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Data Bus

The portion of the memory bus that carries the actual data to and from the memory is known as the data bus. Like the other buses that you have learned about thus far, the performance of the bus, or how fast the data travels, is determined by how fast the data moves over the bus and the width of the bus.

One can think of a bus much like a highway system. The higher the speed limit, the more cars can travel over the highway. Additionally, the more lanes that make up the highway, the more cars can travel over the highway simultaneously. The same holds true for computer buses. Your typical modern PC has a memory bus speed of 100 MHz or greater. This means that there are 100 million cycles per second that data can be sent over the bus. Newer PC's have memory bus speeds of 133 MHz. So we have effectively raised the speed limit on our digital highway 33 million cycles per second.

Taking our analogy further, if we add more lanes to our highway (more wires on the bus), the more cars (bits) can travel over the bus per cycle. The function of the bus speed and width describes how much data can flow over the bus and is known as bandwidth.

 Calculating BandwidthFor those of you who are into math, here is how you can calculate the bandwidth of a given data bus (for this example, we have chosen a Pentium III processor with a data bus of 64 bits, running in a 100 MHz system):Data Bus (in bits) / 8 * Bus Speed (MHz) * 1,000,000 / 1,048,576 = Bandwidth (in MB / sec)64 / 8 * 100 * 1,000,000 / 1,048,576 = 763 MB / sec (Pentium III, 100 MHz bus)Okay, lets see what would happen if we upgraded to a 133 MHz system: 64 / 8 * 133 * 1,000,000 / 1,048,576 = 1015 MB / sec (Pentium III, 133 MHz bus)As you might have expected, the extra 33 MHz afforded us with about 252 MB / sec of additional bandwidth to slightly under 1 GB per second of data transfer.

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