The Linux Virtual Memory System: Page Table Structure

Page-table structure provided by x86

Because we are focused on Linux for x86, our discussion will center on the type of page-table structure provided by x86, as it determines what Linux can and cannot do. As mentioned before, x86 provides a hardware-managed, multi-level page table structure, with one-page table per process; the OS simply sets up mappings in its memory, points a privileged register at the start of the page directory, and the hardware handles the rest. The OS gets involved, as expected, at process creation, deletion, and upon context switches, making sure in each case that the correct page table is being used by the hardware MMU to perform translations.

Shift from 32-bit x86 to 64-bit x86

Probably the biggest change in recent years is the move from 32-bit x86 to 64-bit x86, as briefly mentioned above. As seen in the VAX/VMS system, 32-bit address spaces have been around for a long time, and as technology changed, they were finally starting to become a real limit for programs. Virtual memory makes it easy to program systems, but with modern systems containing many GB of memory, 32 bits were no longer enough to refer to each of them. Thus, the next leap became necessary.

Moving to a 64-bit address affects page table structure in x86 in the expected manner. Because x86 uses a multi-level page table, current 64-bit systems use a four-level table. The full 64-bit nature of the virtual address space is not yet in use, however, rather only the bottom 48 bits. Thus, a virtual address can be viewed as follows: