SWAPPING

•  To replace pages or segments of data in memory.
• Swapping is a useful technique that enables a computer to execute programs and manipulate data files larger than main memory.
• The operating system copies as much data as possible into main memory, and leaves the rest on the disk. When the operating system needs data from the disk, it exchanges a portion of data (called a page or segment) in main memory with a portion of data on the disk.
• Swapping is a mechanism in which a process can be swapped temporarily out of main memory to a backing store , and then brought back into memory for continued execution.
• Backing store is a usually a hard disk drive or any other secondary storage which fast in access and large enough to accommodate copies of all memory images for all users.
•  It must be capable of providing direct access to these memory images.
• DOS does not perform swapping, but most other operating systems, including OS/2, Windows, and UNIX, do.
• Swapping is often called paging.
• Major time consuming part of swapping is transfer time.
• Total transfer time is directly proportional to the amount of memory swapped.
•  Let us assume that the user process is of size 100KB and the backing store is a standard hard disk with transfer rate of 1 MB per second.