HP ProLiant Servers AIS: Official Study Guide and Desk Reference

The original PC system architecture started out with one bus with a single speed and the same bus width to all peripherals, as illustrated in Figure 2-1.

Two main performance bottlenecks resulted from this architecture. First, because only one system bus was available for all devices, only one device could use the system bus at a time. When one device was using the bus, all other devices had to wait their turn for attention. Second, all bus transfers, both system and I/O, were restricted to the same bus speed.

In this architecture, approximately 96% of all data transfers were between the processor and memory.