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 segment descriptors has been loaded with non-standard values, like 32-bit limits allowing to access entire memory. 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. 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.

Source: en.wikipedia.org