2021 BLSS Computing P1 Solutions

BLS/PRELIM/2021/4E/COMP/7155/01 BOON LAY SECONDARY SCHOOL PRELIMINARY EXAMINATION 2021 Name CCA Subject : COMPUTING Paper No : 1 Subject Code : 7155/01 Level : SECONDARY FOUR EXPRESS Date/Day : 30th AUGUST 2021 / MONDAY Time : 1100 – 1300 Duration : 2 HOURS Candidates answer on the Question Paper. No Additional Materials are required. READ THESE INSTRUCTIONS FIRST Before you start your exam, check that you have received the correct paper and the number of printed pages are correct. Write your name, index number, and CCA in the spaces at the top of this page. Write in dark blue or black pen. You may use an HB pencil for any diagrams or graphs or rough working. Do not use staples, paper clips, glue or correction fluid. Approved calculators are allowed. Answer all questions. The number of marks is given in brackets [ ] at the end of each question or part question. You should show all your working. The total number of marks for this paper is 80. This document consists of 12 printed pages.

2 BLS/PRELIM/2021/4E/COMP/7155/01 1 Digital systems make use of several different methods to store data. a (i) Explain what is meant by “hexadecimal digits”. Base 16, numbers are from 0 to 9, A to F. …………………………….…………………………………………………………………...[1] (ii) Describe two examples where a hexadecimal storage system would be preferred over a denary system. file compression, MAC addresses, colour codes, encryption [Any two] …………………………….…………………………………………………………………...[2] (iii) Justify the use of hexadecimal storage systems in one of your examples in a(ii). Able to represent more data with the same number of bits. [1] 4-bit binary word: max – 1111 = 15. 4-bit hexadecimal word: FFFF = 65535 [1] b (i) The character Omega, “Ω” is represented by the binary number 11101010 in ASCII. Convert 11101010 into a denary number. Show your working. 7 6 5 4 3 2 1 0 128 64 32 16 8 4 2 1 1 1 1 0 1 0 1 0 128 64 32 0 8 0 2 0 128 + 64 + 32 + 8 + 2 = 234 …………………………….…………………………………………………………………...[1]

3 BLS/PRELIM/2021/4E/COMP/7155/01 (ii) The character “j” is represented by the denary number 106 in ASCII. Describe a method to convert 106 into its hexadecimal representation. 1 – convert 106 to binary = 01101010 2 – break into words of 4-bits = [0010] [1010] 3 – convert the binary words into their hexadecimal equivalents = 6A …………………………….……………………………………………………………………… …………………………….……………………………………………………………………… …………………………….……………………………………………………………………… …………………………….……………………………………………………………………… …………………………….…………………………………………………………………...[3] 2 a Where appropriate, draw a line to match the terms to its correct description. Terms Description DoS attack ¢ Cookie ¢ ¢ A small piece of data used by websites to store personal information on a user’s web browser. Encryption ¢ ¢ Hidden program that secretly collects personal information. Phishing ¢ ¢ Flooding network with useless traffic, making the network slow or inaccessible. Software piracy ¢ ¢ Sending fake or spoof emails. Worms ¢ [4] b (i) What are cybersecurity threats and how can they affect our data security? They threaten our data security. Might lose personal or important data.

4 BLS/PRELIM/2021/4E/COMP/7155/01 …………………………….…………………………………………………………………...[2] (ii) In the context of computer systems, explain the difference between a computer worm, and a computer virus. A worm does not require a host program to replicate itself whereas a virus requires one before it can replicate itself and infect other programs. …………………………….…………………………………………………………………...[2] (iii) A key-logger is software that records every keystroke that has been entered into a computer system. Explain how a key-logger can be used as a part of a system’s security, or as spyware. As a part of a system’s security It can be used to record unauthorised activities on the computer. In the event of a breach, the modifications or access can be tracked easily. …………………………….……………………………………………………………………… …………………………….…………………………………………………………………...[2] As a spyware It can be used to record our login credentials and we might lose access to our online identities or our financial accounts. …………………………….……………………………………………………………………… …………………………….…………………………………………………………………...[2] (c) (i) What is ‘software piracy’? Unauthorised use or distribution of copyrighted software …………………………….…………………………………………………………………...[1] (ii) Provide two possible reasons that causes software piracy to occur. High or prohibitive costs. Lack of intellectual property rights. Lack of education. …………………………….…………………………………………………………………...[2]

5 BLS/PRELIM/2021/4E/COMP/7155/01 (iii) Describe possible positive and negative effects of software piracy on the software industry. Positive Proliferation of the use of the software / gain in popularity. …………………………….…………………………………………………………………...[1] Negative Software company loses revenue and is unable to continue developing good products. …………………………….…………………………………………………………………...[1] 3 a Decomposition, Pattern Recognition and Generalisation are three common problem solving techniques. (i) Describe how decomposition helps in problem-solving and algorithm design. It helps to simplify the problem, breaking it into smaller ones or similar that can be solved more easily. …………………………….…………………………………………………………………...[1] (ii) One approach to decomposing is modularity. Describe what this is. Solve simple examples of the problem manually and identify tasks that are of different natures. Usually, these tasks can be separated from each other to become distinct (and sometimes unrelated) sub-problems. …………………………….…………………………………………………………………...[1] (iii) What is Pattern Recognition? How is it useful in solving problems? Identifying patterns among two or more problems. These problems may come from different situations or they may be sub-problems obtained from decomposition. These patterns are clues that the solutions to these problems will most likely be similar. Thus, if we already know the solution for one problem, we can probably modify that solution to solve other problems that follow a similar pattern. …………………………….…………………………………………………………………...[2] b Read the following lines of code.

6 BLS/PRELIM/2021/4E/COMP/7155/01 1 string_1 = 'this is a string 1234AZ!@#$%' 2 count = 0 3 for i in range(len(string_1)): 4 if string_1[i].isalpha(): count += 1 5 print(count) (i) What is the function of this code? To count the number of characters in the string that are letters. …………………………….…………………………………………………………………...[1] (ii) Explain how this code can be used for other similar situations with minimal modifications. modifying line 4. .isalpha() can be changed to isdigit() or isspace() to output the number of digits or space in the string. [Accept other reasonable answers] …………………………….……………………………………………………………………… …………………………….…………………………………………………………………...[2] (iii) In the code provided, suggest a replacement for the variable name count that is more self-explanatory. count_alpha [accept other reasonable answers] …………………………….…………………………………………………………………...[1] 4 a (i) Besides magnetic storage devices, name two other types of external storage technologies found in a computer system. List their corresponding example and describe each of them in terms of their advantage and disadvantage. Type 1: Optical storage Example: Compact disc Advantage: Portable. Able to store large amounts of data (GBs) Disadvantage: Vulnerable to scratches and fingerprints. Limited write/rewrite capacity[3] Type 2: Solid-state

7 BLS/PRELIM/2021/4E/COMP/7155/01 Example: Flashdrives / SD cards Advantage: Faster than magnetic media. Not as vulnerable to