Incomputer data storage,drive letter assignment is the process of assigning alphabetical identifiers tovolumes. Unlike the concept ofUNIXmount points, where volumes are named and located arbitrarily in a single hierarchical namespace, drive letter assignment allows multiple highest-level namespaces. Drive letter assignment is thus a process of using letters to name the roots of the "forest" representing the file system; each volume holds an independent "tree" (or, for non-hierarchical file systems, an independent list of files).
The concept of drive letters, as used today, presumably[citation needed] owes its origins toIBM'sVM family of operating systems, dating back toCP/CMS in 1967 (and its research predecessorCP-40), by way ofDigital Research's (DRI)CP/M. The concept evolved through several steps:
A:README.TXT. (This was the era of8-inch floppy disks, where such small namespaces did not impose practical constraints.) This usage was influenced by the device prefixes used inDigital Equipment Corporation's (DEC)TOPS-10 operating system.[2]The important capability ofhierarchical directories within each drive letter was initially absent from these systems. This was a major feature ofUNIX 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 bylong filenames. In file systems lacking such naming mechanisms, drive letter assignment proved a useful, simple organizing principle.
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MS-DOS/PC DOS since version 5.0, and later operating systems, assigns drive letters according to the following algorithm:[4]
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 driveE:. Starting with DOS 5.0, the system ensures that driveC: 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 theApricot PC assigns letters to hard drives, starting withA:, before considering floppy drives. A system with two of each drive would call the hard drivesA: andB:, and the floppiesC: andD:.
On the JapanesePC-98, 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 theWindows 95 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 withC:, 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 fromA: orC: 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 theCONFIG.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 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.
InWindows NT andOS/2, the operating system uses the aforementioned algorithm to automatically assign letters tofloppy disk drives,optical disc drives, theboot disk, and other recognized volumes that are not otherwise created by anadministrator 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 alocal volume (like a newly installed CD/DVD drive or a USB stick). For example, if the last local drive is driveD: and a network drive would have been assigned asE:, then a newly attached USB mass storage device would also be assigned driveE: 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.[5]Another condition that can cause problems onWindows 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.[citation needed]
Applying the scheme discussed above on a fairly modern Windows-based system typically results in the following drive letter assignments:
diskpart. MS-DOS typically uses parameters on the line loading device drivers inside theCONFIG.SYS file.Case-specific drive letters:
\DEV fordevice files underMiNT,MagiC, andMultiTOS.[8][9]When there is no second physical floppy drive, driveB: can be used as a "virtual" floppy drive mapped onto the physical driveA:, whereby the user would be prompted to switch floppies every time a read or write was required to whichever was the least recently used ofA: orB:. 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 DOSDRIVER.SYS inCONFIG.SYS.
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 lettersA: toP: only. The PalmDOS PCMCIA driver stack supports drive letters0:,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.SYSLASTDRIVE=32 directive in order to allocate up to 32 drive letters, namedA: toZ:,[:,\:,]:,^:,_: and`:. (DR-DOS 7.02-7.07 also supportsHILASTDRIVE andLASTDRIVEHIGH directives in order to relocate drive structures into upper memory.) Some DOS application programs do not expect drive letters beyondZ: and will not work with them, therefore it is recommended to use them for special purposes or search drives.
JP Software's4DOS command line processor supports drive letters beyondZ: 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 forLASTDRIVE=32 andLASTDRIVEHIGH=32 as well.
If access to more filesystems thanZ: is required under Windows NT,Volume Mount Points must be used.[11] However, it is possible to mount non-letter drives, such as1:,2:, or!: using the command lineSUBST 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 lettersA: toZ:.
Drive letters are not the only way of accessing different volumes.DOS offers aJOIN command that allows access to an assigned volume through an arbitrary directory, similar to theUnixmount command. It also offers aSUBST command which allows the assignment of a drive letter to a directory. One or both of these commands were removed in later systems likeOS/2 orWindows NT, but starting withWindows 2000, both are again supported: The SUBST command exists as before, while JOIN's functionality is subsumed inLINKD (part of the WindowsResource Kit). InWindows Vista, the new commandMKLINK can be used for this purpose. Also, Windows 2000 and later support mount points, accessible from theControl Panel.
Many operating systems originating from Digital Research provide means to implicitly assign substitute drives, calledfloating drives in DRI terminology, by using the CD/CHDIR command in the following syntax:
CD N:=C:\SUBDIR
DOS Plus supports this for drive lettersN:,O:, andP:. This feature is also present inConcurrent DOS,Multiuser DOS, System Manager 7, andREAL/32, however, these systems extend the concept to all unused drive letters from A: to Z:, except for the reserved drive letter L:.[7]DR DOS 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 letterL: 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.[7] Thisload 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 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 theINSTALL[HIGH]/LOADHIGH option/D[:loaddrive] (forB:TSR.COM instead of, say,C:\DIR\TSR.COM). This can be used to minimize a driver's effectivememory 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.[12][13][14][15]
MPDOSTIP.ZIP collection maintained up to 2001 and distributed on many sites at the time. The provided link points to a HTML-converted older version of theNWDOSTIP.TXT file.)