Computer System
Senior 1 · ICT
Computer System
Lesson 4 of 5
20 min
Let's begin
With the power switched off and the cable unplugged, the ICT teacher opens the side of a desktop computer. Inside is a large green circuit board with chips, fans and cables. These internal parts do the real work, and the storage devices keep your files safe when the computer is switched off.
| Part | What it does |
|---|---|
| Motherboard | The main circuit board. Every other part plugs into it or is connected to it, and it carries data between them. |
| CPU (central processing unit, the processor) | Carries out the instructions of programs: the "brain" of the computer. It has a control unit, which directs the other parts, and an arithmetic and logic unit (ALU), which calculates and compares. Its speed is measured in gigahertz (GHz). A heat sink and fan stop it overheating. |
| RAM (random access memory) | The computer's working memory. It holds the programs and data in use right now. It is volatile: everything in it is lost when the power goes off. More RAM lets more programs run smoothly at once. |
| ROM (read-only memory) | A chip holding the start-up instructions the computer needs when it is switched on. It is non-volatile: it keeps its contents without power. |
| Power supply unit (PSU) | Changes the mains electricity from the wall socket (230 volts in Rwanda) into the low, steady voltages the parts need. A laptop uses a charger and a battery instead. |
| Expansion cards | Extra boards that add features, such as a graphics card or a network card. |
RAM forgets when the power goes, so files must be kept on storage devices (also called secondary storage), which keep data without power.
| Device | How it works | Good points | Weak points |
|---|---|---|---|
| Hard disk drive (HDD) | Spinning magnetic disks (platters) read and written by a head on a moving arm | Large capacity, low cost per gigabyte | Slower; moving parts can be damaged if dropped; makes some noise |
| Solid-state drive (SSD) | Flash memory chips, with no moving parts | Much faster; silent; survives knocks better; uses less power | Costs more per gigabyte |
| USB flash drive | Flash memory in a small case with a USB connector | Small, cheap, easy to carry between computers | Easy to lose; can spread viruses between computers |
| Memory card (SD, microSD) | Flash memory on a tiny card | Fits phones and cameras | Very small and easy to lose or break |
| Unit | Size |
|---|---|
| 1 bit | a single 0 or 1 |
| 1 byte (B) | 8 bits: enough for one letter |
| 1 kilobyte (KB) | 1,024 bytes |
| 1 megabyte (MB) | 1,024 KB |
| 1 gigabyte (GB) | 1,024 MB |
| 1 terabyte (TB) | 1,024 GB |
Each photo from Keza's phone is about 4 MB. How many fit on a 16 GB flash drive?
Disk makers count 1 GB as 1,000 MB, not 1,024, so a disk sold as 1 TB shows only about 931 GB on a computer. Nothing is missing; the two counts use different units.
Tip
RAM forgets, storage remembers. Work in RAM disappears in a power cut; that is why you save to storage often.
Byte ladder: 8 bits make a byte; after that, multiply by 1,024 at each step: KB, MB, GB, TB.
Which part (a) carries out program instructions, (b) connects all the parts together, (c) changes mains electricity for the computer?
Solution: (a) the CPU; (b) the motherboard; (c) the power supply unit.
Mugisha is typing an essay when the power goes off. He had not saved it. Why is the work lost?
Solution: Unsaved work is held only in RAM, which is volatile, so it is wiped when the power goes. Saved work is on the hard disk or SSD, which keeps data without power.
A laptop is often carried around on a motorbike taxi. Is an HDD or an SSD better for it? Why?
Solution: An SSD. It has no moving parts, so it survives bumps much better than a hard disk, whose spinning platters and moving arm can be damaged.
How many 4 MB songs fit on a 2 GB flash drive?
Solution: 2 × 1,024 = 2,048 MB, and 2,048 ÷ 4 = 512 songs.
Remember
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
