Figure 8 4 the system crystal sets the speed clock

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Unformatted text preview: bridge, all timed by a 133-MHz crystal (Figure 8-4). 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. ch08.indd 291 12/14/09 2:49:17 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 8 CompTIA A+Certification All-in-One Exam Guide 292 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...
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This note was uploaded on 04/27/2010 for the course COMPTIA 1201 taught by Professor N/a during the Spring '10 term at Galveston College.

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