Real mode
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Real mode is an operating mode of 80286 and later x86-compatible CPUs. Real mode is characterized by a 16 bit segmented memory address space (meaning that only 1 MB of memory can be addressed), direct software access to BIOS routines and peripheral hardware, and no concept of memory protection or multitasking at the hardware level. All x86 CPUs in the 80286 series and later start up in real mode at power-on; 80186 CPUs and earlier had only one operational mode, which is equivalent to real mode in later chips.
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 3.0, 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 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). Windows 3.1 removed support for Real Mode, and was the first mainstream operating environment which required at least an 80286 processor (not counting Windows/286 which was not a mainstream product). Almost all modern x86 operating systems (Linux, Windows 95 and later, OS/2, etc.) switch the CPU into protected mode at startup.