What’s Actually In The Page Table?

Linear page table

Let’s talk a little about page table organization. The page table is just a data structure that is used to map virtual addresses (or really, virtual page numbers) to physical addresses (physical frame numbers). Thus, any data structure could work. The simplest form is called a linear page table, which is just an array. The OS indexes the array by the virtual page number (VPN) and looks up the page-table entry (PTE) at that index in order to find the desired physical frame number (PFN). For now, we will assume this simple linear structure; in later chapters, we will make use of more advanced data structures to help solve some problems with paging.

Valid bits

As for the contents of each PTE, we have a number of different bits in there worth understanding at some level. A valid bit is common to indicate whether the particular translation is valid; for example, when a program starts running, it will have code and heap at one end of its address space, and the stack at the other. All the unused space in-between will be marked invalid, and if the process tries to access such memory, it will generate a trap to the OS which will likely terminate the process. Thus, the valid bit is crucial for supporting a sparse address space; by simply marking all the unused pages in the address space invalid, we remove the need to allocate physical frames for those pages and thus save a great deal of memory.

Protection bits

We also might have protection bits, indicating whether the page could be read from, written to, or executed from. Again, accessing a page in a way not allowed by these bits will generate a trap to the OS.

Other bits

There are a couple of other bits that are important but we won’t talk about much for now. A present bit indicates whether this page is in physical memory or on disk (i.e., it has been swapped out). We will understand this machinery further when we study how to swap parts of the address space to disk to support address spaces that are larger than physical memory; swapping allows the OS to free up physical memory by moving rarely-used pages to disk. A dirty bit is also common, indicating whether the page has been modified since it was brought into memory.

A reference bit (a.k.a. accessed bit) is sometimes used to track whether a page has been accessed, and is useful in determining which pages are popular and thus should be kept in memory; such knowledge is critical during page replacement, a topic we will study in great detail in subsequent chapters.

The figure below shows an example page table entry from the x86 architecture"Intel 64 and IA-32 Architectures Software Developer’s Manuals" Intel, 2009. Available: http://www.intel.com/products/processor/manuals. In particular, pay attention to “Volume 3A: System Programming Guide Part 1” and “Volume 3B: System Programming Guide Part 2”.. It contains a present bit §; a read/write bit (R/W) which determines if writes are allowed to this page; a user/supervisor bit (U/S) which determines if user-mode processes can access the page; a few bits (PWT, PCD, PAT, and G) that determine how hardware caching works for these pages; an accessed bit (A) and a dirty bit (D); and finally, the page frame number (PFN) itself.