MULTI-MICROPROCESSORS; AN OVERVIEW AND WORKING EXAMPLE 467

be a loosely coupled computer network. Closely coupled microcomputer networks might provide an attractive organization for reliable systems,* systems that must manage a large data base on many disks or other secondary storage, or even as a computational structure tailored to the data flow of a specialized application. It is questionable, however, whether a multiple microprocessor organized in the form of a network could replace a large conventional uniprocessor.

Figure 3 shows the basic structure of a multiprocessor. Its distinguishing characteristic is that, unlike the processors in computer net works, the processors in a multiprocessor share primary memory. Note that in the computer network of Figure 2, each processor has its own, private primary memory. Data is shared in a computer network by passing inter- processor messages, whereas in a multiprocessor, the central processors can directly share data in primary memory. The concept of a multiprocessor is not new; the Burroughs

Figure 3. The basic structure of a multiprocessor.

D825 (1962), Bendix G-21 (1963), GE 645 (1969), and IBM 360/65 (1969) provide early examples. In these multiprocessors, conventional, relatively expensive central processors were used, making it uneconomical to have more than a few processors. With small numbers of processors, it is not mandatory to decompose a single job into a set of concurrent, cooperating processes to use all the central processors at once; enough independent programs are usually resident in the primary memory of a conventional multiprogramming system to keep a few processors busy. More recently, multiprocessors using minicomputers have been implemented, and configurations now exist with as many as 14 to 16 processors in a single computer system [Wulf and Bell, 1972; Heart et al., 1973]. To effectively utilize the processors in such a system, a task must be explicitly decomposed to run concurrently on different processors.

One of the most challenging problems in de signing and implementing the hardware of multiprocessor systems, especially for large number of processors, is the processor/memory switching structure. Many techniques have been tried and used successfully in particular systems: multiple ports per memory unit, electronic crossbar switches, time-multiplexed common buses, and combinations and hierarchies of simpler switches.

The third form of computer organization that incorporates multiple processing elements is the multi-arithmetic logic unit (ALU) processor. The fundamental difference between this type

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* Examples of closely coupled computer networks built with minicomputers and designed for ultra-reliable applications include the Tandem computer [1977] and the five processor system for NASA's space shuttle [Sklaroff. 1976; Cooper. Chow. 1976].