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Connecting to the Processor

We have considered various ways to speed up processor operations, including having more stages in the processor, increasing the speed of the clock, and sending more data in the same amount of time. Accessing L2 cache and motherboard components was a bottleneck in older systems because the CPU used the same bus to communicate with RAM and other motherboard components as it did with L2 and motherboard cache. The solution is DIB (dual independent bus). With DIB, two buses are used: a back side bus and a front side bus. The back side bus connects the CPU to the L2 cache. The FSB (front side bus) connects the CPU to the motherboard components. The FSB is considered the speed of the motherboard. Figure 3.11 illustrates the concept of a front side bus. Remember that the front side bus is more detailed than what is shown; the figure simply illustrates the difference between the back side bus and the front side bus.

Figure 3.11

Figure 3.11 Front and back side bus

Many people think that the higher the CPU speed, the faster the computer. This is seldom true. Several factors contribute to computer speed. One factor is bus speed. Bus speed describes how fast the CPU can communicate with motherboard components, such as memory, the chipset, or the PCI/PCIe bus. The first Pentium CPUs ran at the same speed as the bus (60MHz); in time, CPUs got faster and buses stayed the same. Advances in technology have not reached the rest of the motherboard components (and it would cost too much to try to have them keep pace).

Intel and AMD have technologies to replace the front side bus in some parts. AMD’s solution is Direct Connect. Direct Connect allows each of the processor cores to connect directly to memory, to the other motherboard components such as the expansion slots, and to other processor cores using a high-speed bus called HyperTransport. Figure 3.13, later in this chapter, shows HyperTransport connectivity. Intel has QuickPath Interconnect (QPI), Serial Peripheral Interface (SPI), and Direct Media Interface (DMI), which are full-duplex (that is, traffic can flow in both directions simultaneously) point-to-point connections between the processor and one or more motherboard components. This type of connectivity used with Intel-based processors and chipsets is shown later in Figure 3.38.

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