Intel 80386 manual Virtual Memory, Linear to Physical Address Translation

SYSTEM ARCHITECTURE

of reference principle suggests that the new entry is likely to be used again in the near future.

While enabling paging does not increase address translation time, it does make instruction execu- tion time vary slightly, due to the occasional TLB misses. By disabling paging, real-time systems can eliminate this potential response time variable.

Figure 3-7 shows the basic content of a page table entry (PTE). Directory entries are identical, except that the page address field is interpreted as the physical address of a page table, rather than a page.

Tasks can share individual pages or entire page tables. Entries in different page tables that point to the same page are aliases of one another just as descriptors with the same base address are aliases of one another. The 80386's two-level page table structure makes it easier to share pages between tasks by sharing entire page tables. Since the

address of a page shared in this way exists in a single page table, the operating system has one page table entry to update when it moves the page.

3.3.4 Virtual Memory

Virtual memory allows very large programs, or groups of programs, to run in much smaller amounts of physical memory without overlays. Virtual memory systems can be based on either segments or pages. In either case, the basic idea of virtual memory is to exploit the much lower cost of disk storage compared to semiconductor memory. A virtual memory operating system stores all segments or pages in a large disk area, often called the swap area. The much smaller physical ("real') memory holds only the most frequently used segments or pages. So long as the segments or pages stored on disk are used

4GB

				liNEAR ADDRESS
			31	21	11		0
			DIRECTORY		PAGE	OFFSET	J
			J			l		PHYSICAL	4KB
									] PAGE
							~	ADDRESS
					.Jl	:J::
I	CR3	~ DIR. ENTRY		n'-PGf. TBl. ENTRY ~
(PAGE TABLE			PAGE TABLE
			DIRECTORY
DIRECTORY BASE)			(1024 ENTRIES)
					~	~		....	.... ...
					""'1:	:....f'"		'-'C	>r
					PAGE TABLES			o PHYSICAL
					(1024 ENTRIES EACH)			ADDRESS
								SPACE

Figure 3-6. Linear to Physical Address Translation

3-8