The CPU (Central Processing Unit)

The CPU is the main IC chip on your computer's motherboard. They come in different shapes, sizes and packages. Older CPUs came in the DIP format, and some 286s and early 386s were QSOPs (see Integrated Circuits), but what you'll see the most are the flat, square PGA or SPGA chips.
The PGA chip used to be installed in a friction fit socket. Installation and removal had to be done carefully to avoid bending or breaking the pins on the CPU. First, you had to line up the chip. There is always some way to designate Pin#1, both on the chip and on the socket. There may be a dot in one corner, or an arrow, or a small, silk-screened '1'. Usually, a CPU will have one of its corners beveled, and this has to line up with the designated corner on the socket. You have to be sure all the pins on the chip are straight and lined up properly, then set the chip in the socket, making sure all the pins fit into all the holes. To install the chip, you press down firmly and evenly. If any pins aren't quite in their appropriate holes, or if you didn't push straight down, pins will bend sideways. Trying to straighten the pins is not impossible, but usually results in them snapping off. You learn pretty quickly to take your time and be careful.
Removal is another problem. A chip extractor is rather narrow and doesn't allow for much control when trying to lift the chip out (we're talking about a fair amount of friction holding this chip in). Some upgrade or 'turbo' packages included a special, widened chip extractor that was better, but still didn't provide much control. The best way is to carefully insert a narrow flat-head screwdriver between the chip and the socket, being careful not to touch the pins underneath, and twist ever so slightly lifting the chip a very minimal amount. Then go to the next side and do the same. By the time you work your way completely around the chip, it'll come out fairly easy.
The problem was recognized and a new Zero Insertion Force socket (ZIF) was developed. This socket makes a big difference to the removal and installation of the PGA and SPGA CPUs. It has a small handle or lever on the side of the socket that unclips and lifts, releasing the pins completely. You can just lift the old CPU out, drop another in, and pull the lever down, inserting it under its little clip. No bent or broken pins, and no chipped edges on the CPU or socket.
For Intel's Pentium II, Pentium III, and some of it's Celeron chips, the CPU is put on a small circuit board with some external cache memory, and encased or packaged in a plastic cassette. This 'Slot 1' style CPU clips into a connector on the motherboard called a 'Slot 1' connector.
As should be obvious now, you must match the CPU with the motherboard. Most motherboards will accept more than one type of CPU as far as model, manufacturer, and speed is concerned, but they generally only accept one socket type. This is a consideration that should be addressed and decided before you purchase either. The motherboard's manual will tell you what different types of CPUs it will accommodate. If you don't have a manual, then visit the manufacturer's website. They usually provide that type of information. The major CPU manufacturers include Intel, Motorola, IBM, Advanced Micro Devices (AMD), and Cyrix.
It should be noted that Intel was making CPUs before there were Personal Computers. They made the first chips for IBM's PCs. Most the BIOS chips (Basic Input/Output System) on the early motherboards were developed based on Intel's architecture. Intel is still considered to be the #1 manufacturer and has set the standard for processors. Other CPU manufacturers and distributors tend to compare their product to Intel's, saying it's "comparable to the Pentium II-350" or it's "better than the PIII-500".
A specific CPU is described by it's manufacturer, model, and speed in megahertz (MHz). A computer's system board has a quartz crystal on it that emits a constant signal or beat like a metronome. Each beat is called a clock cycle and is measured in MHz, or millions of cycles per second. A CPU runs at a multiple of the motherboard's clock speed and the instructions it performs are synchronized with each cycle of the CPU's internal clock. So, if your CPU can perform a single instruction with each cycle, and you have a 500 MHz CPU, then that chip can perform five hundred million instructions per second. Your heart won't beat that many times in a lifetime. Current technology by Intel and AMD provides us with 1.5 GHz (Gigahertz) CPUs this year. That's one and one-half billion instructions per second!
The Central Processing Unit is considered the 'brain' of your computer. It controls and directs all the activities of the computer, transmitting, receiving and processing data constantly. But like the 'brain' of any project or organization, it relies very heavily on its support group and advisors. There are a lot of factors involved that are related to the CPU and have an effect on the speed and performance of your machine. Some of these factors include:
Whether there's a math coprocessor present and if it's internal or external.
The clock speed of the system and of the CPU. The amount of internal cache and external cache available.
The bus architecture or supporting circuitry on the motherboard.
I'm going to talk about the Central Processing Unit a bit more later, but to help you better understand the changes, improvements and history of the microprocessor and your computer, let's spend some time on the above support group.