Legacy Mode - Examples

Examples

• x86-64 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 to run in Windows.

• 32-bit x86 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 cards have a VGA and a SVGA 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 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 can be configured to emulate Windows 2000 and Windows 98; Mac OS X 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)
  • PS2/RS-232 mouse (USB mouse)
  • PS2/AT keyboard (USB keyboard)
  • Many SATA disk controllers offer a legacy mode of operation for compatibility i.e. Parallel ATA emulation