computing Computing is any goal-oriented activity requiring, benefiting from, or creating computer, computing machinery. It includes the study and experimentation of algorithmic processes, and the development of both computer hardware, hardware and softw ...

, legacy mode is a state in which a

computer A computer is a machine that can be Computer programming, programmed to automatically Execution (computing), carry out sequences of arithmetic or logical operations (''computation''). Modern digital electronic computers can perform generic set ...

system A system is a group of interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its open system (systems theory), environment, is described by its boundaries, str ...

, component, or

application software Application software is any computer program that is intended for end-user use not operating, administering or programming the computer. An application (app, application program, software application) is any program that can be categorized as ...

behaves in a way that is different from its standard operation in order to support older

software Software consists of computer programs that instruct the Execution (computing), execution of a computer. Software also includes design documents and specifications. The history of software is closely tied to the development of digital comput ...

data Data ( , ) are a collection of discrete or continuous values that convey information, describing the quantity, quality, fact, statistics, other basic units of meaning, or simply sequences of symbols that may be further interpreted for ...

, or expected behavior. It differs from

backward compatibility In telecommunications and computing, backward compatibility (or backwards compatibility) is a property of an operating system, software, real-world product, or technology that allows for interoperability with an older legacy system, or with Input ...

in that an item in legacy mode will often sacrifice newer features or

performance A performance is an act or process of staging or presenting a play, concert, or other form of entertainment. It is also defined as the action or process of carrying out or accomplishing an action, task, or function. Performance has evolved glo ...

, or be unable to access data or run programs it normally could, in order to provide continued access to older data or functionality. Sometimes it can allow newer technologies that replaced the old to emulate them when running older

operating systems An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ...

Examples

x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit extension of the x86 instruction set architecture, instruction set. It was announced in 1999 and first available in the AMD Opteron family in 2003. It introduces two new ope ...

processors can be run in one of two states: **

Long mode In the x86-64 computer architecture, long mode is the mode where a 64-bit operating system can access 64-bit instructions and registers. 64-bit programs are run in a sub-mode called 64-bit mode, while 32-bit programs and 16-bit protected mod ...

: provides larger physical address spaces and the ability to run

64-bit In computer architecture, 64-bit integers, memory addresses, or other data units are those that are 64 bits wide. Also, 64-bit central processing units (CPU) and arithmetic logic units (ALU) are those that are based on processor registers, a ...

applications Application may refer to: Mathematics and computing * Application software, computer software designed to help the user to perform specific tasks ** Application layer, an abstraction layer that specifies protocols and interface methods used in a ...

which can use larger virtual address spaces and more registers. ** Legacy mode: allows these processors to act as if they were 16- or

32-bit In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in a maximum of 32- bit units. Compared to smaller bit widths, 32-bit computers can perform la ...

x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel, based on the 8086 microprocessor and its 8-bit-external-bus variant, the 8088. Th ...

processors with all of the abilities and limitations of them in order to run legacy 16-bit and 32-bit

, and to run programs requiring

virtual 8086 mode In the 80386 microprocessor and later, virtual 8086 mode (also called virtual real mode, V86-mode, or VM86) allows the execution of real mode applications that are incapable of running directly in protected mode while the processor is running ...

to run (e.g., in

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 ...

). * 32-bit

processors have two legacy modes:

real mode Real mode, also called real address mode, is an operating mode of all x86-compatible CPUs. The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode is characterized by a 20- bit s ...

and virtual 8086 mode. Real mode causes the processor to mostly act as if it was an original 8086, while virtual 8086 mode allows the creation of a

virtual machine In computing, a virtual machine (VM) is the virtualization or emulator, emulation of a computer system. Virtual machines are based on computer architectures and provide the functionality of a physical computer. Their implementations may involve ...

, enabling programs that require real mode to run within a

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 ...

environment. Protected mode is the non-legacy mode of 32-bit x86 processors, introduced with the

Intel 80286 The Intel 80286 (also marketed as the iAPX 286 and often called Intel 286) is a 16-bit microprocessor that was introduced on February 1, 1982. It was the first 8086-based CPU with separate, non- multiplexed address and data buses and also the f ...

. * Most PC graphic cards have a VGA and a

SVGA Super VGA (SVGA) or Extended VGA is a broad term that covers a wide range of computer display standards that extended IBM's Video Graphics Array, VGA specification. When used as shorthand for a resolution, as VGA and XGA often are, SVGA refers to ...

mode that allows their use on systems without their dedicated

device drivers 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 ...

, which means they can function in basic mode even when the system hasn't fully recognized the card's advanced capabilities. * Operating systems often have a special mode allowing them to emulate an older release in order to support software applications dependent on the specific interfaces and behavior of that release.

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 ...

can be configured to emulate

Windows 2000 Windows 2000 is a major release of the Windows NT operating system developed by Microsoft, targeting the server and business markets. It is the direct successor to Windows NT 4.0, and was Software release life cycle#Release to manufacturing (RT ...

and

Windows 98 Windows 98 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of Microsoft Windows operating systems. It was the second operating system in the 9x line, as the successor to Windows 95. It was Software ...

Windows 11 Windows 11 is a version of Microsoft's Windows NT operating system, released on October 5, 2021, as the successor to Windows 10 (2015). It is available as a free upgrade for devices running Windows 10 that meet the #System requirements, Windo ...

can run programs in "compatibility mode" for

Windows 8 Windows 8 is a major release of the Windows NT operating system developed by Microsoft. It was Software release life cycle#Release to manufacturing (RTM), released to manufacturing on August 1, 2012, made available for download via Microsoft ...

Windows 7 Windows 7 is a major release of the Windows NT operating system developed by Microsoft. It was Software release life cycle#Release to manufacturing (RTM), released to manufacturing on July 22, 2009, and became generally available on October 22, ...

, Windows Vista (Service Pack 2), Windows Vista (Service Pack 1),

Windows Vista Windows Vista is a major release of the Windows NT operating system developed by Microsoft. It was the direct successor to Windows XP, released five years earlier, which was then the longest time span between successive releases of Microsoft W ...

Mac OS X macOS, previously OS X and originally Mac OS X, is a Unix, Unix-based operating system developed and marketed by Apple Inc., Apple since 2001. It is the current operating system for Apple's Mac (computer), Mac computers. With ...

used to support the execution of

Mac OS 9 Mac OS 9 is the ninth and final major release of the classic Mac OS operating system for Macintosh computers, made by Apple Computer. Introduced on October 23, 1999, it was promoted by Apple as "The Best Internet Operating System Ever", highlight ...

applications on

PowerPC PowerPC (with the backronym Performance Optimization With Enhanced RISC – Performance Computing, sometimes abbreviated as PPC) is a reduced instruction set computer (RISC) instruction set architecture (ISA) created by the 1991 Apple Inc., App ...

-based Macintoshes. *

Computer buses In computer architecture, a bus (historically also called a data highway or databus) is a communication system that transfers Data (computing), data between components inside a computer or between computers. It encompasses both Computer hardw ...

emulated through legacy mode: ** Emulated bus (Host bus) ** ISA ( LPC) ** PCI (

PCI Express PCI Express (Peripheral Component Interconnect Express), officially abbreviated as PCIe, is a high-speed standard used to connect hardware components inside computers. It is designed to replace older expansion bus standards such as Peripher ...

) ** PS/2 or RS-232 mouse ( USB mouse) ** PS/2 or AT keyboard ( USB keyboard) ** Many

SATA SATA (Serial AT Attachment) is a computer bus interface that connects host bus adapters to mass storage devices such as hard disk drives, optical drives, and solid-state drives. Serial ATA succeeded the earlier Parallel ATA (PATA) standard ...

disk controllers offer a legacy mode of operation for compatibility i.e.

parallel ATA Parallel ATA (PATA), originally , also known as Integrated Drive Electronics (IDE), is a standard interface designed for IBM PC-compatible computers. It was first developed by Western Digital and Compaq in 1986 for compatible hard drives and C ...

emulation * Some niche markets have enabled

Compact Flash CompactFlash (CF) is a flash memory mass storage device used mainly in portable electronic devices. The format was specified and the devices were first manufactured by SanDisk in 1994. CompactFlash became one of the most successful of the ea ...

and

SD card Secure Digital (SD) is a proprietary, non-volatile, flash memory card format developed by the SD Association (SDA). Owing to their compact size, SD cards have been widely adopted in a variety of portable consumer electronics, including dig ...

s to emulate IDE hard drives for old DOS and

Windows 95 Windows 95 is a consumer-oriented operating system developed by Microsoft and the first of its Windows 9x family of operating systems, released to manufacturing on July 14, 1995, and generally to retail on August 24, 1995. Windows 95 merged ...

computers. * The

Wii U The Wii U ( ) is a home video game console developed by Nintendo as the successor to the Wii. Released in late 2012, it is the first eighth-generation video game console and competed with Microsoft's Xbox One and Sony's PlayStation 4. The W ...

can be run in a special " Wii Mode" that activates an emulated version of the

Wii Menu The Wii system software is a set of updatable firmware versions and a software frontend on the Wii, a home video game console. Updates, which could be downloaded over the Internet or read from a Nintendo optical discs#Wii Optical Disc, game dis ...

as a means of playing games made for the latter system (it is not compatible with

GameCube The is a PowerPC-based home video game console developed and marketed by Nintendo. It was released in Japan on September 14, 2001, in North America on November 18, 2001, in Europe on May 3, 2002, and in Australia on May 17, 2002. It is the suc ...

games without system modification, however).

References

Backward compatibility Legacy hardware Legacy systems {{computer-stub

Examples

See also

References