In computing, (abbreviated ) is an internal

DOS DOS (, ) is a family of disk-based operating systems for IBM PC compatible computers. The DOS family primarily consists of IBM PC DOS and a rebranded version, Microsoft's MS-DOS, both of which were introduced in 1981. Later compatible syste ...

command Command may refer to: Computing * Command (computing), a statement in a computer language * command (Unix), a Unix command * COMMAND.COM, the default operating system shell and command-line interpreter for DOS * Command key, a modifier key on A ...

in COMMAND.COM that is used to load a program into the

upper memory area In DOS memory management, the upper memory area (UMA) is the memory between the addresses of 640 KB and 1024 KB ( 0xA0000–0xFFFFF) in an IBM PC or compatible. IBM reserved the uppermost 384 KB of the 8088 CPU's 1024 KB ...

(UMA) instead of

conventional memory In DOS memory management, conventional memory, also called base memory, is the first 640 kilobytes of the memory on IBM PC or compatible systems. It is the read-write memory directly addressable by the processor for use by the operating system ...

. The command was introduced with MS-DOS 5.0 / PC DOS 5.0 in 1991, copying the built-in command earlier introduced with DR DOS 5.0 in 1990. DR DOS 6.0 added support for this naming variant as well in 1991.

Overview

Due to design of the

IBM PC The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first microcomputer released in the List of IBM Personal Computer models, IBM PC model line and the basis for the IBM PC compatible ''de facto'' standard. Released on ...

, DOS suffered from what was known as the 640 KB barrier. The size of this memory area, known as

, was fixed and independent of the amount of system memory actually installed. Various schemes were developed to support extra memory (see also

EMS Ems or EMS may refer to: Places and rivers * Domat/Ems, a Swiss municipality in the canton of Grisons * Ems (river) (Eems), a river in northwestern Germany and northeastern Netherlands that discharges in the Dollart Bay * Ems (Eder), a river o ...

, XMS) and

DOS extender A DOS extender is a computer software program running under DOS that enables software to run in a protected mode environment even though the host operating system is only capable of operating in real mode. DOS extenders were initially developed ...

s, but conventional memory was still an issue due to compatibility issues. It was a scarce resource as many applications demanded a large part of this basic memory fragment at runtime. Therefore, it was often necessary to move high some TSR programs like the

mouse A mouse (: mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus'' ...

driver or the disk caching driver (like

SMARTDRV SmartDrive (or SMARTDRV) is a disk caching program shipped with MS-DOS versions 4.01 through 6.22 and Windows 3.0 through Windows 3.11. It improves data transfer rates by storing frequently accessed data in random-access memory (RAM). Early ...

) prior to running a memory-hungry application. This was achieved by using called with the program's name as the parameter. To load TSRs high within CONFIG.SYS, the INSTALLHIGH directive must be used instead of the command. The equivalent of for

device driver In the context of an operating system, a device driver is a computer program that operates or controls a particular type of device that is attached to a computer or automaton. A driver provides a software interface to hardware devices, enabli ...

s is DEVICEHIGH (usable only within CONFIG.SYS). These are also supported since DR DOS 6.0. DR DOS 5.0 and higher also support HIINSTALL and HIDEVICE, respectively. Most modern operating systems now run in

protected mode In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units (CPUs). It allows system software to use features such as Memory_segmentation, segmentation, virtual mem ...

with support for an unsegmented (flat) memory model and do not have a 640 KB constraint. and other methods of freeing conventional memory have largely become obsolete. is part of the

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 successor to Windows 2000 for high-end and business users a ...

MS-DOS subsystem to maintain MS-DOS and MS OS/2 version 1.x syntax compatibility only. It is not available at all on

Windows XP 64-Bit Edition Windows XP, which is the next version of Windows NT after Windows 2000 and the successor to the consumer-oriented Windows Me, has been released in several editions since its original release in 2001. Windows XP is available in many languages. In ...

and also no longer available in the

command interpreter A command-line interface (CLI) is a means of interacting with software via commands each formatted as a line of text. Command-line interfaces emerged in the mid-1960s, on computer terminals, as an interactive and more user-friendly alternativ ...

of newer

Windows Windows is a Product lining, product line of Proprietary software, proprietary graphical user interface, graphical operating systems developed and marketed by Microsoft. It is grouped into families and subfamilies that cater to particular sec ...

operating systems.

Overview

See also

References

Further reading