Enabling 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.
Undocumented features of the CPU, such as LOADALL, can be used to enter unreal mode more quickly.
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