CompTIA A+ Certification All-in-One Exam Guide

including the chips on your expansion cards. Suppose you buy a device that
did not come with your computer—say, a sound card. The chips on the sound
card need to be pushed by a CLK signal from a crystal. If PCs were designed
to use the system crystal to push that sound card, sound card manufacturers
would need to make sound cards for every possible motherboard speed. You
would have to buy a 100-MHz sound card for a 100-MHz system or a 200-
MHz sound card for a 200-MHz system.
That would be ridiculous, and IBM knew it when they designed the PC.
They had to make an extension to the external data bus that ran at its own
standardized speed. You would use this part of the external data bus to snap
new devices into the PC. IBM achieved this goal by adding a different
crystal, called the expansion bus crystal, which controlled the part of the
external data bus connected to the expansion slots (see Figure 6-21).
Figure 6-21 Function of system and expansion bus crystals
The expansion slots run at a much slower speed than the frontside bus.
The chipset acts as the divider between the two buses, compensating for the
speed difference with wait states and special buffering (storage) areas. No
matter how fast the motherboard runs, the expansion slots run at a standard
speed. In the original IBM PC, that speed was about 14.318 MHz ÷ 2, or
about 7.16 MHz. Luckily, modern expansion buses run much faster! Let’s
start with the oldest of the modern expansion slots, PCI.
PCI
Intel introduced the Peripheral Component Interconnect (PCI) bus