History
The 286 architecture introduced protected mode, allowing for (among other things) hardware-level memory protection. Using these new features, however, required a new operating system that was specifically designed for protected mode. Since a primary design specification of x86 microprocessors is that they are 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 systems written for the 8086 and the 80186. As of August 2011, even the newest x86 CPUs (including x86-64 CPUs) start in real mode at power-on and can run software written for almost any previous x86 chip.
The PC BIOS which IBM introduced operates in real mode, as do the DOS operating systems (MS-DOS, DR-DOS, etc.). Early versions of Microsoft Windows ran in real mode, until Windows 386, which ran in protected mode, and the more fully realized Windows 3.0, 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, Linux, OS/2, Windows 95 and later, etc.) switch the CPU into protected mode at startup, but 64-bit operating systems will use this only as another stepping stone to get to long mode. It is worth noting that the protected mode of the 80286 is considerably more primitive than the improved protected mode introduced with the 80386; the latter is sometimes called 386 protected mode, and is the mode most modern 32-bit x86 operating systems run in.
Read more about this topic: Real Mode
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