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The 286 architecture introduced protected mode, allowing for (among other things) hardware-level memory protection. Using these new features, however, required extra software instructions not previously necessary. Since a primary design specification of x86 microprocessors is that they be fully backwards compatible with software written for all x86 chips before them, the 286 chip was made to start up in 'real mode' — that is, in a mode which turned off the new memory protection features, so that it could run software written for older microprocessors. To this day, even the newest x86 CPUs start up in real mode at power-on, and can run software written for any previous chip.
The DOS operating systems ( MS-DOS, DR-DOS, etc.) operate in real mode. Early versions of Microsoft Windows (which were essentially just graphical user interface shells running on top of DOS, and not actually operating systems per se) ran in real mode, until Windows 3.1, which could run in either real or protected mode. Windows 3.1's protected mode environment was called 386 enhanced mode, because in addition to using protected mode, it also used 32 bit addressing and thus would not run on a 286 (despite having protected mode, the 286 was still a 16 bit chip; 32 bit registers were introduced in the 80386 series.) Almost all modern x86 operating systems ( LinuxThis article is about Linux-based operating systems, GNU/Linux, and related topics. See Linux kernel for the kernel itself. See Linux (washing powder) for the Swiss brand of washing powder. Tux, a plump penguin, is the official Linux mascot Linux is the n, Windows 95Windows 95 ( codename Chicago is a hybrid 16-bit/ 32-bit graphical operating system released on October 13, 1995 by the Microsoft Corporation. Windows 95 is a direct descendant of Microsoft's formerly separate MS-DOS and Windows products. It is the first and later, OS/2OS/2 is an operating system created by Microsoft and IBM and later developed by IBM exclusively. The name stands for "Operating System/2", because it was intended as the preferred operating system for IBM's " Personal System/2 ( PS/2)" line of second-gene, etc.) switch the CPU into protected mode at startup.