| x86 processor modes | ||
| Mode | First supported | |
| Intel 8086 | ||
|
NEC V20/V30 only | |
| Intel 80286 | ||
| Intel 80386 | ||
|
Intel 80386 | |
| Intel 386SL | ||
| AMD Opteron | ||
| AMD Athlon 64, varies | ||
In x86 computing, unreal mode, also big real mode, huge real mode, or flat real mode, is a variant of real mode (PE=0), 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 x86-32 and x86-64 processors can operate in. It is used in the 80386 and later x86 processors.
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For efficiency reasons, the 80386 and later x86 processors use the base address stored in their internal segment descriptor cache whenever accessing memory, regardless of whether they are operating in real or protected mode. The "selector", i.e. the 16-bit "segment number" visible to the programmer is used once, while reloading a segment register, to update the base address of the respective descriptor.
Some DOS extenders use this feature to address the high memory. It was used by many computer games in the 1990-to-1995 time frame, because it allowed programmers to address more memory than in real mode (only 1 MiB; only 640 KiB usable on IBM PC-compatible machines), without losing access to the DOS operating system (which doesn't work in protected mode). 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. 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 SMM mode in Intel 386SL and later processors places the processor in unreal mode.
To enable unreal mode without using any undocumented features of the CPU, the program has to 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.
There is a time penalty for leaving protected mode on the 286, because the CPU must be reset. Undocumented features of the CPU, such as LOADALL[1], can be used to enter unreal mode more quickly.
As described above, unreal mode usually involves using one or more data selectors to address data in memory more efficiently. This has been common practice and often referred to as "big" real mode. "Huge" real mode is attained by, in addition, loading the code selector (CS) from a descriptor allowing access to the whole memory and having the 32-bit attribute ("D" bit) set to one. This rarely used "mode" presents some advantages but it is more difficult to set up, since real mode interrupts do not automatically preserve the high sixteen bits of the extended instruction pointer, EIP.