Real Mode -- On 80x8x CPUs 80386 and higher, Real Mode is the initial start up mode of the CPU. In real mode, 1 Megabyte of memory is accessible through a 20 bit address composed of a 16 bit segment number and a 16 bit address. I/O ports are directly addressable and no address translation is done. An additional 64KB of "high" memory addressing is accessible through an addressing trick. In Real Mode, early MSDOS programs will (well, should) directly execute just as they would on an 8088 or 80286 CPU. Real mode is used by MSDOS and is supported by Windows 3.0, but not Windows 3.1. Under Windows 95, "MSDOS mode" MSDOS runs in real mode unless a program such as Windows 3 that initiates protected mode is started from MSDOS.

Some programs and some old hardware may run only in Real Mode. Under Windows 9x, protected mode drivers are not accessible from Real Mode since no interrupts are enabled to initiate I/O operations through the protected mode device drivers.

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