Protected mode - paging

Paging is a more convenient memory management scheme introduced in 80386 and has been a bit enhanced since then. The idea behind paging is memory virtualization--this is the mechanism that makes it possible for different processes to have the same memory layout. In fact, the addresses we use in pointers (if we are writing in C, C++, or any other high-level language that compiles into native code) are virtual and do not correspond to physical addresses. The translation of a virtual address into a physical address is implemented in hardware and is performed by the CPU (however, some operating system interventions are possible).

By default, a 32-bit CPU uses a two-level translation scheme for the derivation of a physical address from the supplied virtual one.

The following table explains how a virtual address is used in order to find a physical address:

Most, if not all, modern processors based on the Intel architecture also support Page Size Extension (PSE), which makes it possible to use the so-called large pages of 4 MB. In this case, the translation of a virtual address into a physical address is a bit different, as there is no page table any more. The following table shows the meaning of bits in a 32-bit virtual address:

Furthermore, the Physical Address Extension (PAE) was introduced, which significantly changes the scheme and allows access to a much bigger range of memory. In protected mode, PAE adds a page directory pointer table of four entries and the virtual to physical address conversion would be as per the following table:

Enabling PSE in addition to PAE forces each entry in the page directory to point directly to a 2 MB page instead of an entry in a page table.