The System Unit & Its Internal Components
What is inside the system unit, and what does each part do?
- Define the system unit and list its main internal components
- State the function of the motherboard, CPU, RAM, PSU, storage drive and expansion cards
- Trace how data moves between the CPU, memory and storage
Overview
The system unit is the case that holds the working parts of a computer — everything except the monitor, keyboard and other peripherals. Open it and you find a flat board covered in chips (the motherboard), a fan-covered square (the CPU), thin sticks of memory (RAM), a metal box of cables (the power supply) and one or more drives. Knowing each part by sight, and knowing what it does, is the base skill for installing, upgrading and repairing computers.
What counts as the system unit?
The system unit (sometimes called the tower, chassis or CPU case) is the enclosure holding the components that process and store data. Input and output devices plug into it from outside. In a laptop or all-in-one the same components are simply packed into a thinner case.
Main internal components
Six groups of parts do almost all the work inside the case.
Inside a typical desktop system unit:
- Motherboard: The main circuit board. Every other part connects to it, and it carries the buses that move data between them.
- CPU (processor): The 'brain'. Fetches, decodes and executes instructions. Sits in a socket under a heatsink and fan.
- RAM (main memory): Fast, volatile memory holding the programs and data in use right now. Contents vanish when power is lost.
- Storage drive (HDD / SSD): Non-volatile storage for the operating system, programs and files. Keeps data when the power is off.
- Power supply unit (PSU): Converts 240V AC mains power into the low-voltage DC (+3.3V, +5V, +12V) the components need.
- Expansion cards & ports: Graphics, sound, network or Wi-Fi cards in PCIe slots; rear ports (USB, HDMI, Ethernet, audio) connect peripherals.
Cooling (case fans, heatsinks, thermal paste) and the CMOS battery that keeps BIOS settings and the clock alive are also part of the unit.
The buses — how parts talk
A bus is a set of electrical pathways on the motherboard. The data bus carries the data itself, the address bus carries the location in memory, and the control bus carries timing and command signals. Wider and faster buses mean more data moved per clock cycle.
The machine cycle in practice
When you open a file: the CPU asks storage for the file → the operating system copies it into RAM → the CPU fetches instructions from RAM, decodes and executes them → results go back to RAM and, when you save, back to the storage drive. Only RAM is fast enough to feed the CPU directly, which is why more RAM often feels faster than a faster CPU.
Safety when working inside
Always shut down, unplug the mains lead and hold the metal chassis (or wear an anti-static strap) before touching components. Static electricity from your body can destroy a RAM stick or CPU without any visible sign of damage.
Label the system unit
- In pairs, open a switched-off, unplugged desktop case (or use a labelled photo).
- Identify and point to: motherboard, CPU heatsink, RAM slots, PSU, storage drive, expansion card, case fan, CMOS battery.
- Draw the case layout and label all 8 parts, adding a one-line function for each.
- Write down which single upgrade you would recommend for that machine and why.
- What is the system unit?
Reveal answer
The case holding the components that process and store data — motherboard, CPU, RAM, storage, PSU and cards.
- State one difference between RAM and a storage drive.
Reveal answer
RAM is volatile and very fast; a storage drive is non-volatile and keeps data when power is off.
- What does the PSU do?
Reveal answer
Converts AC mains electricity into the low-voltage DC supplies the components need.
- Name the three types of bus and what each carries.
Reveal answer
Data bus (the data), address bus (memory locations), control bus (timing and command signals).
- Why must you touch the metal chassis before handling RAM?
Reveal answer
To discharge static electricity that could silently destroy the component.
Photograph the inside of a real system unit and produce an annotated poster naming every part you can identify.