Drive letter assignment
Encyclopedia
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 mostly found in Microsoft
operating system
s. 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).
'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:
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.
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:.
Some versions of DOS don't 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.
Some late versions of the DR-DOS IBMBIO.COM provide a pre-boot config structure, holding bit flags to select (beside others) between various drive letter assignment strategies. These strategies can be pre-selected by a user or OEM or be changed by a boot loader on the fly when launching DR-DOS.
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 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.
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, optical disc drives, the boot disk
, and other recognized 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. 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 we have assigned to a network drive as E:, then when we connect a USB mass storage device it will 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 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.
The C: drive usually contains all of the Windows operating system
files required for operation of the computer. On many modern personal computers, only one hard drive with one partition is present, 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. These drives can, however, be different.
When there is no second physical floppy drive, the B: can be used as a "virtual" floppy drive mapped onto the physical A: drive, 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.
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. This 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.
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 to this, 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:-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 don't expect drive letters beyond Z: and won't work with them, therefore it is recommended to use them for special purposes or search drives.
JP Software's 4DOS 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
s must be used. However, it is possible to mount non-letter drives, such as 1:, 2:, or !: using the command line subst utility in Windows XP or Vista (i.e. "subst 1: c:\temp"), but this is not officially supported and may break programs that assume that all drive letters are "A-Z".
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 were 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
). In Windows Vista
, the new command mklink 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
supports this for drive letters N:, O:, and P:. This feature is also present in Concurrent DOS, Multiuser DOS
, System Manager 7, and REAL/32, however, these systems extend the concept to all unused drive letters from A: to Z:, except for the reserved drive letter L:. DR DOS 3.31 - 6.0 (up to initial 1991 release only) also supports this including drive letter L:. This feature is not available under DR DOS 6.0 (1992 upgrade), PalmDOS 1, 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 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.)
Microsoft
Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s. Unlike the concept of UNIX mount points
Unix directory structure
In Unix and Unix-like operating systems, the Unix directory structure is a convention for filesystem layout.Several attempts exist to standardize the Unix filesystem layout, such as the "Filesystem Hierarchy Standard," however, it is targeted primarily at Linux...
, 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 IBMIBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
's VM
VM (operating system)
VM refers to a family of IBM virtual machine operating systems used on IBM mainframes System/370, System/390, zSeries, System z and compatible systems, including the Hercules emulator for personal computers. The first version, released in 1972, was VM/370, or officially Virtual Machine Facility/370...
family of operating systems, dating back to CP/CMS
CP/CMS
CP/CMS was a time-sharing operating system of the late 60s and early 70s, known for its excellent performance and advanced features...
in 1967 (and its research predecessor CP-40
IBM CP-40
CP-40 was a research precursor to CP-67, which in turn was part of IBM's then-revolutionary CP[-67]/CMS – a virtual machine/virtual memory time-sharing operating system for the IBM System/360-67, and the parent of IBM's VM family. CP-40 ran multiple instances of client operating systems...
), by way of CP/M
CP/M
CP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...
. 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 disksFloppy diskA floppy disk is a disk storage medium composed of a disk of thin and flexible magnetic storage medium, sealed in a rectangular plastic carrier lined with fabric that removes dust particles...
, where such small namespaces did not impose practical constraints.) This usage was influenced by the device prefixes used in Digital Equipment CorporationDigital Equipment CorporationDigital Equipment Corporation was a major American company in the computer industry and a leading vendor of computer systems, software and peripherals from the 1960s to the 1990s...
's TOPS-10TOPS-10The TOPS-10 System was a computer operating system from Digital Equipment Corporation for the PDP-10 mainframe computer launched in 1967...
operating system.
- The drive letter syntax chosen for CP/M was inherited by MicrosoftMicrosoftMicrosoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
for its operating system MS-DOSMS-DOSMS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating...
and later, OS/2OS/2OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...
and the Microsoft WindowsMicrosoft WindowsMicrosoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
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
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
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.
Operating systems that use drive letter assignment
- Digital ResearchDigital ResearchDigital Research, Inc. was the company created by Dr. Gary Kildall to market and develop his CP/M operating system and related products. It was the first large software company in the microcomputer world...
CP/MCP/MCP/M was a mass-market operating system created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...
, MP/MMP/MMP/M was a 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 using a separate terminal....
, FlexOSFlexOSFlexOS was a modular real-time multi-user multi-tasking operating system designed for computer-integrated manufacturing, laboratory, retail and financial markets...
, Concurrent CP/M, Concurrent DOS, Multiuser DOSMultiuser DOSMultiuser DOS is a soft real-time multi-user multi-tasking operating system for IBM PC-compatible microcomputers.An evolution of the older Concurrent CP/M-86 and Concurrent DOS operating systems, it was originally developed by Digital Research and later further developed by Novell...
, System Manager, REAL/32, Personal CP/M, DOS PlusDOS PlusDOS Plus is an operating system written by Digital Research, first released in 1985. It can be seen as an intermediate step between CP/M-86 and DR-DOS....
, Novell DOS, PalmDOS, OpenDOS, and DR-DOSDR-DOSDR-DOS is an MS-DOS-compatible operating system for IBM PC-compatible personal computers, originally developed by Gary Kildall's Digital Research and derived from Concurrent PC DOS 6.0, which was an advanced successor of CP/M-86... - 86-DOS
- IBM PC DOS and MicrosoftMicrosoftMicrosoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...
MS-DOSMS-DOSMS-DOS is an operating system for x86-based personal computers. It was the most commonly used member of the DOS family of operating systems, and was the main operating system for IBM PC compatible personal computers during the 1980s to the mid 1990s, until it was gradually superseded by operating... - MSX-DOSMSX-DOSMSX-DOS is a Disk operating system developed by Microsoft for the 8-bit home computer standard MSX, and is a cross between MS-DOS rev 1.0 and CP/M.-MSX-DOS:...
- Elektronika BKElektronika BKThe Elektronika BK was a series of 16-bit PDP-11-compatible Soviet home computers developed by NPO Scientific Center, the leading Soviet microcomputer design team at the time. It was also responsible for the more powerful UKNC and DVK micros...
operating systems: ANDOSANDOSANDOS is a Russian operating system for Electronika BK-0010, Electronika BK-0011 and Electronika BK-0011M series computers. It was created in 1990 and first released in 1992. Initially it was developed by Alexey Nadezhin and later also by Sergey Kamnev, who joined the project...
, CSI-DOSCSI-DOSCSI-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 graphics video mode. The system supported both hard and...
, MK-DOSMK-DOSMK-DOS was one of the most widespread operating systems for Elektronika BK Soviet personal computers, developed by Mikhail Korolev and Dmitriy Butyrskiy from 1993. Like ANDOS, the system provided full compatibility of operating environments for all models of BK, emulating environments of the... - PTS-DOSPTS-DOSPTS-DOS is a disk operating system, a DOS clone, developed in Russia by PhysTechSoft.- History and versions :PhysTechSoft was formed in 1991 in Moscow, Russia by graduates and members of MIPT, informally known as PhysTech. At the end of 1993, PhysTechSoft released the first commercially available...
- Atari TOSAtari TOSTOS is the operating system of the Atari ST range of computers. This range includes the 520 and 1040ST, their STF/M/FM and STE variants and the Mega ST/STE. Later, 32-bit machines were developed using a new version of TOS, called MultiTOS, which allowed multitasking...
- SpartaDOS XSpartaDOS XSpartaDOS X is a disk operating system for the Atari 8-bit family of computers that closely resembles MS-DOS. It was developed and sold by ICD, Inc. in 1987-93.- History :...
- GEOSGEOS (8-bit operating system)GEOS is an operating system from Berkeley Softworks . Originally designed for the Commodore 64 and released in 1986, it provided a graphical user interface for this popular 8-bit computer.GEOS closely resembled early versions of Mac OS and included a graphical word processor and paint program...
- SymbOSSymbOSSymbOS is a free multitasking operating system for Z80-based 8-bit computer systems. At present it is available for the Amstrad CPC series of computers, as well as for all MSX models starting from the MSX2 standard and for most Amstrad PCW models...
- OS/2OS/2OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...
/ eComStationEComStationeComStation or eCS is a PC operating system based on OS/2, published by Serenity Systems. It includes several additions and accompanying software not present in the IBM version of the system.-Differences between eComStation and OS/2:... - Microsoft WindowsMicrosoft WindowsMicrosoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
- Microsoft XboxXboxThe Xbox is a sixth-generation video game console manufactured by Microsoft. It was released on November 15, 2001 in North America, February 22, 2002 in Japan, and March 14, 2002 in Australia and Europe and is the predecessor to the Xbox 360. It was Microsoft's first foray into the gaming console...
video game console operating system - FreeDOSFreeDOSFreeDOS is an operating system for IBM PC compatible computers. FreeDOS is made up of many different, separate programs that act as "packages" to the overall FreeDOS Project...
- ReactOSReactOSReactOS is an open source computer operating system intended to be binary compatible with application software and device drivers made for Microsoft Windows NT versions 5.x and up...
- Symbian OS
- DexOS
- MiraculixMiraculixMiraculix is a 32-bit operating system that originated from Russia.-History:The first version of this system was published in 2003, and named "UCS" . Created using a C++-like language, UCS had its own file system and provided interaction through a single-user console.In 2004, the second edition of...
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:- Assign the drive letter A: to the first floppy disk drive (drive 0), and B: to the second floppy disk drive (drive 1).
- Assign a drive letter, beginning with C: to the first active primary partition recognized upon the first physical hard diskHard diskA hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
. - Assign subsequent drive letters to the first primary partition upon each successive physical hard disk drive.
- Assign subsequent drive letters to every recognized logical partition, beginning with the first hard drive and proceeding through successive physical hard disk drives.
- Assign subsequent drive letters to any RAM DiskRAM diskA RAM disk or RAM drive is a block of RAM that a computer's software is treating as if the memory were a disk drive...
. - Assign subsequent drive letters to any additional floppy or optical disc 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:.
Some versions of DOS don't 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.
Some late versions of the DR-DOS IBMBIO.COM provide a pre-boot config structure, holding bit flags to select (beside others) between various drive letter assignment strategies. These strategies can be pre-selected by a user or OEM or be changed by a boot loader on the fly when launching DR-DOS.
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 MS-DOS requires drivers loaded through the CONFIG.SYS
CONFIG.SYS
CONFIG.SYS is the primary configuration file for the DOS, OS/2 as well as similar operating systems. It is a special file that contains setup or configuration instructions for the computer system.- Usage :...
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.
In Windows NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
, Windows 2000
Windows 2000
Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the...
, Windows XP
Windows XP
Windows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base...
and OS/2
OS/2
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...
, the operating system uses the aforementioned algorithm to automatically assign letters to floppy disk drives, optical disc drives, the boot disk
Boot disk
A boot disk is a removable digital data storage medium from which a computer can load and run 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 certain standards.Boot disks are used for:* Operating...
, and other recognized volumes that are not otherwise created by an administrator
System administrator
A system administrator, IT systems administrator, systems administrator, or sysadmin is a person employed to maintain and operate a computer system and/or network...
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 we have assigned to a network drive as E:, then when we connect a USB mass storage device it will 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 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 drives, 3.5" or 5.25", and possibly other types of disk drives, if present.
- B: — Reserved for a second floppy drive, if present.
- C: — First hard diskHard diskA hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
partition. - D: to Z: — Other disk partitions get labeled here. The letter D: or E: are often assigned to CD-ROM, DVD drives but not always. In fact, Windows assigns the next free drive letter to the next drive it encounters while enumerating the disk drives on the system during installation. 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 Manager (Windows NT, 2000, XP and later) or through the Device Manager found in the Control Panel. MS-DOS typically uses parameters on the line loading device drivers inside the CONFIG.SYSCONFIG.SYSCONFIG.SYS is the primary configuration file for the DOS, OS/2 as well as similar operating systems. It is a special file that contains setup or configuration instructions for the computer system.- Usage :...
file. - F: — First network drive if using Novell NetWareNovell NetWareNetWare is a network operating system developed by Novell, Inc. It initially used cooperative multitasking to run various services on a personal computer, with network protocols based on the archetypal Xerox Network Systems stack....
. - L: - Dynamically assigned "load drive" 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 drives" under CP/M-86 4.x, Personal CP/M-86 2.x, DOS PlusDOS PlusDOS Plus is an operating system written by Digital Research, first released in 1985. It can be seen as an intermediate step between CP/M-86 and DR-DOS....
1.2-2.1 (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.
- Z: — First network drive if using Banyan VINESBanyan VINESBanyan VINES was a computer network operating system and the set of computer network protocols it used to talk to client machines on the network. The Banyan company based the VINES operating system on Unix, and the network protocols on the archetypical Xerox XNS stack...
, and the initial drive letter assignment for the virtual disk network in the DOSBoxDOSBoxDOSBox is emulator software that emulates an IBM PC compatible computer running MS-DOS. It is intended especially for use with old PC games. DOSBox is free software....
x86 emulator.
The C: drive usually contains all of the Windows operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
files required for operation of the computer. On many modern personal computers, only one hard drive with one partition is present, so it is designated C:. On such a computer, all of a user's personal files are often stored in directories
Directory (file systems)
In computing, a folder, directory, catalog, or drawer, is a virtual container originally derived from an earlier Object-oriented programming concept by the same name within a digital file system, in which groups of computer files and other folders can be kept and organized.A typical file system may...
on this drive as well. These drives can, however, be different.
When there is no second physical floppy drive, the B: can be used as a "virtual" floppy drive mapped onto the physical A: drive, 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.
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. This 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.
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 to this, 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:-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 don't expect drive letters beyond Z: and won't work with them, therefore it is recommended to use them for special purposes or search drives.
JP Software's 4DOS 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
Volume Mount Point
Volume Mount Points are specialized NTFS filesystem objects which are used to mount and provide an entry point to other volumes. Mount points can be created in a directory on an NTFS file system, which gives a reference to the root directory of the mounted volume. In fact, any empty directory can...
s must be used. However, it is possible to mount non-letter drives, such as 1:, 2:, or !: using the command line subst utility in Windows XP or Vista (i.e. "subst 1: c:\temp"), but this is not officially supported and may break programs that assume that all drive letters are "A-Z".
ASSIGN, JOIN and SUBST in DOS and Windows
Drive letters are not the only way of accessing different volumes. DOSDOS
DOS, short for "Disk Operating System", is an acronym for several closely related operating systems that dominated the IBM PC compatible market between 1981 and 1995, or until about 2000 if one includes the partially DOS-based Microsoft Windows versions 95, 98, and Millennium Edition.Related...
offers a JOIN command that allows access to an assigned volume through an arbitrary directory, similar to the Unix
Unix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
mount
Mount (Unix)
The Unix command line utility mount instructs the operating system that a file system is ready to use, and associates it with a particular point in the system's file system hierarchy . The counterpart umount instructs the operating system that the file system should be disassociated from its mount...
command. It also offers a SUBST
Subst
subst is a command on the DOS, IBM OS/2 and Microsoft Windows operating systems used for substituting paths on physical and logical drives as virtual drives...
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
OS/2 is a computer operating system, initially created by Microsoft and IBM, then later developed by IBM exclusively. The name stands for "Operating System/2," because it was introduced as part of the same generation change release as IBM's "Personal System/2 " line of second-generation personal...
or Windows NT
Windows NT
Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...
, but starting with Windows 2000
Windows 2000
Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the...
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
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...
). In Windows Vista
Windows Vista
Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...
, the new command mklink can be used for this purpose. Also Windows 2000 and later support mount points, accessible from the Control Panel
Control Panel (Windows)
The Control Panel is a part of the Microsoft Windows graphical user interface which allows users to view and manipulate basic system settings and controls via applets, such as adding hardware, adding and removing software, controlling user accounts, and changing accessibility options...
.
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
DOS Plus is an operating system written by Digital Research, first released in 1985. It can be seen as an intermediate step between CP/M-86 and DR-DOS....
supports this for drive letters N:, O:, and P:. This feature is also present in Concurrent DOS, Multiuser DOS
Multiuser DOS
Multiuser DOS is a soft real-time multi-user multi-tasking operating system for IBM PC-compatible microcomputers.An evolution of the older Concurrent CP/M-86 and Concurrent DOS operating systems, it was originally developed by Digital Research and later further developed by Novell...
, System Manager 7, and REAL/32, however, these systems extend the concept to all unused drive letters from A: to Z:, except for the reserved drive letter L:. DR DOS 3.31 - 6.0 (up to initial 1991 release only) also supports this including drive letter L:. This feature is not available under DR DOS 6.0 (1992 upgrade), PalmDOS 1, 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
4DOS
4DOS is a command line interpreter by JP Software, designed to replace the default command interpreter COMMAND.COM in DOS and Windows 95/98/Me. The 4DOS family of programs are meant to replace the default command processor. 4OS2 and 4NT replace CMD.EXE in OS/2 and Windows NT respectively...
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 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.)
See also
- Drive mapping
- FilenameFilenameThe filename is metadata about a file; a string used to uniquely identify a file stored on the file system. Different file systems impose different restrictions on length and allowed characters on filenames.A filename includes one or more of these components:...
- Net useNet useIn Microsoft operating-systems like DOS and Windows, the net use command In Microsoft operating-systems like DOS and Windows, the net use command In Microsoft operating-systems like DOS and Windows, the net use command (a sub-command of the net command, has several network-related functions...
(A command in Microsoft Windows for viewing/controlling drive-letter assignments for network drives)