Unreal mode

In x86 computing, unreal mode, also big real mode, huge real mode, or flat real mode, is a variant of real mode, in which one or more data segment registers have been loaded with 32-bit addresses and limits. Contrary to its name, it is not a separate addressing mode that the x86 processors can operate in. It is used in the 80386 and later x86 processors.

For efficiency reasons, the 80286 and all later x86 processors use the base address, size and other attributes stored in their internal segment descriptor cache whenever computing effective memory addresses, even in real mode.

Contrary to simplified descriptions found in early Intel manuals, a "selector", i.e. a 16-bit "segment number" stored and kept in a (visible to the programmer) "segment register", is not used to calculate addresses. HIMEM.SYS uses this feature to address extended memory. After the introduction of Windows 95, unreal mode quickly fell out of favor because programs using it cannot run in the DOS prompt of Microsoft Windows; they require a "Restart in MS-DOS mode" in Windows 95 and 98, and cannot be run at all on NT and later Windows systems, which run MS-DOS programs in virtual 8086 mode that is incompatible with unreal mode. For those operating systems, an emulator such as DOSBox is the only way to run programs designed for unreal mode.

Unreal mode is still extensively used by BIOS code. In particular, the System Management Mode (SMM) in Intel 386SL and later processors places the processor in unreal mode.

To put an 80386 or higher microprocessor into unreal mode, a program must first enter protected mode, find or create a flat descriptor in the GDT or LDT, load some of the data segment registers with the respective protected mode "selector", and then switch back to real mode. After returning to real mode, the processor will continue using the cached descriptors as established in protected mode, thus allowing access to 4 GiB of "extended" memory from real mode.

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