In this chapter, we have examined memory management. We saw that the simplest systems do not swap or page at all. Once a program is loaded into memory, it remains there in place until it finishes. Some operating systems allow only one process at a time in memory, while others support multiprogramming. This model is still common in small, embedded real-time systems.

The next step up is swapping. When swapping is used, the system can handle more processes than it has room for in memory. Processes for which there is no room are swapped out to the disk or SSD. Free space in memory and on nonvolatile storage can be kept track of with a bitmap or a hole list.

Modern computers often have some form of virtual memory. In the simplest form, each process’ address space is divided up into uniform-sized blocks called pages, which can be placed into any available page frame in memory. There are many page replacement algorithms; two of the better algorithms are aging and WSClock.

To make paging systems work well, choosing an algorithm is not enough; attention to such issues as determining the working set, memory allocation policy, and page size is required.

Segmentation helps in handling data structures that can change size during execution and simplifies linking and sharing. It also facilitates providing different protection for different segments. Sometimes segmentation and paging are combined to provide a two-dimensional virtual memory. The MULTICS system and the 32-bit Intel x86 support segmentation and paging. Still, it is clear that few operating system developers care deeply about segmentation (because they are married to a different memory model).