Information Technology Division

Internal Structure of a PC

Although some people regard personal computers with a mixture of fear and awe, PC's are not magic machines. A few minutes, this document, and an open mind are the only requirements for a very broad understanding of what goes on inside that mysterious metal box.

The heart of any personal computer system is the technological marvel known as the Central Processing Unit, or CPU. The CPU is an integrated circuit of more than 3 million transistors. Manufacturers grow CPU's as silicon crystals before etching on circuits with lasers.

It is the CPU number that gives a personal computer its name; for instance, a 486 PC contains an Intel 486 CPU. A computer's Central Processing Unit is in charge of processing all mathematical and logical operations -- at a speed of several million per second.

Random Access Memory, or RAM, is the place where instructions to the CPU are stored. One instruction might be to add three numbers. The CPU would first read in the addition instruction. Then it would read in the two numbers to be added. The addition would be performed, and the CPU would stick the answer back at a specified location in RAM.

Of course, even the simplest computer programs are elaborate chains of such instructions. (Immense programs such as Microsoft Word consist of billions.) When a program is loaded into memory, the instructions composing it are lined up in RAM, ready to be executed. When a computer is turned off, its RAM is wiped clean. That's the reason why it takes a few seconds for a program to load when you click on its icon.

People marvel at the mind-boggling graphics of which current PC's are capable. In reality, there is nothing magical about the phosphorescent images in video games, Web pages, and multimedia.

The screen is really just a window into RAM. Several times per second, the CPU looks at a specified region in RAM (an area appropriately known as "video memory"), and tells the monitor to draw a dot on the screen according to what value is stored in its corresponding RAM location. Programs manipulate pictures on the screen by shuttling numbers into and out of video memory.

Disk drives (hard disks, floppy disks, and CD-ROMS) provide away of storing programs and data permanently; the programs remain on disks even after you turn the computer off.

These days, most programs are longer than a computer's available RAM, and the program has to be drawn into RAM from the disk section-by-section as needed. From the perspective of the lightening-fast CPU, disk accesses take a lot of time. The more RAM a computer has, the fewer disk accesses will be required, and the faster the computer will run. That's the reason why a Pentium computer with 32 megs of RAM runs faster than a Pentium computer with 16 megs of RAM.

Let's take a break from all this technical jargon to talk about something with which you are probably familiar: the competition between IBM computers and Macintosh computers. Internally, IBM's and Mac's don't differ much philosophically; both have RAM, CPU's, and hard disks.

One of the things that they do differ on is operating systems. IBM operating systems include DOS, Windows 3.1, and Windows 95. Macintosh operating systems are the various versions of Mac OS.

Think of the operating system as a waiter at a pizzeria. The customer (user) tells the waiter (the operating system) that he wants a pizza with everything on it (some program to execute). The waiter writes down the customer's request, runs to the back of the restaurant, makes, bakes, and serves a piping hot pizza. The customer is glad when he receives his pepperoni pizza; he had to wait a while to receive it, but he really couldn't be bothered with the complex, detailed process of rolling out the dough, putting on the pepperoni's, warming up the oven, etc.

A PC user has the same relationship with an operating system that the customer had with his waiter. For instance, if the user needs to read a file from the hard disk into RAM, he will be willing to sacrifice some speed for the convenience of having the operating system open the file, read each tiny bit of information into the appropriate area in memory, check for errors, and close the file again.

The operating system serves as a go-between for the user and primitive system resources. When a Mac or Windows user clicks on a program or file icon, that user is making a request of the operating system.