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.