← IGCSE Computer Science Paper 2

Sub-system

A smaller part of a computer system that does one job within the whole system, such as the CPU, memory or storage in a computer. It is not the whole system.

Part of: IGCSE Computer Science 0478 — 7b Decomposition and Sub-systems — Module 2

Understanding Sub-system in IGCSE Computer Science Paper 2

What it does: Memory is a part of the computer that does one job (holding data in use), which is what makes it a sub-system. A sub-system does one job within the whole system. The whole system does its overall job by combining its sub-systems. The storage sub-system has the job of keeping data saved long term. The CPU's job is to process instructions.

Common mistakes: The whole system is the computer. The CPU is only one sub-system inside it. Line 1 should be System ← "Computer". A PROCEDURE must be closed with ENDPROCEDURE, not ENDFUNCTION.

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This module's key terms are also in a printable crossword (Modules 94–106) and word search (Modules 94–118).

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Learning
Duo
Which statement describes a sub-system?
A smaller part of a system that does one jobThe whole computer system with all its parts
Matches
The computer is a sub-system of the CPU
↓
This reverses the relationship. The CPU is a sub-system of the computer, not the other way round.
Applying
Meanings
Term
sub-system
Meaning
A smaller part of a computer system that does one job within the whole system, such as the CPU, memory or storage in a computer. It is not the whole system.
Practice
// System: computer
Testing
Gaps
computer = + memory + storage
Check Answer
Spots
1System ← "CPU"
2Part1 ← "CPU"

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Quick Questions

1. Which statement describes a sub-system?
A. A smaller part of a system that does one job ✓
B. The whole computer system with all its parts
A sub-system is one smaller part that does one job within the whole system. It is not the whole system.
2. Which of these is a sub-system of a computer?
A. The CPU ✓
B. The whole computer
The CPU is one part of a computer that does one job: it processes instructions. The whole computer is the system itself.
3. What is the job of the memory sub-system in a computer?
A. To hold data and instructions the computer is using now ✓
B. To process every instruction in the whole computer
Memory holds data and instructions in use. Processing instructions is the job of a different sub-system, the CPU.
4. A computer system is made up of the CPU, memory and storage. What are the CPU, memory and storage called?
A. Whole systems
B. Sub-systems ✓
Each one is a smaller part of the computer system, so each is a sub-system.
5. True or false: a sub-system does many unrelated jobs for the whole system.
A. True: it does many unrelated jobs
B. False: it does one job ✓
A sub-system does one job within the whole system. The whole system does its overall job by combining its sub-systems.

Match the Pairs

CPU → A sub-system of a computer that processes instructions, doing just that one job within the whole system
Storage → A sub-system of a computer that keeps data and programs permanently, even when the power is off
A sub-system is a smaller part of a system that does one job → Memory sub-system -> holds data and instructions being used
The whole system is made up of its sub-systems working together → Computer = CPU + Memory + Storage
The computer is a sub-system of the CPU → This reverses the relationship. The CPU is a sub-system of the computer, not the other way round.
A sub-system is the whole computer system → A sub-system is only one smaller part of the system, not the whole system.

Key Term

sub-system
A smaller part of a computer system that does one job within the whole system, such as the CPU, memory or storage in a computer. It is not the whole system.

Fill in the Blanks

1. computer = CPU + memory + storage
Hint: Type the sub-system that runs program instructions.
2. sub-system storage: keeps files when off
Hint: Type the sub-system that keeps data saved after the power is switched off.
3. computer = whole, CPU = sub-system
Hint: Type the term for a smaller part of a system that does one job.
4. A sub-system is not the whole system
Hint: Type the word that shows a part is different from the whole system.
5. Each sub-system does one job
Hint: Type the number of jobs a single sub-system does.

Spot the Error

Question 1
A System ← "CPU"
B Part1 ← "CPU"
C Part2 ← "Memory"
D Part3 ← "Storage"
Line A — The whole system is the computer. The CPU is only one sub-system inside it. Line 1 should be System ← "Computer".
Question 2
A PROCEDURE ShowCPU()
B OUTPUT "CPU: processes data"
C OUTPUT "It is one part"
D ENDFUNCTION
Line D — A PROCEDURE must be closed with ENDPROCEDURE, not ENDFUNCTION.
Question 3
A OUTPUT "Computer parts:"
B OUTPUT "CPU: processes data"
C OUTPUT "Memory: keeps files"
D OUTPUT "Storage: keeps files"
Line C — Memory holds data that is currently in use. Keeping files long term is the job of the storage sub-system. Each sub-system does its own job.
Question 4
A INPUT Part
B IF Part = "CPU"
C OUTPUT "Sub-system"
D ENDIF
Line B — An IF statement needs the keyword THEN after the condition. It should be IF Part = "CPU" THEN.
Question 5
A PROCEDURE Storage()
B OUTPUT "Saves files"
C OUTPUT "Does every job"
D ENDPROCEDURE
Line C — A sub-system does one job, not every job. The line should say that storage has one job only.

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