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Figure 8-1 Expansion slots connecting to Southbridge

Figure 8-2 Expansion slots connecting to Northbridge

Figure 8-3 Expansion slots connecting to both Northbridge and Southbridge

Figure 8-4 The system crystal sets the speed.

Clock crystals aren’t just for CPUs and chipsets. Pretty much every chip in your computer has a CLK wire and needs to be pushed by a clock chip, 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 133-MHz sound card for a 133-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 (Figure 8-5).

Figure 8-5 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. The latest expansion buses run much faster, but remember that old speed of roughly 7 MHz; as you learn more about expansion slots, you’ll see that it’s still needed on even the most modern systems.

PC Bus

On first-generation IBM PCs, the 8088 CPU had an 8-bit external data bus and ran at a top speed of 4.77 MHz. IBM made the expansion slots on the first PCs with an 8-bit external bus connection. IBM wanted the bus to run as fast as the CPU, and even way back then, 4.77 MHz was an easy speed to achieve. IBM settled on a standard expansion bus speed of about 7 MHz—faster than the CPU! (This was the only occurrence in the history of PCs when the expansion bus was faster than the CPU.) This expansion bus was called the PC bus or XT bus. Figure 8-6 shows these ancient 8-bit expansion slots.

Figure 8-6 Eight-bit PC/XT slots

IBM certainly didn’t invent the idea of the expansion bus—plenty of earlier computers, including many mainframes, had expansion slots—but IBM did something no one had ever done. They allowed competitors to copy the PC bus and make their own PCs without having to pay a licensing or royalty fee. They also allowed third parties to make cards that would snap into their PC bus. Remember that IBM invented the PC bus—it was (and still is) a patented product of IBM Corporation. By allowing everyone to copy the PC expansion bus technology, however, IBM established the industry standard and fostered the emergence of the clone market. If IBM had not allowed others to copy their patented technologies for free, companies such as Compaq, Dell, and Gateway never would have existed. Equally, component makers such as Logitech, Creative, and 3Com would never be the companies they are today without the help of IBM. Who knows? If IBM had not opened the PC bus to the world, this book and the A+ Certification exams might have been based on Apple computers.

PC Bus

8 bits wide

7-MHz speed

Manual configuration

ISA

When Intel invented the 286 processor, IBM wanted to create a new expansion bus that took advantage of the 286’s 16-bit external data bus, yet also supported 8-bit cards. IBM achieved this by simply adding a set of connections to the end of the PC bus, creating a new 16-bit bus (Figure 8-7). Many techs