  :A system with 32-bit physical addresses would be limited to 4 GiB of main memory.  A system with 36-bit physical addresses, such as a 32-bit x86 processor with Physical Address Extension, would be limited to 64 GiB of physical memory, even if, at any given time, a single address space could only include 4 GiB of that memory.  Many OSes running on x86 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 4 GiB of memory, it can inconveniently do so on many OSes (for example, most UN*Xes, as well as Windows NT; I don't think Windows 9x supported PAE) by mapping files in and out of the address space, and multiple processes can, together, access more than 4 GiB of memory.   