A system with bit physical addresses would be limited to  GiB of main memory  A system with bit physical addresses such as a bit x processor with Physical Address Extension would be limited to  GiB of physical memory even if at any given time a single address space could only include  GiB of that memory  Many OSes running on x processors would give different processes different address spaces and many of them would let the code running in that process map files and other objects into and out of the address space so whilst one process cannot conveniently access more than  GiB of memory it can inconveniently do so on many OSes for example most UNXes as well as Windows NT I dont think Windows x supported PAE by mapping files in and out of the address space and multiple processes can together access more than  GiB of memory