In computing, legacy mode is a state in which a computer system, component, or software application behaves in a way that is different from its standard operation in order to support older software, data, or expected behavior. It differs from

backward compatibility Backward compatibility (sometimes known as backwards compatibility) is a property of an operating system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especiall ...

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

Examples

x86-64 x86-64 (also known as x64, x86_64, AMD64, and Intel 64) is a 64-bit version of the x86 instruction set, first released in 1999. It introduced two new modes of operation, 64-bit mode and compatibility mode, along with a new 4-level paging ...

processors can be run in one of two states: long mode provides larger physical address spaces and the ability to run 64-bit applications which can use larger virtual address spaces and more registers, and legacy mode. These processors' legacy mode allows these processors to act as if they were 16- or 32-bit x86 processors with all of the abilities and limitations of them in order to run legacy 16-bit and 32-bit operating systems, 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 a ...

to run in Windows. * 32-bit

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 Intel 8086 microprocessor and its 8088 variant. The 8086 was intr ...

processors themselves have two legacy modes: real mode 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 to allow the running of programs that require real mode in order to run under a protected mode environment. Protected mode is the non-legacy mode of 32-bit x86 processors and the 80286. * Most PC

graphic card A graphics card (also called a video card, display card, graphics adapter, VGA card/VGA, video adapter, display adapter, or mistakenly GPU) is an expansion card which generates a feed of output images to a display device, such as a computer mo ...

s have a

VGA Video Graphics Array (VGA) is a video display controller and accompanying de facto graphics standard, first introduced with the IBM PS/2 line of computers in 1987, which became ubiquitous in the PC industry within three years. The term can no ...

and a

SVGA Super VGA (SVGA) is a broad term that covers a wide range of computer display standards that extended IBM's VGA specification. When used as shorthand for a resolution, as VGA and XGA often are, SVGA refers to a resolution of 800×600. History I ...

mode that allows them to be used on systems that have not loaded the device driver necessary to take advantage of their more advanced features. *

Operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...

s 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 upgrade to its predecessors, Windows 2000 for high-end and ...

can be configured to emulate

Windows 2000 Windows 2000 is a major release of the Windows NT operating system developed by Microsoft and oriented towards businesses. It was the direct successor to Windows NT 4.0, and was released to manufacturing on December 15, 1999, and was officiall ...

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. The second operating system in the 9x line, it is the successor to Windows 95, and was released to ...

;

Mac OS X macOS (; previously OS X and originally Mac OS X) is a Unix operating system developed and marketed by Apple Inc. since 2001. It is the primary operating system for Apple's Mac computers. Within the market of desktop and la ...

can support the execution of Mac OS 9 applications on PowerPC-based Macintoshes. * Computer buses emulated through legacy mode: ** Emulated bus (Host bus) ** ISA ( LPC) ** PCI ( PCI Express) ** PS/2 or RS-232 mouse ( USB mouse) ** PS/2 or AT keyboard (

USB keyboard The technology of computer keyboards includes many elements. Among the more important of these is the switch technology that they use. Computer alphanumeric keyboards typically have 80 to 110 durable switches, generally one for each key. The ch ...

) ** 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 t ...

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

and

SD card Secure Digital, officially abbreviated as SD, is a proprietary non-volatile flash memory card format developed by the SD Association (SDA) for use in portable devices. The standard was introduced in August 1999 by joint efforts between SanDis ...

s to emulate IDE hard drives for old

DOS DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems. DOS may also refer to: Computing * Data over signalling (DoS), multiplexing data onto a signalling channel * Denial-of-service attack (DoS), an attack on a communicat ...

and

Windows 95 Windows 95 is a consumer-oriented operating system developed by Microsoft as part of its Windows 9x family of operating systems. The first operating system in the 9x family, it is the successor to Windows 3.1x, and was released to manufacturi ...

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. Th ...

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

Wii Menu The Wii Menu is the graphical shell of the Wii and Wii U game console, as part of the Wii system software. It has four pages, each with a 4:3 grid, and each displaying the current time and date. Available applications, known as "channels", are ...

as a means of playing games made for the latter system (it is not compatible with GameCube games without system modification, however).

Examples

See also