The system in Fig. 16.4.1 is quite misleading. For one thing, it has no input/output devices, without which no computer can do anything useful. Fig. 16.1.1 at the beginning of the chapter shows several I/O controllers plugged into the system bus. These boards are really small special-purpose computers which communicate with the memory and the main CPU through the system bus. In fact, the standard way for many systems is to communicate solely through the main memory.

I/O controllers often act as bus initiators because they need to initiate transfers of data from their attached devices (mice, disk drives, keyboards, tape drives, light pens) to memory, or from memory to various output devices (monitors, printers, motors.)