Cambridge (CIE) IGCSE · Computer Science
Computer Sub-Systems &
Problem Decomposition
Big idea: Big, complicated computer problems become manageable when you
break them into smaller working parts — a computer is just several sub-systems working
together, and every good programmer solves problems by splitting them up the same way.
Summary — Everything In This Chapter
- A sub-system is a smaller part of a bigger system that works together with other parts to make the whole thing function.
- A computer has five main sub-systems: CPU, Memory, Storage, Input devices, Output devices.
- Sub-systems can be broken down further — e.g. the CPU itself contains the Control Unit, Registers, and the ALU.
- Using sub-systems makes it easier to troubleshoot, isolate, and fix problems, and helps designers see the "big picture."
- Decomposition is the process of breaking a large problem into a set of smaller, easier-to-manage problems.
- Decomposed problems are easier to solve, can be worked on and tested independently, and are combined at the end into the full solution.
- Every decomposed problem can be described using four building blocks: Inputs, Processes, Outputs, Storage (IPOS).
What Is a Sub-System?
Think about your body for a second. You don't work as one single blob — you have a
digestive system, a circulatory system, a nervous system, and so on. Each system has
its own job, but none of them are useful alone. Your circulatory system pumping
blood is pointless if your digestive system isn't giving that blood any nutrients to carry.
They only work because they work together.
A sub-system is exactly this idea applied to computers (or any complex
system): it's a smaller, self-contained part of a bigger system that has to cooperate with
the other parts to make the whole thing function.
Analogy — The Car
A car doesn't work because of one giant "car part." It works because separate sub-systems
all do their own job and hand off to each other:
Engine → creates power | Brakes → stops the car | Wheels → move the car
And you can zoom in even further — the Engine itself is made of smaller
sub-systems: spark plugs, sensors, pistons. This is important: sub-systems aren't a
"one level only" idea. You can keep breaking things down into smaller and smaller pieces
until each piece is simple enough to fully understand and build.
The Five Main Computer Sub-Systems
Every computer — whether it's a phone, a laptop, or a supercomputer — is built from the same five core sub-systems:
| Sub-System |
Job |
Real Examples |
| Central Processing Unit (CPU) |
Executes instructions — this is the "brain" that actually does the thinking/calculating |
Intel Core i7, Apple M-series chip |
| Memory |
Stores data and instructions temporarily, only while the CPU needs them right now |
RAM |
| Storage |
Stores data and software permanently, even when the power is off |
HDD, SSD |
| Input devices |
Let a user put information into the computer |
Keyboard, mouse |
| Output devices |
Show information out of the computer, or create a physical result |
Monitor, printer |