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The Central Processing Unit (CPU) is the heart of your computer as it performs nearlly allfunctions that require extensive processing power. Therefore, it is very important that youchoose a suitable CPU for your function as the choice of CPU directly affects the speed andstability of your system.Before we can explain differences between CPUs, you must first be familiar with certain CPUproperties.•Clock Speed, measured in Gigahertz (GHz), or Megahertz (MHz) on older systems is thenumber of calculation cycles that your CPU can perform per second. Therefore, a higher clockspeed generally points toward a faster system. But not all CPUs perform an equal quantity ofwork per cycle, meaning two CPUs at the same clock speed can potentially perform at verydifferent levels.•IPC, or Instructions Per Cycle is the amount of work a CPU can do in a cycle. This varies withdiffrent properties of the CPU.•Front Side Bus Speed (FSB) is the rate at which the CPU communicates with the motherboardFront Side Bus components in MHz. A larger FSB value shows that your CPU is able tocommunicate with other components on the motherboard (and thus your system) faster.•Interface: CPUs must connect to motherboards via a series of connectors. It is VERYimportant that your CPU interface is a COMPLETE MATCH to your motherboard CPU socketotherwise you would be wasting money on a piece of spare silicon.•Bit-Rate: Most modern CPUs are of the 32-bit system which work fine with most modernoperating system and hardware. Higher end models are the 64-bit system which may allowfaster CPU processing capabilities due to the larger band-width, but not all operating systemsare compatible with the newer 64-bit format -- they will still run, but without a majorperformance boost.•Hyper Threading (HT): Hyper-threading is a new technology of parallel processing whichinstead of one single core, your CPU is able to have separate cores working on different taskswhich greatly speeds up the system speed. However, Hyper Threading technology requires amotherboard that supports Hyperthreading technology which are generally very expensive. Andof course the software you use has to be optimized for Hyperthreading to give more speed.•Manufacture and Model: There are two main manufactures of CPUs Intel and AMD, eachhaving advantages and disadvantages that would be explained below.•L2-Cache: the amount of Memory dedicated for the CPU in MB, generally, the larger the L2cache, the faster your system would run. However, L2 cache uses a lot of transistors, and thelarger the cache, the transistors needed, which consums more electricity, and outputs more heat.•The Core of the CPU is the heart of the CPU. Often several cores will be marketed under thesame name, so look at what core you are buying.Now, one of the most common mistakes of choosing a CPU is by ignoring the fine print ofCPU specifications while relying completely on the clock speed. CPUs specs are written in full,I give a brief explanation of the spec, eg.Intel Pentium 4 3.2GHz LGA775 FSB800 HT L2-2MBModel: Intel Pentium 4Clock Speed: 3.2GHz (=3200MHz)Interface: Land Grid Array 775L2-Cache: 2MB (=2048 kB)Other Spec: HT technologyThe consumer logic for processor speed may be misleading because many consumers think thatclock speed, which is measured in gigahertz (GHz) or megahertz (MHz) is equal to systemspeed. While the higher the clock speed the CPU is able to do cycles more frequently and itdoes have a fundamental effect on speed, it is not the sole factor as the number of calculationsper cycle is different for each different manufacturer and model.Intel classifies its CPUs using a series of numbers. 3xx, 4xx, 5xx, 6xx and 7xx of which 7xxbeing the highest end products. Generally, the higher the number, the faster the CPU and themore expensive. usually, models and ratings correspond.•3xx Series: Intel Celeron (L2-128KB)•4xx Series: Intel Celeron D (L2-512KB)•5xx Series: Intel Pentium 4 / Celeron D (L2-1MB)•6xx Series: Intel Pentium 4 / Pentium 4 XE (L2-2MB)•7xx Series: Intel Pentium 4 XE•8xx Series: Intel Pentium D•9xx Series: Intel Pentium Dthe number followed by suffix J signifies XD technologyeg.Intel Pentium 4 3.0GHz L2-1MB with HT --> Intel Pentium 4 530JAMD CPUs are even more confusing in classification. The AMD Athlon CPU rating are NOTof the actual clock speed but rather the equivalence bench mark performance corresponding toa comparason to the AMD Athlon Thunderbird 1.0Ghz. The conversion Table is as follows,AMD Athlon 1500+ = Actually runs at 1.33 GHzAMD Athlon 1600+ = Actually runs at 1.40 GHzAMD Athlon 1700+ = Actually runs at 1.47 GHzAMD Athlon 1800+ = Actually runs at 1.53 GHzAMD Athlon 1900+ = Actually runs at 1.60 GHzAMD Athlon 2000+ = Actually runs at 1.67 GHzAMD Athlon 2100+ = Actually runs at 1.73 GHzAMD Athlon 2200+ = Actually runs at 1.80 GHzAMD Athlon 2400+ = Actually runs at 1.93 GHzAMD Athlon 2500+ = Actually runs at 1.833 GHzAMD Athlon 2600+ = Actually runs at 2.133 GHzAMD Athlon 2700+ = Actually runs at 2.17 GHzAMD Athlon 2800+ = Actually runs at 2.083 GHzAMD Athlon 3000+ = Actually runs at 2.167 GHzAMD Athlon 3200+ = Actually runs at 2.20 GHzIn choosing different manufacturers and models, the CPU would generally be dictated by theway you intend to use your computer. AMD CPUs are generally less expensive than Intel CPUsof the same clock speed but there are great differences in the CPU architechture. Intel Pentium4 is specifically designed to optimise clock speed while the number of calculations per cycle isreduced. Therefore, benchmark tests of the speed of the AMD Athlon XP 3000+ isapproximately the same as Intel Pentium 4 3.2GHz. Most gamers prefer AMD CPUs becauseof cheaper price for a faster speed, however, graphic designers and professionals prefer IntelCPUs due to their Hyper Threading technology.You may wish to purchase a high end AMD64/emt64 processor, which provides support for64-bit operating system (eg. Windows XP Professional 64-bit Edition). A 64-bit system is veryefficient in handling large amounts of RAM. A 32-bit system efficiency drops beyond about512 to 864MB of RAM, and becomes significantly less efficient beyond 4GB of RAM. Mostprocessors for gaming range in about the 2.8Ghz-3.2GhzA 64-bit processor is currently an expensive investment as most applications run on the 32-bitsystem. However, there is no doubt that the 32-bit system would gradually be replaced by the64-bit system when the prices fall over a few years time. It is unlikely that the 64-bit systemwould completely replace the 32-bit system within 5 years but Linux users might find a greatimprovement in speed when a 64-bit processor is used.Smaller processors are generally preferred for overclocking, as they run cooler, and can achievehigher clock speeds. Retail CPU's come in a package containing a HSF (Heat Sink Fan),instructions, and a warranty, often 3 years. OEM CPUs do not include these.The current CPU speeds and advantages change frequently, so for up-to-date comparisons, youmay want to check a website that specializes in Hardware reviews, such as Tom's HardwareGuide or Anandtech. A good, current (as of 14 February 2006) beginner's explanation can befound at [1]CPU CoolingCPU cooling is very important and should not be overlooked. A less than average CPUtemperature prolongs CPU life (up to more than 10 years). On the other hand high CPUtemperatures can cause unreliable operation, such as computer freezes, or slow operation.Extremely high temperatures can cause immediate CPU destruction by melting the materials inthe chip and changing the physical shape of the sensitive transistors on the CPU. Because ofthis, never switch on the computer if your CPU has no cooling at all. It is an extremely stupidthing to do, the scenario of 'I'll just test whether my CPU works!' as by doing so, you wouldfind that the CPU fries in less than 5 seconds and you will be off to buy a new one.Most CPU installations use forced-air cooling, but convection cooling and water cooling arealso options. For traditional forced-air cooling, the heat sink and fan (HSF) included in mostretail CPUs is usually sufficient to cool the CPU at stock speed. Overclockers might want touse a more powerful aftermarket fan, or even try water cooling because they need additionalcooling ability given the increased heat of overclocking.HSFs with decent performance are usually copper-based. The cooling effect is enhanced if theHSF has heatpipes. Silent (i.e. Fanless) HSFs are there to provide users a nearly silent cooling.Many retail heatsink+fan units have a thermal pad installed, which transfers heat from the CPUto the fan helping diffuse the heat created by the CPU. This pad is usable only once. If youwish to remove the fan from another CPU so that you can use it on your new one, or need totake it off for some reason, you will need to remove it, and apply a thermal paste or anotherthermal pad. Note that some of the cheaper pads can melt in unexpected heat and may causeproblems and potentially even damage if you are overclocking. In either case, thermal paste isusually more effective, just harder to apply. If you plan to do any high performance computing,or removing and replacing the HSF, often thermal paste is suggested. If you are planning a longterm installation a thermal pad is suggested. Non-conductive thermal pastes made up of siliconare the cheapest and safest.Silver-based thermal pastes sometimes perform better than normal thermal pastes, and carbonbasedones perform better still. When applied improperly both can be conductive, causingelectrical shorts upon contact with the motherboard. A thin properly-applied layer will usuallyprevent this problem, though some pastes can become runny when they get hot. Users shouldalso beware that many "silver" thermal pastes do not actually contain any silver metal.For quiet operation, start with a low-heat (low number of watts) CPU. Processors made byVIA, such as the VIA C3, tend to produce low amounts of heat. The Pentium M givesperformance that is on par with many of the desktop processors, but gives off more heat thanthe Via processors. You can also underclock your CPU, giving up some unneeded performancefor some peace and quiet. Another option is to choose a large copper heat sink with an open finpattern. However, true fanless operation is difficult to achieve in most case designs. You canposition a case fan to blow across the heat sink, or mount a fan on the heat sink. With eitherchoice of fan placement, choose a large and slow fan over a small and fast fan to decrease noiseand increase air flow.Some low-noise CPU cooling fans require special mounting hardware on the motherboard. Besure that the cooling fan you choose is compatible with your motherboard.