data storage, drive letter assignment is the process of assigning alphabetical identifiers to volumes. Unlike the concept of UNIX mount points, where volumes are named and located arbitrarily in a single hierarchical namespace, driv ...
s under CP/M-86 4.x, Personal CP/M-86 2.x,
DOS Plus
DOS Plus (erroneously also known as DOS+) was the first operating system developed by Digital Research's OEM Support Group in Newbury, Berkshire, UK, first released in 1985. DOS Plus 1.0 was based on CP/M-86 Plus combined with the PCMODE e ...
1.1-2.1 (via BDOS call 0Fh), a concept later extended to any unused drive letters under Concurrent DOS, Multiuser DOS, System Manager, REAL/32 and DR DOS up to 6.0.
*Q: — Microsoft Office Click-to-Run virtualization.
*U: — Unix-like ''unified filesystem'' with virtual directory
\DEV
for
device file
In Unix-like operating systems, a device file or special file is an interface to a device driver that appears in a file system as if it were an ordinary file. There are also special files in DOS, OS/2, and Windows. These special files allow a ...
s under
MiNT
MiNT is Now TOS (MiNT) is a free software alternative operating system kernel for the Atari ST system and its successors. It is a multi-tasking alternative to TOS and MagiC. Together with the free system components fVDI device drivers, XaAES g ...
,
MagiC
Magic or Magick most commonly refers to:
* Magic (supernatural), beliefs and actions employed to influence supernatural beings and forces
* Ceremonial magic, encompasses a wide variety of rituals of magic
* Magical thinking, the belief that unrela ...
, and
MultiTOS
MiNT is Now TOS (MiNT) is a free software alternative operating system kernel for the Atari ST system and its successors. It is a multi-tasking alternative to TOS and MagiC. Together with the free system components fVDI device drivers, XaAES g ...
.
*Z: — First network drive if using
Banyan VINES, and the initial drive letter assignment for the virtual disk network in the
DOSBox
DOSBox is a free and open-source emulator which runs software for MS-DOS compatible disk operating systems—primarily video games. It was first released in 2002, when DOS technology was becoming obsolete. Its adoption for running DOS games i ...
x86
x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was introd ...
emulator. It is also the first letter selected by Windows for network resources, as it automatically selects from Z: downwards. By default,
Wine
Wine is an alcoholic drink typically made from fermented grapes. Yeast consumes the sugar in the grapes and converts it to ethanol and carbon dioxide, releasing heat in the process. Different varieties of grapes and strains of yeasts are m ...
maps Z: to the root of the UNIX directory tree.
When there is no second physical floppy drive, drive B: can be used as a "virtual" floppy drive mapped onto the physical drive A:, whereby the user would be prompted to switch floppies every time a read or write was required to whichever was the least recently used of A: or B:. This allows for much of the functionality of two floppy drives on a computer that has only one. This concept of multiple drive letters sharing a single physical device (optionally with different "views" of it) is not limited to the first floppy drive, but can be utilized for other drives as well by setting up additional block devices for them with the standard DOS
DRIVER.SYS in
CONFIG.SYS
CONFIG.SYS is the primary configuration file for the DOS and OS/2 operating systems. It is a special ASCII text file that contains user-accessible setup or configuration directives evaluated by the operating system's DOS BIOS (typically residing ...
.
Network drives are often assigned letters towards the end of the alphabet. This is often done to differentiate them from local drives: by using letters towards the end, it reduces the risk of an assignment conflict. It is especially true when the assignment is done automatically across a network (usually by a logon script).
In most DOS systems, it is not possible to have more than 26 mounted drives. Atari GEMDOS supports 16 drive letters A: to P: only. The PalmDOS PCMCIA driver stack supports drive letters 0:, 1:, 2:, ... to address PCMCIA drive slots.
Some Novell network drivers for DOS support up to 32 drive letters under compatible DOS versions. In addition, Novell DOS 7, OpenDOS 7.01, and DR-DOS 7.02 genuinely support a CONFIG.SYS
LASTDRIVE=32
directive in order to allocate up to 32 drive letters, named A: to Z:,
, \:, , ^:, _: and `:. (DR-DOS 7.02-7.07 also supports
HILASTDRIVE
and
LASTDRIVEHIGH
directives in order to relocate drive structures into upper memory.) Some DOS application programs do not expect drive letters beyond Z: and will not work with them, therefore it is recommended to use them for special purposes or search drives.
JP Software's
4DOS
4DOS is a command-line interpreter by JP Software, designed to replace the default command interpreter COMMAND.COM in Microsoft DOS and Windows. It was written by Rex C. Conn and Tom Rawson and first released in 1989. Compared to the default, ...
command line processor supports drive letters beyond Z: in general, but since some of the letters clash with syntactical extensions of this command line processor, they need to be escaped in order to use them as drive letters.
Windows 9x (MS-DOS 7.0/MS-DOS 7.1) added support for
LASTDRIVE=32
and
LASTDRIVEHIGH=32
as well.
If access to more filesystems than Z: is required under Windows NT,
Volume Mount Point
NTFS volume mount points are specialized NTFS filesystem objects which are used to mount and provide an entry point to other volumes.
Description
NTFS volume mount points are NTFS filesystem objects—implemented as NTFS reparse points—which ...
s must be used.
However, it is possible to mount non-letter drives, such as 1:, 2:, or !: using the command line
SUBST
In computing, SUBST is a command on the DOS, IBM OS/2, Microsoft Windows and ReactOS operating systems used for substituting paths on physical and logical drives as virtual drives.
Overview
In MS-DOS, the SUBST command was added with the rele ...
utility in Windows XP or later (i.e.
SUBST 1: C:\TEMP
), but it is not officially supported and may break programs that assume that all drives are letters A: to Z:.
ASSIGN, JOIN and SUBST in DOS and Windows
Drive letters are not the only way of accessing different volumes.
DOS
DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems.
DOS may also refer to:
Computing
* Data over signalling (DoS), multiplexing data onto a signalling channel
* Denial-of-service attack (DoS), an attack on a communicatio ...
offers a
JOIN Join may refer to:
* Join (law), to include additional counts or additional defendants on an indictment
*In mathematics:
** Join (mathematics), a least upper bound of sets orders in lattice theory
** Join (topology), an operation combining two topo ...
command that allows access to an assigned volume through an arbitrary directory, similar to the
Unix
Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, and ot ...
''
mount
Mount is often used as part of the name of specific mountains, e.g. Mount Everest.
Mount or Mounts may also refer to:
Places
* Mount, Cornwall, a village in Warleggan parish, England
* Mount, Perranzabuloe, a hamlet in Perranzabuloe parish, C ...
'' command. It also offers a
SUBST
In computing, SUBST is a command on the DOS, IBM OS/2, Microsoft Windows and ReactOS operating systems used for substituting paths on physical and logical drives as virtual drives.
Overview
In MS-DOS, the SUBST command was added with the rele ...
command which allows the assignment of a drive letter to a directory. One or both of these commands were removed in later systems like
OS/2
OS/2 (Operating System/2) is a series of computer operating systems, initially created by Microsoft and IBM under the leadership of IBM software designer Ed Iacobucci. As a result of a feud between the two companies over how to position OS/2 ...
or
Windows NT
Windows NT is a proprietary graphical operating system
An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs.
Time-sharing operating systems sc ...
, but starting with
Windows 2000
Windows 2000 is a major release of the Windows NT operating system developed by Microsoft and oriented towards businesses. It was the direct successor to Windows NT 4.0, and was Software release life cycle#Release to manufacturing (RTM), releas ...
, both are again supported: The SUBST command exists as before, while JOIN's functionality is subsumed in
LINKD (part of the Windows
Resource Kit Resource Kit is a term used by Microsoft for a set of software resources and documentation released for their software products, but which is not part of that product. Resource kits offer supplementary resources such as technical guidance, compatibi ...
). In
Windows Vista
Windows Vista is a major release of the Windows NT operating system developed by Microsoft. It was the direct successor to Windows XP, which was released five years before, at the time being the longest time span between successive releases of ...
, the new command
MKLINK
The NTFS file system defines various ways to redirect files and folders, e.g., to make a file point to another file or its contents without making a copy of it. The object being pointed to is called the target. Such file is called a hard or symbo ...
can be used for this purpose. Also, Windows 2000 and later support mount points, accessible from the
Control Panel.
Many operating systems originating from Digital Research provide means to implicitly assign substitute drives, called ''floating drives'' in DRI terminology, by using the CD/CHDIR command in the following syntax:
CD N:=C:\SUBDIR
DOS Plus
DOS Plus (erroneously also known as DOS+) was the first operating system developed by Digital Research's OEM Support Group in Newbury, Berkshire, UK, first released in 1985. DOS Plus 1.0 was based on CP/M-86 Plus combined with the PCMODE e ...
supports this for drive letters N:, O:, and P:. This feature is also present in
Concurrent DOS
Multiuser DOS is a Real-time operating system, real-time multi-user multi-tasking operating system for IBM Personal Computer, IBM PC-compatible microcomputers.
An evolution of the older Concurrent CP/M-86, Concurrent DOS and Concurrent DOS 386 ...
,
Multiuser DOS
Multiuser DOS is a real-time multi-user multi-tasking operating system for IBM PC-compatible microcomputers.
An evolution of the older Concurrent CP/M-86, Concurrent DOS and Concurrent DOS 386 operating systems, it was originally developed by ...
, System Manager 7, and
REAL/32
Multiuser DOS is a real-time multi-user multi-tasking operating system for IBM PC-compatible microcomputers.
An evolution of the older Concurrent CP/M-86, Concurrent DOS and Concurrent DOS 386 operating systems, it was originally developed by ...
, however, these systems extend the concept to all unused drive letters from A: to Z:, except for the reserved drive letter L:.
DR DOS
DR-DOS (written as DR DOS, without a hyphen, in versions up to and including 6.0) is a disk operating system for IBM PC compatibles. Upon its introduction in 1988, it was the first DOS attempting to be compatible with IBM PC DOS and MS-D ...
3.31 - 6.0 (up to the 1992-11 updates with BDOS 6.7 only) also supports this including drive letter L:. This feature is not available under DR DOS 6.0 (1992 upgrade), PalmDOS 1.0, Novell DOS 7, OpenDOS 7.01, DR-DOS 7.02 and higher. Floating drives are implemented in the BDOS kernel, not in the command line shell, thus they can be used and assigned also from within applications when they use the "change directory" system call. However, most DOS applications are not aware of this extension and will consequently discard such directory paths as invalid. JP Software's command line interpreter 4DOS supports floating drives on operating systems also supporting it.
In a similar feature, Concurrent DOS, Multiuser DOS, System Manager and REAL/32 will dynamically assign a drive letter L: to the load path of a loaded application, thereby allowing applications to refer to files residing in their load directory under a standardized drive letter instead of under an absolute path.
This
load drive
In computer data storage, drive letter assignment is the process of assigning alphabetical identifiers to volumes. Unlike the concept of UNIX mount points, where volumes are named and located arbitrarily in a single hierarchical namespace, driv ...
feature makes it easier to move software installations on and across disks without having to adapt paths to overlays, configuration files or user data stored in the load directory or subsequent directories.
(For similar reasons, the appendage to the environment block associated with loaded applications under DOS 3.0 (and higher) contains a reference to the load path of the executable as well, however, this consumes more resident memory, and to take advantage of it, support for it must be coded into the executable, whereas DRI's solution works with any kind of applications and is fully transparent to users as well.)
In some versions of DR-DOS, the load path contained in the appendage to the environment passed to drivers can be shortened to that of a temporary substitute drive (e.g.
SUBST B: C:\DIR
) through the
INSTALL ">IGH/code>/LOADHIGH
In computing, (abbreviated ) is an internal DOS command in COMMAND.COM that is used to load a program into the upper memory area (UMA) instead of conventional memory.
The command was introduced with MS-DOS 5.0 / PC DOS 5.0 in 1991, copyin ...
option /D loaddrive/code> (for B:TSR.COM
instead of, say, C:\DIR\TSR.COM
). This can be used to minimize a driver's effective memory footprint, if the executable is located in a deep subdirectory and the resident driver happens to not need its load path after installation any more.
See also
* Drive mapping Drive mapping is how Microsoft Windows associates a local drive letter (A through Z) with a shared storage area to another computer (often referred as a File Server) over a network. After a drive has been mapped, a software application on a clien ...
* Filename
A filename or file name is a name used to uniquely identify a computer file in a directory structure. Different file systems impose different restrictions on filename lengths.
A filename may (depending on the file system) include:
* name &ndas ...
* net (command)
In computing, net is a command in IBM OS/2 (including eComStation and ArcaOS), Microsoft Windows and ReactOS used to manage and configure the operating system from the command-line. It is also part of the IBM PC Network Program for DOS.
Ove ...
(A command in Microsoft Windows that can be used for viewing/controlling drive-letter assignments for network drives)
* Portable application
A portable application (portable app), sometimes also called standalone, is a program designed to read and write its configuration settings into an accessible folder in the computer, usually in the folder where the portable application can be f ...
References
External links
Change Drive Letter in Windows 8
{{DEFAULTSORT:Drive Letter Assignment
Windows architecture
DOS technology
Computer peripherals
Assignment operations
de:Laufwerk (Computer)#Laufwerksbuchstaben