Our coverage of operating-system issues thus far has focused mainly on general-purpose computing systems. There are, however, specialpurpose systems with requirements different from those of many of the systems we have described.

A real-time system is a computer system that requires not only that computed results be "correct" but also that the results be produced within a specified deadline period. Results produced after the deadline has passed—even if correct—may be of no real value. For such systems, many traditional operating-system scheduling algorithms must be modified to meet the stringent timing deadlines.

A multimedia system must be able to handle not only conventional data, such as text files, programs, and word-processing documents, but also multimedia data. Multimedia data consist of continuous-media data (audio and video) as well as conventional data. Continuous-media data—such as frames of video—must be delivered according to certain time restrictions (for example, 30 frames per second). The demands of handling continuous-media data require significant changes in operatingsystem structure, most notably in memory, disk, and network management.