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HKDSE ICT Practice Paper: Data Representation, Computer Systems and Network Security (12 Questions, 100 Marks, 75 Minutes, with Answers)
HKDSE ICT: Data Representation, Systems and Network Security
HKDSE ICT practice paper: binary and hexadecimal conversion, image and audio file sizes, network transfer, CPU speed, memory, passwords, search algorithms, IoT data, with worked answers.
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Paper details Level: S6 | Subject: ICT | Time: 75 minutes | Marks: 100 | Questions: 12 | Answers included
Section A: Data Representation and Multimedia (32 marks)
Answer all questions in Section A.
| # | Question | Answer and marking notes |
|---|---|---|
| 1 | Data representation. (a) Convert the decimal number 45 into an 8-bit binary number (3 marks). (b) Convert the hexadecimal number 5C into decimal (3 marks). (c) State one advantage of displaying memory addresses in hexadecimal (2 marks). | Answer: (a) 45 = 32 + 8 + 4 + 1, that is 101101, written as the 8-bit pattern 00101101. (b) 5 x 16 + 12 = 92. (c) Every 4 bits map to one hexadecimal digit, so addresses are shorter and easier to read with fewer copying errors. Marking: the answer in (a) must be padded to 8 bits. |
| 2 | Image file. A 1920 x 1080 pixel 24-bit true colour bitmap is stored uncompressed. (a) Calculate the number of bytes used by one pixel (2 marks). (b) Calculate the file size in MB, where 1 MB = 1,000,000 bytes (3 marks). (c) Give two ways to reduce the file size (3 marks). | Answer: (a) 24 bits = 3 bytes. (b) 1920 x 1080 x 3 = 6,220,800 bytes = 6.22 MB. (c) Save as JPEG or another compressed format, lower the colour depth or resolution, or split the image and load parts on demand. Marking: forgetting to divide by 1,000,000 loses half the marks in (b). |
| 3 | Audio file. A 30-second stereo sound clip is sampled at 44,100 Hz with a 16-bit sample resolution and is uncompressed. (a) Calculate the number of bytes used per second (3 marks). (b) Calculate the file size in MB, where 1 MB = 1,000,000 bytes (3 marks). (c) State one effect of halving the sampling rate on sound quality and file size (2 marks). | Answer: (a) 44,100 x 16 x 2 bits = 176,400 bytes. (b) 176,400 x 30 = 5,292,000 bytes = 5.29 MB. (c) Halving the sampling rate halves the file size but loses high frequency detail, so quality falls. Marking: stereo means a factor of 2. |
| 4 | Network transfer. A 1.2 GB file is uploaded over a 100 Mbps connection, where 1 GB = 1,000 MB = 1,000,000,000 bytes. (a) Convert 1.2 GB into Mb (3 marks). (b) Calculate the ideal transfer time in seconds (3 marks). (c) Explain two reasons why the actual time is longer (2 marks). | Answer: (a) 1.2 x 1,000 x 8 = 9,600 Mb. (b) 9,600 / 100 = 96 seconds. (c) Network congestion lowers the real bandwidth; TCP retransmission, server processing and disk access also take time. Marking: bits and bytes must be unified first. |
Section B: Computer Systems, Data Processing and Security (36 marks)
Answer all questions in Section B.
| # | Question | Answer and marking notes |
|---|---|---|
| 5 | CPU. A processor runs at 3.2 GHz and needs 2.5 clock cycles per instruction on average. A program executes 4.0 x 10 to the power 9 instructions. (a) Calculate the number of clock cycles per second (2 marks). (b) Calculate the total cycles needed (3 marks). (c) Calculate the execution time in seconds to 3 decimal places (2 marks). (d) Suggest one hardware upgrade that raises performance and explain it (2 marks). | Answer: (a) 3.2 GHz = 3.2 x 10 to the power 9 cycles per second. (b) 4.0 x 10 to the power 9 x 2.5 = 1.0 x 10 to the power 10 cycles. (c) 1.0 x 10 to the power 10 / 3.2 x 10 to the power 9 = 3.125 seconds. (d) Add more cores or a larger cache so that instructions run in parallel and main memory is accessed less often. Marking: (c) must be in seconds to 3 decimal places. |
| 6 | Memory hierarchy. A computer has 4 GB of main memory and 512 KB of cache memory, where 1 GB = 1,048,576 KB. (a) Calculate how many times larger main memory is than the cache (3 marks). (b) Explain why the cache hit rate matters for overall performance (3 marks). (c) State one cost of using virtual memory when RAM runs short (3 marks). | Answer: (a) 4 x 1,048,576 / 512 = 8,192 times. (b) A high hit rate means most data comes from fast cache instead of waiting for main memory, so total execution time falls sharply. (c) Swapping pages needs disk reads and writes, which are far slower than memory, so the machine slows down. Marking: use 1,048,576 KB for 1 GB. |
| 7 | Spreadsheet. The grades of 40 students are: grade A 6, grade B 12, grade C 14, grade D 6 and grade E 2. (a) Calculate the percentage of the class in each grade (3 marks). (b) Write a spreadsheet formula that counts the students who pass, that is grades A, B and C (3 marks). (c) Explain why an absolute reference is used when a formula is copied (3 marks). | Answer: (a) A 15.0 per cent, B 30.0 per cent, C 35.0 per cent, D 15.0 per cent, E 5.0 per cent, totalling 100 per cent. (b) For example =SUM(B2:B4) or =COUNTIF(range,”A”)+COUNTIF(range,”B”)+COUNTIF(range,”C”), giving 32 students, that is 80.0 per cent. (c) An absolute reference such as $B$2 keeps pointing at the same total or range when the formula is copied. Marking: the formula and the value must both be right in (b). |
| 8 | Password security. A system accepts only 8 lowercase letters and an attacker can try 1,000,000,000 times per second. (a) Calculate the number of possible passwords (3 marks). (b) Calculate the time in seconds to try every combination, to 1 decimal place (3 marks). (c) Explain how adding uppercase letters and symbols improves security (3 marks). | Answer: (a) 26 to the power 8 = 208,827,064,576 combinations. (b) 208,827,064,576 / 1,000,000,000 = 208.8 seconds. (c) The alphabet grows from 26 to more than 90 characters, so the number of combinations rises exponentially and exhaustive search takes far longer. Marking: (b) in seconds to 1 decimal place. |
Section C: Algorithms, Programming and Emerging Technology (32 marks)
Answer all questions in Section C.
| # | Question | Answer and marking notes |
|---|---|---|
| 9 | Search algorithms. A target value is searched in 1,024 sorted records. (a) State the maximum number of comparisons made by binary search in the worst case (3 marks). (b) How many times more comparisons does linear search need in the worst case (2 marks)? (c) State two preconditions for using binary search (3 marks). | Answer: (a) log2 1024 = 10 comparisons, because each comparison halves the range. (b) 1,024 / 10 = 102.4 times more. (c) The records must be sorted and the middle element must be directly accessible, as in an array. Marking: (b) refers to the worst case. |
| 10 | Program tracing. Study this program: total = 0; for i in range(1, 6): total = total + i * i; print(total). (a) State the output and show the working (4 marks). (b) What is the output if range(1, 6) becomes range(1, 7) (2 marks)? (c) If the same loop is rewritten with while and the counter is never updated, what goes wrong (2 marks)? | Answer: (a) 1 + 4 + 9 + 16 + 25 = 55. (b) Add 36 and the output is 91. (c) The counter never changes, the condition stays true and the loop runs forever. Marking: (a) must show the five squares. |
| 11 | IoT data. Each of 500 sensors sends 60 records per hour and each record is 200 bytes. The system runs for 30 days. (a) Calculate the number of records collected per day (3 marks). (b) Calculate the data volume for 30 days in GB, where 1 GB = 1,000,000,000 bytes (2 marks). (c) Give two benefits of running edge computing on the sensors first (3 marks). | Answer: (a) 500 x 60 x 24 = 720,000 records. (b) 720,000 x 30 x 200 = 4,320,000,000 bytes = 4.32 GB. (c) Less upload bandwidth and cloud storage cost, and faster local responses that still work when the link drops. Marking: (b) multiply by 200 bytes before converting to GB. |
| 12 | AI and data literacy. A school trains a model on ten years of alumni data to predict how current students will progress. (a) Explain two biases the training data may introduce (3 marks). (b) Suggest two measures that protect students personal data (3 marks). (c) Explain why human review is still required for such predictions (2 marks). | Answer: (a) Alumni records cover one past group of students and ignore family background and curriculum change, so the model repeats and even amplifies existing bias. (b) Anonymise the data and keep only necessary fields, restrict access with audit logs, and inform students of the purpose and obtain consent. (c) The model only gives a probability, and promotion is a high stakes decision, so teachers must judge each case. Marking: (b) both measures must be concrete. |
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