Most microprocessors are thinner than a dime and less than 1/3 of a square inch in size. They are made of silicon and contain millions of transistors (electronic wires), which are each approximately 1/500th the width of a human hair. The Pentium 4 image at the beginning of this article shows not just the microprocessor (chip), but the packaging surrounding it. The packaging is what enables the microprocessor to be connected to the motherboard. Some processors have packaging designed to plug into a motherboard that has a socket, while others are designed for a motherboard with a slot.

Intel developed the first microprocessor in 1971. It was called the 4004. However, it wasn't until 1978 that the first generation of PCs came to market. They were based on Intel's 8086 microprocessor, which had 29,000 transistors. Today we are in the 7th generation of PCs and Intel's latest microprocessor, the Pentium 4 with Northwood core, has 55,000,000 transistors. Yet, the P4 is still based on the same architecture as the 8086, now known as x86 architecture. In fact, even microprocessors developed by Intel's main competitors, AMD (Advanced Micro Devices) and Via (who recently bought out Cyrix), are based on the x86 platform.

If your CPU and motherboard are compatible in the ways mentioned below there is a good chance they will be compatible overall. However, there are some exceptions to that rule so don't rely on it. Ending up with a CPU and motherboard that are incompatible is a terrible drag. Therefore, before purchasing a CPU call your motherboard's manufacturer and double check that the two will be compatible. If you're not sure how to find out your motherboard's manufacturer and model number, Motherboard HomeWorld (motherboards.org) has some "Mobo ID Tools".

1. Does your motherboard support Intel or AMD CPUs? A motherboard will support one or the other, but not both. Even though Intel and AMD CPUs are based on the same x86 platform and support the same applications and operating systems, they aren't compatible with the same motherboards.

2. Does your motherboard have a slot or a socket, and what type? The slot or socket on a motherboard is where the CPU plugs into it. If the CPU is in packaging designed for a slot, you won't be able to plug it into a socket.* Though Intel and AMD have alternated between slots and sockets throughout the years, both companies are currently working with sockets.
   

   Socket packaging has pins, like the picture of the Intel CPU above, and slot packaging looks more like a game cartridge. Intel's latest Pentium 4 and Celeron processors are in packaging designed for Socket478 motherboards. AMD's current AthlonXP, Athlon, and Duron processors are all designed for SocketA motherboards.
   

3. Can your motherboard support the CPU's core voltage? If the CPU's core voltage is too high for the motherboard's cooling powers, your board could get too hot resulting in serious damage. In addition, the CPU's voltage could be too low for the motherboard. For instance, older Socket 370 motherboards designed for PIIIs and Celerons having a core voltage of 2.0V will not support newer PIIIs and Celerons, which have a core voltage of approximately 1.65V.

4. Can your motherboard support the CPU's speed? The motherboard must also be able to support the CPU's clock speed. A motherboard's specifications will list the speeds it supports.

• CPU Clock Speed: In the title "Intel Pentium 4 3GHz/800MHz," 3Ghz represents the CPU's clock speed, also called the backside bus speed. Clock speed is measured in Megahertz (millions of pulses per second), or Gigahertz (1000 MHz). Every time there is a clock pulse the microprocessor accomplishes something. It would seem to be true that a 600 MHz CPU will be a better performer than a 500 MHz CPU. But that is not necessarily correct. Performance also depends on how much the CPU can accomplish per clock pulse, which is a factor of internal design.
  

  For instance, despite the fact that they both run at 600 MHz, an Intel PIII 600 MHz CPU is a better performer than an Intel Celeron 600 MHz CPU. That's because the PIII has a more sophisticated design. So, remember not to rely too heavily on clock speed when comparing CPUs of different designs.

  In fact, consumers' tendancy to gage a CPU's performance ability on clock speed alone has long been a source of frustration for AMD. AMD's processors typically have lower clock speeds than Intel's, though they perform similarly. In an attempt to address this marketing dilema AMD gave their AthlonXP processors names such as "AthlonXP 1800+" and "AthlonXP 2000+". From the names you would assume the processors run at 1.8 GHz and 2.0GHz respectively. In fact, these processors run slower than that (1.533GHz and 1.666GHz), but nevertheless they perform similarly to processors with higher clock speeds.

• System Bus Speed: As mentioned earlier, the system bus is also referred to as the frontside bus (FSB). It is the most important bus in your computer because it serves as the data thoroughfare between your computer's CPU and main memory. The CPU and main memory constantly exchange information in order to complete tasks.
  

  When looking at a CPU title you will see two speeds listed--for example "Intel Pentium 4 3Ghz/ 800MHz." The 3GHz represents the CPU's clock speed, and the 800MHz represents the system bus speed. The most common system bus speeds are 266, 333, 400, 533, and 800 MHz.

• Floating Point Calculations (FPU): A CPU's ability to perform floating point calculations is important if you are a gamer or interested in 3D manipulation. FPU performance is not as important for basic business oriented tasks such as word processing, creating spreadsheets, or surfing the Internet. Those tasks rely on the CPUs ability to perform integer calculations; all modern PCs are pretty good at integer calculations.

• Memory Cache: Memory cache is a small amount of high-speed memory located near the microprocessor. It holds the microprocessor's most frequently requested instructions and data. It is much quicker for the microprocessor to access data from the cache memory than from main memory. The concept behind memory cache is the 80/20 rule, which says that of all the programs and data in a computer, 20 percent of it is used 80 percent of the time.
  

  Memory cache is described by its distance from the microprocessor. The L1 (level 1) cache is located right on the microprocessor itself, and runs at full clock speed. L2 (level 2) cache is the next closest. Formerly L2 cache was located on the motherboard because there wasn't room for it on the CPU. Now, for the most part it is located inside the CPU's packaging, and runs at either 1/3, 1/2 or full clock speed. Sometimes Intel will refer to ATC (advanced transfer cache), which is their way of saying that the L2 cache runs at full processor speed.
  

  Cache is critical to gaming and 3D computing, but not necessary for tasks like word processing or surfing the Web. If you'll be doing the former, be sure to consider both the size and speed of the cache.

• Multimedia Instructions: It's also worth mentioning multimedia instruction sets, which you'll run into and may wonder about. Multimedia instruction sets are designed to improve a computer's performance when running 3D applications. However, in order to benefit, the 3D application must also support the special instruction set. This used to be a problem, but now Microsoft offers support for both Intel and AMD's instruction sets.
  

  The first multimedia instruction set was developed by Intel and called MMX. All current generation CPUs have support for MMX. In addition, AMD has developed it's own set called 3DNow!, which is included in all their current processors. Intel has also developed a set called SSE, which is available in current PIIIs, and SIMD2, available in P4s.

• Hyper-Threading: Intel has introduced a technology called Hyper-Threading (or HT) in their most recent Pentium 4 processors. Hyper-Threading enables the processor to execute two threads (part of the program) in parallel, allowing you to multitask more effectively than ever before. If you often run several heavy-duty applications at once, or if your multitasking needs are likely to increase in the near future, then HT Technology is something you may want to consider in your processor.
  

• Graphics Intensive Tasks: Anything that has to do with graphics, such as playing 3D games, image editing (for instance in Photoshop), creating 3D images, or editing video, is CPU intensive.
  

  However, you should also consider the following: some video cards have a graphics processor (GPU) for the specific purpose of helping the CPU out with such tasks. Therefore, if you have one of these high-end video cards, you may not need as strong of a CPU as you would otherwise. In addition, the amount and speed of memory in your system plays an extremely important role in its overall performance, as does your motherboard. So, if you need to purchase more than just a CPU, remember that those other areas are just as important, and not to spend it all on a high speed processor.

• Heatsink and Fan: CPUs generate heat. If your computer's insides get too hot, your entire system could be ruined. That is why you always need to install a heatsink and fan along with your CPU. If you purchase a retail box version of an Intel or AMD processor, a heatsink and fan will be included. Generally, an OEM version of either processor will not include a heatsink or fan.

• Warranty: The retail versions of Intel and AMD processors come with a 3-year warranty. The warranties on OEM processors of either manufacturer will vary. You may also be interested to know that CPUs have a very low return rate. In general microprocessors are very reliable because they are mass produced by machines.