Drive letter assignment

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Drive letter assignment is the process of assigning drive letters to primary and logical partitions (drive volumes) in the root namespace; this usage is found in Microsoft operating systems. Unlike the concept of UNIX mount points, where the user can create directories of arbitrary name and content in a root namespace, drive letter assignment constrains the highest-level namespace to single letters. Drive letter assignment is thus a process of using letters to name the roots of the "forest" representing the file system; each volume holds an independent "tree" (or, for non-hierarchical file systems, an independent list of files).

1 Origin
2 JOIN and SUBST
3 Operating systems that use drive letter assignment
4 Order of assignment
5 Common assignments
6 References

[edit] Origin

The concept of drive letters, as used today, probably owes its origins to IBM's VM family of operating systems, dating back to CP/CMS in 1967 (and its research predecessor CP-40). The concept evolved through several steps:

CP/CMS used drive letters to identify minidisks attached to a user session. A full file reference (pathname in today's parlance) consisted of a filename, a filetype, and a disk letter called a filemode. Minidisks could correspond to physical disk drives, but more typically referred to logical drives, which were mapped automatically onto shared devices by the operating system as sets of virtual cylinders of fixed-size blocks. This was vastly easier to use than other mainframe file reference mechanisms, e.g. JCL.

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

The drive letter syntax chosen for CP/M was also adopted by Microsoft for its ubiquitous microcomputer operating systems MS-DOS and, later, Microsoft Windows. Originally, drive letters always represented physical volumes, but support for logical volumes eventually appeared.

Note that the important capability of hierarchical directories within each drive letter was initially absent from these systems. This was a major feature of UNIX and other robust operating systems, where hard disk drives held thousands (rather than tens or hundreds) of files. Increasing microcomputer storage capacities led to their introduction, eventually followed by long filenames. In file systems lacking such naming mechanisms, drive letter assignment proved a useful, simple organizing principle.

[edit] JOIN and SUBST

Drive letters are not the only way of accessing different volumes. MS-DOS offers a JOIN command that allows access to an assigned volume through an arbitrary directory, similar to the Unix mount command. It also offers a SUBST command which allows the assignment of a drive letter to a directory. One or both of these commands are removed in later systems like OS/2 or Windows NT, but starting with Windows 2000 both are again supported: the SUBST command exists as before, while JOIN's functionality is subsumed in LINKD (part of the Windows Resource Kit). The JUNCTION command (available from the third-party sysinternals) provides similar functionality. Also Windows 2000 and later supports mount points.

[edit] Operating systems that use drive letter assignment

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

MS-DOS versions 3 and earlier assign letters to all of the floppy drives before considering hard drives, so a system with four floppy drives would call the first hard drive 'E'.

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

In Windows NT, Windows 2000, Windows XP and OS/2, the operating system uses the aforementioned algorithm to automatically assign letters to floppy disk drives, CD-ROM drives, DVD drives, the boot disk, and other recognised volumes that are not otherwise created by an administrator within the operating system. Volumes that are created within the operating system are manually specified, and some of the automatic drive letters can be changed. Unrecognised volumes are not assigned letters, and are usually left untouched by the operating system.

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

[edit] Common assignments

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

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

The C: drive usually contains all of the operating system files required for operation of the computer. On many modern personal computers only one hard drive is included in the design so it is designated C:. On such a computer, all of a user's personal files are often stored in directories on this drive as well. Keep in mind, that these drives can, however, be different.

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

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

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