Legacy mode
Encyclopedia
In computing, legacy mode is a state in which a computer system, component, or software application behaves in a way different from its standard operation in order to support older software, data, or expected behavior. It differs from backward compatibility
in that an item in this state 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.
Backward compatibility
In the context of telecommunications and computing, a device or technology is said to be backward or downward compatible if it can work with input generated by an older device...
in that an item in this state 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
- x86-64X86-64x86-64 is an extension of the x86 instruction set. It supports vastly larger virtual and physical address spaces than are possible on x86, thereby allowing programmers to conveniently work with much larger data sets. x86-64 also provides 64-bit general purpose registers and numerous other...
processors can be run in one of two states: long modeLong modeIn the x86-64 computer architecture, long mode is the mode where a 64-bit application can access 64-bit instructions and registers...
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 modeVirtual 8086 modeIn the 80386 microprocessor and later, virtual 8086 mode allows the execution of real mode applications that are incapable of running directly in protected mode while the processor is running a protected mode operating system.VM86 mode uses a segmentation scheme identical to that of real mode In...
to run in Windows.
- 32-bit x86 processors themselves have two legacy modes: real modeReal modeReal mode, also called real address mode, is an operating mode of 80286 and later x86-compatible CPUs. Real mode is characterized by a 20 bit segmented memory address space and unlimited direct software access to all memory, I/O addresses and peripheral hardware...
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 modeProtected modeIn computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units...
environment. Protected mode is the non-legacy mode of 32-bit x86 processors and the 80286.
- Most PCPersonal computerA personal computer is any general-purpose computer whose size, capabilities, and original sales price make it useful for individuals, and which is intended to be operated directly by an end-user with no intervening computer operator...
graphic cards have a VGAVideo Graphics ArrayVideo Graphics Array refers specifically to the display hardware first introduced with the IBM PS/2 line of computers in 1987, but through its widespread adoption has also come to mean either an analog computer display standard, the 15-pin D-subminiature VGA connector or the 640×480 resolution...
and a SVGASuper Video Graphics ArraySuper Video Graphics Array or Ultra Video Graphics Array, almost always abbreviated to Super VGA, Ultra VGA or just SVGA or UVGA is a broad term that covers a wide range of computer display standards....
mode that allows them to be used on systems that have not loaded the device driverDevice driverIn computing, a device driver or software driver is a computer program allowing higher-level computer programs to interact with a hardware device....
necessary to take advantage of their more advanced features.
- Operating systemOperating systemAn operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
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 XPWindows XPWindows XP is an operating system produced by Microsoft for use on personal computers, including home and business desktops, laptops and media centers. First released to computer manufacturers on August 24, 2001, it is the second most popular version of Windows, based on installed user base...
can be configured to emulate Windows 2000Windows 2000Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the...
and Windows 98Windows 98Windows 98 is a graphical operating system by Microsoft. It is the second major release in the Windows 9x line of operating systems. It was released to manufacturing on 15 May 1998 and to retail on 25 June 1998. Windows 98 is the successor to Windows 95. Like its predecessor, it is a hybrid...
; Mac OS XMac OS XMac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...
can support the execution of Mac OS 9Mac OS 9Mac OS 9 is the final major release of Apple's Mac OS before the launch of Mac OS X. Introduced on October 23, 1999, Apple positioned it as "The Best Internet Operating System Ever," highlighting Sherlock 2's Internet search capabilities, integration with Apple's free online services known as...
applications on PowerPC-based Macintoshes.
- Computer buses emulated through legacy mode:
- Emulated bus (Host bus)
- ISAIndustry Standard ArchitectureIndustry Standard Architecture is a computer bus standard for IBM PC compatible computers introduced with the IBM Personal Computer to support its Intel 8088 microprocessor's 8-bit external data bus and extended to 16 bits for the IBM Personal Computer/AT's Intel 80286 processor...
(LPCLow Pin CountThe Low Pin Count bus, or LPC bus, is used on IBM-compatible personal computers to connect low-bandwidth devices to the CPU, such as the boot ROM and the "legacy" I/O devices . The "legacy" I/O devices usually include serial and parallel ports, PS/2 keyboard, PS/2 mouse, floppy disk controller...
) - PCI (PCI ExpressPCI ExpressPCI Express , officially abbreviated as PCIe, is a computer expansion card standard designed to replace the older PCI, PCI-X, and AGP bus standards...
) - PS2PS/2 connectorThe PS/2 connector is a 6-pin Mini-DIN connector used for connecting some keyboards and mice to a PC compatible computer system. Its name comes from the IBM Personal System/2 series of personal computers, with which it was introduced in 1987...
/RS-232RS-232In telecommunications, RS-232 is the traditional name for a series of standards for serial binary single-ended data and control signals connecting between a DTE and a DCE . It is commonly used in computer serial ports...
mouse (USB mouse) - PS2PS/2 connectorThe PS/2 connector is a 6-pin Mini-DIN connector used for connecting some keyboards and mice to a PC compatible computer system. Its name comes from the IBM Personal System/2 series of personal computers, with which it was introduced in 1987...
/AT keyboard (USB keyboard) - Many SATASataSata is a traditional dish from the Malaysian state of Terengganu, consisting of spiced fish meat wrapped in banana leaves and cooked on a grill.It is a type of Malaysian fish cake, or otak-otak...
disk controllers offer a legacy mode of operation for compatibility i.e. Parallel ATA emulation