Memory Management part II

Swapping

The process of organising memory into fixed partitions on batch system is simple compared to time sharing systems or graphically oriented personal computers. On batch systems, each job is loaded into a partition when it gets to the head of the queue. It stays in memory until it has finished. As long as enough jobs can be kept in memory to keep the CPU busy all the time, there is no reason to use anything more complicated. On time-sharing machines, sometimes there is not enough main memory to hold all the currently active processes, therefore excess processes must be kept on disk and brought in to run dynamically.

Swapping as an approach to memory management consists of bringing each process in its entirety, running it for a while, then putting it back on the disk. For details and illustrations, see OS design and implementation, A.S. Tanenbaum & A.S. Woodhull, Prentice Hall 1997, pg.310.

Virtual Memory

Another strategy for managing memory is the virtual memory, which allows programs to run even when they are only partially in main memory. The basic idea behind this strategy is that the combine size of the program, data, and stack may exceed the amount of physical memory available for it. The operating system keeps those parts of the program currently in use in main memory, and the rest on the disk.

Virtual memory can also work in a multiprogramming system, with bits and pieces pf many programs in memory at once. While a program is waiting for a part of itself to be brought in, it is waiting for I/O and cannot run, so the CPU can be given to another process, the same way as for any other multiprogramming system. Most virtual memory systems use a technique called paging.

Diagram of memory management