Understanding the Linux Kernel

2.4. Paging in Hardware

The paging unit translates linear addresses into physical ones. It checks the requested access type against the access rights of the linear address. If the memory access is not valid, it generates a page fault exception (see Chapter 4, and Chapter 6).

For the sake of efficiency, linear addresses are grouped in fixed-length intervals called pages; contiguous linear addresses within a page are mapped into contiguous physical addresses. In this way, the kernel can specify the physical address and the access rights of a page instead of those of all the linear addresses included in it. Following the usual convention, we shall use the term "page" to refer both to a set of linear addresses and to the data contained in this group of addresses.

The paging unit thinks of all RAM as partitioned into fixed-length page frames (they are sometimes referred to as physical pages). Each page frame contains a page, that is, the length of a page frame coincides with that of a page. A page frame is a constituent of main memory, and hence it is a storage area. It is important to distinguish a page from a page frame: the former is just a block of data, which may be stored in any page frame or on disk.

The data structures that map linear to physical addresses are called page tables; they are stored in main memory and must be properly initialized by the kernel before enabling the paging unit.

In Intel processors, paging is enabled by setting the PG flag of the cr0 register. ...