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Real mode, also called real address mode, is an operating mode of 80286 and later x86-compatible CPUs
Central processing unit

A central processing unit is an electronic circuit that can execute computer programs. This broad definition can easily be applied to many early computers that existed long before the term "CPU" ever came into widespread usage....
. Real mode is characterized by a 20 bit segmented memory address space (meaning that a maximum of 1 MB
Megabyte

Megabyte is a SI prefix-multiple of the unit byte for digital information computer storage or transmission and is equal to 106 bytes....
 of memory can be addressed), direct software access to BIOS
BIOS

In computing, the Basic Input/Output System , also known as the System BIOS, is a de facto standard defining a firmware interface for IBM PC Compatible computers....
 routines and peripheral hardware, and no concept of memory protection
Memory protection

Memory protection is a way to control memory usage on a computer, and is core to virtually every modern operating system. The main purpose of memory protection is to prevent a process running on an operating system from accessing memory beyond that allocated to it....
 or multitasking
Computer multitasking

In computing, multitasking is a method by which multiple tasks, also known as Computer process, share common processing resources such as a Central processing unit....
 at the hardware level. All x86 CPUs in the 80286 series and later start in real mode at power-on; 80186 CPUs and earlier had only one operational mode, which is equivalent to real mode in later chips.

286 architecture introduced protected mode
Protected mode

In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units ....
, allowing for (among other things) hardware-level memory protection.






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Real mode, also called real address mode, is an operating mode of 80286 and later x86-compatible CPUs
Central processing unit

A central processing unit is an electronic circuit that can execute computer programs. This broad definition can easily be applied to many early computers that existed long before the term "CPU" ever came into widespread usage....
. Real mode is characterized by a 20 bit segmented memory address space (meaning that a maximum of 1 MB
Megabyte

Megabyte is a SI prefix-multiple of the unit byte for digital information computer storage or transmission and is equal to 106 bytes....
 of memory can be addressed), direct software access to BIOS
BIOS

In computing, the Basic Input/Output System , also known as the System BIOS, is a de facto standard defining a firmware interface for IBM PC Compatible computers....
 routines and peripheral hardware, and no concept of memory protection
Memory protection

Memory protection is a way to control memory usage on a computer, and is core to virtually every modern operating system. The main purpose of memory protection is to prevent a process running on an operating system from accessing memory beyond that allocated to it....
 or multitasking
Computer multitasking

In computing, multitasking is a method by which multiple tasks, also known as Computer process, share common processing resources such as a Central processing unit....
 at the hardware level. All x86 CPUs in the 80286 series and later start in real mode at power-on; 80186 CPUs and earlier had only one operational mode, which is equivalent to real mode in later chips.

History

The 286 architecture introduced protected mode
Protected mode

In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units ....
, allowing for (among other things) hardware-level memory protection. Using these new features, however, required a new operating system
Operating system

An operating system is an interface between hardware and applications; it is responsible for the management and coordination of activities and the sharing of the limited resources of the computer....
 that was specifically designed for it. Since a primary design specification
Design specification

'Design specifications' are the measurements and characteristics of a structure or object which provide for a workable, sustainable, or pleasing creation or construct....
 of x86 microprocessors is that they be fully backwards compatible with software written for all x86 chips before them, the 286 chip was made to start in 'real mode' — that is, in a mode which turned off the new memory protection features, so that it could run operating system
Operating system

An operating system is an interface between hardware and applications; it is responsible for the management and coordination of activities and the sharing of the limited resources of the computer....
s written for the 8086
Intel 8086

The 8086 is a 16-bit microprocessor chip designed by Intel and introduced on the market in 1978, which gave rise to the x86 architecture. Intel 8088, released in 1979, was essentially the same chip, but with an external 8-bit bus , and is notable as the processor used in the original IBM PC....
 and the 80186. To this day, even the newest x86 CPUs (including x86_64 CPUs) start in real mode at power-on, and can run software written for any previous chip.

The DOS
DOS

DOS, short for "Disk Operating System", is a shorthand term 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 Windows 95, Windows 98, and Windows Me....
 operating systems (MS-DOS
MS-DOS

MS-DOS is an operating system commercialized by Microsoft. It was the most commonly used member of the DOS family of operating systems and was the main operating system for personal computers during the 1980s....
, DR-DOS
DR-DOS

DR-DOS is a DOS-type operating system for IBM PC-PC compatible personal computers, originally developed by Gary Kildall's Digital Research and derived from CP/M-86....
, etc.) operate in real mode. Early versions of Microsoft Windows
Microsoft Windows

Microsoft Windows is a series of software operating systems and graphical user interfaces produced by Microsoft. Microsoft first introduced an operating environment named Windows in November 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces ....
 ran in real mode, until Windows 386, which ran in protected mode, and the more fully realized Windows 3.0
Windows 3.0

Windows 3.0 is the third major release of Microsoft Microsoft Windows, and was released on 22 May 1990. It became the first widely successful version of Windows and a powerful rival to Macintosh and the Commodore Amiga on the GUI front....
, which could run in either real or protected mode. Windows 3.0 could actually run in two "flavours" of protected mode - "standard mode", which ran using protected mode, and "386-enhanced mode", which is a virtualized version of standard mode and thus would not run on a 286. Windows 3.1 removed support for Real Mode, and it was the first mainstream operating environment which required at least an 80286 processor. Almost all modern x86 operating systems (Unix
Unix

Unix is a computer operating system originally developed in 1969 by a group of American Telephone & Telegraph employees at Bell Labs, including Ken Thompson , Dennis Ritchie, Douglas McIlroy, and Joe Ossanna....
, Linux
Linux

Linux is a generic term referring to Unix-like computer operating systems based on the Linux kernel. Their development is one of the most prominent examples of free and open source software collaboration; typically all the underlying source code can be used, freely modified, and redistributed by anyone under the terms of the GNU GPL license...
, 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 "IBM Personal System/2 " line of second-generation personal computers....
, Mac OS
Mac OS

Mac OS is the trademarked name for a series of graphical user interface-based operating systems developed by Apple Inc. for their Macintosh line of computer systems....
, Windows 95
Windows 95

Windows 95 is a consumer-oriented graphical user interface-based operating system. It was released on August 24, 1995 by Microsoft, and was a significant progression from the company's previous Microsoft Windows products....
 and later, etc.) switch the CPU into protected mode at startup.

See also

  • IA-32
    IA-32

    IA-32 , often generically called x86 or x86-32, is the instruction set architecture of Intel's most commercially successful microprocessors....
  • x86 assembly language
    X86 assembly language

    x86 assembly language is the family of backwards-compatible assembly languages for the x86 class of processors, which includes Intel's Pentium series and AMD's Athlon series....
  • Conventional memory
    Conventional memory

    In computing, conventional memory is the first 640 kilobytes of the memory on IBM PC compatible systems....