Virtual Memory (Cont.)

For each process, the operating system keeps a table which maps between the process's virtual address space, to physical memory. This mapping is done in pages (e.g. 4KB long on IA32 machines).

When a process allocates memory, the operating system gives it a chunk of physical RAM, and maps a virtual memory segment to this physical memory page.

If more virtual memory is needed by processes, then one of the physical memory pages (used by one of the processes) is chosen and page out (copied) to the hard disk. If that process later attempts to access this page, a different page will be paged out to disk, and this page will be paged in again into memory.