Editing Page replacement algorithm (section)

== Detecting which pages are referenced and modified ==
{{see also|Virtual memory|Page table}}
Modern general purpose computers and some embedded processors have support for [[virtual memory]]. Each process has its own virtual address space. A [[page table]] maps a subset of the process virtual addresses to physical addresses. In addition, in most architectures the page table holds an "access" bit and a "dirty" bit for each page in the page table. The CPU sets the access bit when the process reads or writes memory in that page. The CPU sets the dirty bit when the process writes memory in that page. The operating system can modify the access and dirty bits. The operating system can detect accesses to memory and files through the following means:
* By clearing the access bit in pages present in the process' page table. After some time, the OS scans the page table looking for pages that had the access bit set by the CPU. This is fast because the access bit is set automatically by the CPU and inaccurate because the OS does not immediately receive notice of the access nor does it have information about the order in which the process accessed these pages.
* By removing pages from the process' page table without necessarily removing them from physical memory. The next access to that page is detected immediately because it causes a [[page fault]]. This is slow because a page fault involves a context switch to the OS, software lookup for the corresponding physical address, modification of the page table and a context switch back to the process and accurate because the access is detected immediately after it occurs.
* Directly when the process makes system calls that potentially access the page cache like <code>read</code> and <code>write</code> in POSIX.