Central Processing Units (CPUs, Processors)
Often called the brains of the computer, the CPU is the device that performs the calculations that make computing possible. Although there are several CPU manufacturers, Intel and Advanced Micro Devices (AMD) make the vast majority of processors used in Windows-based PCs, so we will limit our discussion mostly to Intel Celeron and Pentium II, III, and 4 processors, and AMD K6-2, K6-3, Athlon, and Duron processors. For more information, go to intel.com or amd.com. One other processor family worth mentioning is the very inexpensive VIA C3 family from VIA Technologies (via.com.tw), which has started to show up in some similarly inexpensive systems. Time will tell if the quality will match that of Intel or AMD.
Selecting an Appropriate Processor
When selecting a processor, the first consideration is compatibility with the motherboard.
Motherboard-CPU Compatibility
A given motherboard can accept certain CPUs. The most obvious limitation is whether the motherboard has a CPU socket or slot, and which type of either it has (we discuss sockets and slots later in this chapter). Of the two most popular CPU brands, Intel and AMD, some boards can accept one and some can accept both. CPU compatibility is further limited by design elements of the board that require certain models and speed (gigahertz, or GHz) ratings. The motherboard manual will specify the processors that are compatible with it. However, you should check the board manufacturer’s Web site for updated information. Sometimes a motherboard will accept a processor version that didn’t exist when the manual was printed.
CPU Terminology
For you to be able to select the most appropriate CPU, it will be helpful to understand some terminology:
Front Side Bus (FSB): Measured in megahertz (MHz), the FSB is the channel that connects the processor with main memory. The faster this is, the better the performance will be. This number will range between 33 and 800 MHz.
Cache (pronounced “cash”): All new CPUs have cache memory. Cache, as it pertains to CPUs, is expensive high-speed memory used for storing frequently used instructions. This saves the time needed for the CPU to get all of its instructions from slower main memory. All other things being equal, the more cache a CPU has, the better its performance will be. The less expensive CPU lines, Intel Celeron and AMD Duron, have less cache than their otherwise equivalent Pentium and Athlon cousins. L2 Cache, the most variable number, ranges from 128 to 512 KB on relatively recent processor models.
Sockets and slots: As discussed in the motherboard section, processors either fit in a socket or slot, depending on their construction. There are quite a few different socket and slot types.
For more information on slots and sockets, see itp-journals.com/ sasample/T1053.pdf. There is also a very helpful presentation available at ccc. commnet.edu/DL/~moriber/pc_3e_03b.ppt.
Pins: Within the categories of sockets and slots, there are different types of each. The types vary by size, and number and configuration of pins.
Fans and heat sinks: All processors made in the recent past require heat sinks and fans. Heat sinks are little radiators used to radiate the heat away from the processor. Heat sinks are crucial; remove the heat sink from some processors while they’re running and they will melt, or even catch fire almost immediately. Some Intel processors have built-in temperature protection; they will shut down if the temperature gets too high, but they could still sustain damage. The heat sink and fan work together to keep the processor’s temperature within a safe range. It is necessary to use a thermal pad, thermal grease, or silver thermal compound between a processor and the heat sink, which not only helps to transfer heat from the processor but also evens out the surface to reduce the possibility of cracking the processor. Use the compound sparingly. The silver thermal compound has the highest heat transfer capacity of all of these. Figure 3.18 shows the thermal pad on the bottom of the heat sink.
Figure 3.18: The thermal pad goes on the bottom of the heat sink.
Note A very helpful article on these heat-transfer materials appears at antec-inc. com/info_DIYArticle2.html. We highly recommend that you read it.
CPU families: CPU manufacturers create families of processors for both technical and marketing purposes. Examples of family names are Intel’s Pentium 4 and Celeron, and AMD’s Athlon and Duron. Generally, each member of a family has the same internal design with the only differences being speed and perhaps some less-publicized specifications. Celerons don’t fit that definition exactly, however; all of Intel’s lower priced processors since Pentium IIs were current have been called Celerons. Therefore, Celerons have the same general internal design as other Celerons of the same particular generation and form (socket, slot, etc.). Celerons and Durons are less expensive than Pentiums and Athlons of the same speed and form. The differences are in such specifications as cache memory and front side bus speed. They are suitable for basic computing.
Selecting a Replacement CPU
When building or buying a new computer, it might make more sense to select the CPU first, and then select a motherboard to accommodate it. However, in the context of repair, you’ll usually have to find a CPU that matches an existing motherboard. Therefore, the first consideration is to review the motherboard documentation to see which processors are compatible with it, as discussed earlier in the chapter. You’ll then want to select the best of the compatible processors you can afford. “Best” is usually defined as fastest with the most cache. However, you should also consider the usage of the computer. A computer used primarily for e-mail, to read news on the Web, and to write letters does not need as good a processor as one used for heavy-duty number crunching or video production, for example.
By : Book-PC Repair and Maintenance: A Practical Guide
