Modern Usage
The Intel 80386 microprocessor introduced paging - allocating separate physical memory pages (themselves very small units of memory) at the same virtual addresses, with the advantage that disk paging is far faster and more efficient than segment swapping. Therefore, modern 32-bit x86 operating systems use the LDT very little, primarily to run legacy 16-bit code.
Should 16-bit code need to run in a 32-bit environment while sharing memory (this happens e.g. when running OS/2 1.x programs on OS/2 2.0 and later), the LDT must be written in such a way that every flat (paged) address has also a selector in the LDT (typically this results in the LDT being filled with 64 KiB entries). This technique is sometimes called LDT tiling. The limited size of the LDT means the virtual flat address space has to be limited to 512 megabytes (8191 times 64 KiB) - this is what happens on OS/2, although this limitation was fixed in version 4.5. It is also necessary to make sure that objects allocated in the 32-bit environment do not cross 64 KiB boundaries; this generates some address space waste.
If 32-bit code does not have to pass arbitrary memory objects to 16-bit code, e.g. presumably in the OS/2 1.x emulation present in Windows NT or in the Windows 3.1 emulation layer, it is not necessary to artificially limit the size of the 32-bit address space.
Read more about this topic: Local Descriptor Table
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