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2 Basic Computer Architecture     continued...

As an example of a PMS diagram and a relatively simple computer architecture, Figure 1 shows the major components of the original Apple Macintosh personal computer. The first thing one notices is a single communication channel, known as the bus, that connects all the other major components. Since the bus is a switch, only two of these components can communicate at any time. When the switch is configured for an I/O transfer, for example from main memory () to the disk (via K), the processor is unable to fetch data or instructions and remains idle. This organization is typical of personal computers and low end workstations; mainframes, supercomputers, and other high performance systems have much richer (and thus more expensive) structures for connecting I/O devices to internal main memory that allow the processor to keep working at full speed during I/O operations.

Figure 1: PMS Diagram of the Apple Macintosh.