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More on Paging

The mission of Page Table entries is to describe Page Frames (not only to map them). Such description includes access rights, swap status etc. and it's used by the CPU to enforce general protection and to manage swapping, among other functions.

Each entry on a Page Directory (on a Page Table as well) consists of an array of 32 bits cconforming bit-mapped fields. We are not going to get into details but only to underwrite the following:

-- Each Page Table entry occupies 4 Bytes (32 bits). However, it describes a physical memory space of 4KB (one Page Frame).

-- Each Page Table occupies up to 4KB (1024 entries, 4 Bytes each).

-- Each Directory occupies up to 4KB too. In combination to the Page Tables it points, the Directory describes up to 1M page frames, that is, the entire 4GB linear address space as expected.

-- Page Directories and Page Tables hold the necessary information needed for the CPU to enforce protection.


An option for system designers is to allocate one global directory for each process running in the system. There are other options as well.