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Practise 31 exam-style questions on Sub-systems, decomposition and structure diagrams for Cambridge IGCSE Computer Science (Paper 2). Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with an AI tutor on hand if you get stuck.
Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.
Define the term sub-system.
A sub-system is a smaller part of a larger system that carries out one particular task.
Systems are built from sub-systems, and each sub-system can be broken down into further sub-systems. This is what makes decomposition possible.
A part of a larger system that carries out a particular task / a smaller system within a larger system
A computer game keeps a table of the five highest scores ever achieved.
Explain why storage must be identified when this game is decomposed into its component parts.
The high scores must still be there the next time the game is played, so they cannot just be held in variables while the game runs. Identifying storage means the designer plans for the high-score table to be saved in a file, read when the game starts and updated when a new high score is reached.
Storage covers data that must be kept for later, usually in a file. Without identifying it, a program might calculate high scores correctly but lose them every time it closes.
Data such as the high scores must be kept after the program has closed / between games
So the designer plans how the high-score table is saved (e.g. in a file) and read back the next time the game is played, and updated when a new high score is achieved
Must be applied to the high scores
A satellite navigation (satnav) device is a computer system.
Describe, using examples, how a satnav is made up of sub-systems that are themselves made up of further sub-systems.
A satnav has sub-systems such as finding the current location, planning the route and displaying the map. The route-planning sub-system is made up of further sub-systems: finding the possible roads, calculating the fastest route, and recalculating the route if the driver misses a turn.
The question asks for two levels: sub-systems of the satnav, then sub-systems of one of those. Answers that only list parts of the device at one level cannot get full marks.
A suitable sub-system of a satnav, e.g. find current location (GPS), enter destination, plan route, display map, give spoken directions
A second, different sub-system of a satnav
One sub-system broken down into further sub-systems, e.g. plan route into find possible roads, calculate the fastest route, recalculate when a turn is missed
At least two further sub-systems needed
A problem has been decomposed into these component parts.
Inputs: the number of hours worked in a week and the hourly rate of pay
Processes: up to 40 hours are paid at the hourly rate; each hour over 40 is paid at 1.5 times the hourly rate
Output: the total pay for the week
Write pseudocode for this solution.
DECLARE Hours : REAL
DECLARE Rate : REAL
DECLARE Pay : REAL
INPUT Hours
INPUT Rate
IF Hours > 40
THEN
Pay ← 40 * Rate + (Hours - 40) * Rate * 1.5
ELSE
Pay ← Hours * Rate
ENDIF
OUTPUT "Total pay: ", PayThe decomposition gives the structure directly: inputs first, then the processing, then the output. The common mistake is paying every hour at 1.5 times the rate when overtime is worked; only the hours over 40 get the higher rate.
MP1 inputs the hours worked and the hourly rate
max 5
MP2 checks whether the hours are greater than 40
MP3 if 40 hours or fewer, pay is hours × rate
MP4 if more than 40 hours, the first 40 hours are paid at the normal rate (40 × rate)
MP5 ... plus (hours − 40) × rate × 1.5 for the extra hours
Do not accept all hours paid at 1.5 times the rate
MP6 outputs the total pay after it has been calculated
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