Virtual Memory Systems

Virtual memory is a memory management technique that gives applications the illusion of having a very large contiguous memory space, even though the actual physical memory may be much smaller. This is achieved by using secondary storage (typically disk) to extend the available memory. Virtual memory allows processes to execute even if they are only partially resident in physical memory, with the operating system transparently moving pages between disk and RAM as needed. The key components of virtual memory systems are demand paging (pages are loaded into memory only when needed), page replacement algorithms (deciding which pages to remove when memory is full), and working set model (tracking which pages a process is actively using). Virtual memory provides several important benefits: it allows processes to be larger than physical memory, it simplifies memory management for programmers, it enables efficient process creation through copy-on-write techniques, and it provides memory protection between processes. However, virtual memory introduces overhead due to page faults and address translation, and poor implementation can lead to thrashing where the system spends more time paging than executing useful work. Modern operating systems use sophisticated virtual memory implementations with techniques like prepaging, page buffering, and working set tracking to optimize performance.