77 mhz ibm made the expansion slots on the first pcs

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Unformatted text preview: 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. ch08.indd 292 12/14/09 2:49:17 PM All-In-One / CompTIA Network+ All-in-One Exam Guide / Meyers & Jernigan / 170133-8 / Chapter 8 All-In-One Chapter 8: Expansion Bus 293 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 ch08.indd 293 12/14/09 2:49:18 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 294 ISA When Intel invented the 286 processor, IBM wanted to create a new expansion bus that took advantage...
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