The Systems Unit

Lesson Content

Inquire: Welcome to the Machine!


Computers have taken on many forms as technology has moved forward. When you see a desktop computer, there is an encased unit or box to which the monitor, keyboard, and mouse are connected. In this lesson, we are going to answer the question, “what’s in the box?” by opening up that desktop computer and exploring the components therein. Understanding the various parts that make up a typical computer and how they work together will provide a solid foundation for understanding computer-based technology on the whole.


Big Question

What’s in the box, and why does it matter?

Watch: How Can Something That Can Do So Many Complex Things Be So Simple?

Read: A Computer is Truly the Sum of its Parts


DecorativeThe computer case – otherwise known as the box – houses all of the parts that make a computer a useful device. The physical size of the computer case is referred to as its form factor. A computer can have a large, medium, or small form factor, and this will depend, to some extent, on the intended use of the machine.

A computer intended to be used for business purposes is likely to have a larger form factor than one whose use will be limited to just browsing the Internet. Why this is so has to do with the type and amount of components that are installed inside the machine.


The brain of the computer is its CPU (Central Processing Unit) otherwise known as the microprocessor. This is typically a large microchip that is either located on, or attached to, the largest circuit board (called the motherboard) inside the computer case. The CPU is responsible for calculating and timing everything that takes place in the computer’s input and output activities. How many actual circuits there are on the CPU and how they are arranged with respect to communicating with each other determines how many actual calculations can take place per second. As there are typically millions of calculations per second, these must also be timed so as to make certain that there aren’t conflicts with other calculation resources. To facilitate this, a quartz crystal is embedded in the CPU’s circuitry that serves as the clock that keeps everything running on time. This crystal beats to a frequency that is measured in megahertz (MHz) which is millions of beats per second and is referred to as the clock speed of the CPU.


Adjacent to the CPU is a circuit called the BIOS (buy-ose). BIOS stands for Basic Input-Output System. This circuit includes a short instruction set that has all of the details needed to boot your computer up when you turn on the power. The type of memory that the BIOS makes use of is Read-Only Memory, or ROM. The instructions are burned into the BIOS at the factory and are not meant to be changed. Nothing can be further written into the BIOS as there is typically no need — which is why it is “read-only”.

What takes place during the boot process is an evaluation, or self-check, of all the components needed for the computer to perform its duties. This self-check makes certain that everything is where it should be and is performing as needed. Boot-up, or bootstrapping, is a reference to the notion of one putting on their boots to go to work. Once this is done, the BIOS calls the operating system. It is the operating system software that provides us with an interface that we, as human beings and users, can interact with. It’s important that you understand that the BIOS is a chip, or set of chips (called a chipset), and is hardware. The instructions written to the BIOS are considered to be software.

Anything you can physically touch that is inside a computer case or attached to it is hardware. Anything that runs on a computer is software.


When the computer has booted up and software is running on it, the number of applications that can run concurrently is decided by how much RAM (Random-Access Memory) is loaded onto your machine. RAM, also called volatile memory, is dependent upon the system having power. If power suddenly goes out on your system, any software that is running on your machine will stop, and anything you were working on will close without saving your work. When your computer has booted up, any processes – including the operating system – that run on it do so in RAM. These days, RAM is measured in gigabytes (Gb). The good news is that you can typically add more RAM to your system either when it is being built or after the fact, as there are numerous dealers available who can provide you with more should you need it.

The Motherboard and Hard Drive

The motherboard will usually have ribbon cables attached to it. They are called ribbon cables because they are broad, flat, and resemble, to some extent, ribbons. These ribbon cables will connect to the computer’s hard drive, or drives, and other ancillary devices within the computer case.

DecorativeThe hard drive is the main repository for all things software on your computer. When you load a new application onto your machine, it’s actually being put into the hard drive. If you store a photograph or file on your machine, it’s being saved on your hard drive. Hard drives come in a wide variety of types and capacities. These days, the two most typically encountered will be the mechanical and the solid-state drives. Mechanical hard drives (HD) have a series of platters that are mounted internally to them. These are sealed to keep out dust and other contaminants. Data is read from the disks using a read/write head that is in constant motion as it retrieves needed files and information. Some of the problems with this type of hard drive are that they are relatively slow and, for various reasons, can become slower. Also, since they are mechanical, they can break; at which point, they become useless. Solid-state drives (SSD) have no moving parts, and they are fast at retrieval and storage. However, they are also very expensive when compared to mechanical HDs. Also, computers can have more than one hard drive in them. These alternative drives are referred to as slave drives.

The computer’s motherboard will also usually have a physical connection (called a port) to which a network cable can be attached at the back of the computer case. This enables the computer to be connected for use within a network. Sometimes, this port will also have a short antenna for use when connecting to a Wifi network.

Lastly, there is the power supply. In a desktop machine, this is located where the power cord attaches to the back of the computer case. The job of the power supply is to take the voltage coming from the wall and step it down to the micro-voltages that the circuits inside the case run on. A major by-product of this process is heat. This is why there is a large fan attached to the back of the computer case. Also, it’s important to make certain that the vent for this fan is never covered or completely choked with dust as the high heat that results can easily destroy the components inside the machine.


Poll: Is Bigger Necessarily Better?

Do you need a large form factor due to the fact that your work involves massive computing resources (gamers tend to use very powerful computers that often come with a larger form factor — even in laptop form), or can you work from a much smaller machine and enjoy the relative convenience offered by the smaller size?

Expand: How Do These Things Just Keep Getting Smaller?


Technology is constantly moving forward to meet the demands of the buying public. This incessant pressure to build devices with more capacity and abilities has propelled computer scientists to perpetually push the boundaries of accepted knowledge where it applies to the physics of the computer. Read the section below to gain some context to frame your thinking. Then, you’ll be asked to do some research to find out what the magic is behind the development of the devices we have come to take as commonplace.

Computers: A Miracle in Miniature

DecorativeThe capacity for a computer to store files is determined by the size of the hard drive. Over the last several decades, hard drives have become physically smaller, but their capacity for storage has grown larger. The same can be said for all manner of computer-related storage media (SD cards, outboard storage drives, USB drives, etc.). Even more, the devices that make use of these have, themselves, become physically smaller. A large part of the motivation for companies building these products is consumer convenience. No one wants to have to lug around a particularly large or heavy machine. We, as consumers, expect that these appliances will become smaller and lighter as time goes by. The good news is that the computer industry has fully embraced our desire for smaller computers and related devices. The side benefit of this is that the miniaturization of these technologies has provided advantages in other areas such as: medicine, aerospace, the military, communications, entertainment, etc..

One computer-related device that has gone through endless revisions since it was first introduced is the cell phone. If you watch movies from the late 1980s, you will see some of the first “portable” cell phones. A good example is the scene from the very first Lethal Weapon film where Sgt. Murtagh walks onto an overpass while having a cell phone conversation. In one hand, he has the phone, and in the other, he is carrying the cinder-block sized battery for it!

With all of this in mind, how is it that we experience this generational trend toward smaller devices with greater storage capacities? How is this seeming paradox explained? What is the magic that allows this to happen?

Hint: Your answer will revolve around something called Moore’s Law.

Check Your Knowledge

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Lesson Resources

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Additional Resources and Readings


A tutorial that takes this subject into deeper territory

How PCs Work

An article from a website that demystifies much of the world around us

How Computers Work – Hardware

A deeper look at how computers work

Lesson Glossary


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  • BIOS
    Basic Input-Output System; a chip or chipset that contains the instructions needed to boot your computer once the power is turned on. The instructions are written to the BIOS as a part of read-only memory (ROM) and are not meant to be tampered with. The BIOS is powered by a battery so that the(...)
  • CPU
    Central Processing Unit; otherwise known as the microprocessor. The CPU is responsible for every calculation that takes place on your computer as well as the timing of concurrent processes
  • form factor
    the physical size of the computer
  • Moore's Law
    states that the amount of transistors that can be loaded into a physical space on a microchip doubles every 18 months to two years. By extension, this also means that something developed today as computer-related technology will be smaller and have substantially greater computing capacity(...)
  • RAM
    Random-Access Memory; the memory all the software runs in when you launch applications on your computer. This is also called volatile memory for the reason that once the power is turned off, the application software stops running

License and Citations

Content License

Lesson Content:

Authored and curated by John Kormos for The TEL Library. CC BY NC SA

Media Sources

DecorativeTaking NotesDomenico LoiaUnsplashCC 0
DecorativeDeskspace at DuskGreen ChameleonUnsplashCC 0
DecorativeTech Computer ElectronicCarlos Irineu da CostaUnsplashCC 0
DecorativeSmartphone homescreenSaulo MohanaUnsplashCC 0