Cambridge A Level Computer Science 9608 — 2018 Oct/Nov Paper 1 · Variant 2

* 6 0 5 4 1 9 8 9 6 8 * This document consists of 13 printed pages and 3 blank pages. DC (NF/CGW) 144492/3 © UCLES 2018 [Turn over Cambridge International Examinations Cambridge International Advanced Subsidiary and Advanced Level COMPUTER SCIENCE 9608/12 Paper 1 Theory Fundamentals October/November 2018 1 hour 30 minutes Candidates answer on the Question Paper. No Additional Materials are required. No calculators allowed. READ THESE INSTRUCTIONS FIRST Write your Centre number, candidate number and name 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, graphs or rough working. Do not use staples, paper clips, glue or correction fluid. DO NOT WRITE IN ANY BARCODES. Answer all questions. No marks will be awarded for using brand names of software packages or hardware. At the end of the examination, fasten all your work securely together. The number of marks is given in brackets [ ] at the end of each question or part question. The maximum number of marks is 75.

2 9608/12/O/N/18 © UCLES 2018 1 A company is designing a website. (a) The company creates a 4-colour bitmap image for the website as shown. Each colour is represented by a letter, for example, G = grey, K = black. G G G G G W R R R R G W G G G G G W K K K K K W W W W W K K R R R R R R (i) State the minimum number of bits needed to represent each pixel in the image in part (a). … [1] (ii) Calculate the minimum file size of the image shown in part (a). Show your working. Working … … … … File size … [3] (b) The company takes a photograph of their office to put on the website. The photograph has a resolution of 1000 pixels by 1000 pixels. Two bytes per pixel are used to represent the colours. (i) Estimate the file size of the photograph in megabytes. Show your working. Working … … … … Estimated file size … [4]

3 9608/12/O/N/18 © UCLES 2018 [Turn over (ii) The file size of the photograph needs to be reduced before it is placed on the website. Draw lines to link each method of reducing the file size of the image to: • its description and • its compression type, where appropriate. Description Method Compression type Removes pixels Reduces number of pixels per inch Uses fewer bits per pixel Stores colour code and count of repetitions Crop the photograph Use run-length encoding Use fewer colours Lossy Lossless [5] (c) The company has created a logo for the website. The logo is a vector graphic. Describe two reasons why a vector graphic is a sensible choice for the logo. Reason 1 … … … … Reason 2 … … … … [4]

4 9608/12/O/N/18 © UCLES 2018 2 Gopal types the Uniform Resource Locator (URL) of a website into a web browser. (a) The following sequence (1 to 5) describes the steps that take place. There are three missing statements. 1 Gopal types into the web browser. 2 … 3 DNS looks up the URL in table 4 … 5 … Three statements A, B and C are used to complete the sequence. A DNS finds corresponding IP address B Web browser sends URL to Domain Name Service (DNS) C DNS returns IP address to web browser Write one of the letters A to C in the appropriate rows (2, 4 and 5) to complete the sequence. [2] (b) Describe the purpose of an IP address. … … … … [2] (c) A telecommunications operator has installed fibre-optic cables in Gopal’s neighbourhood. (i) Give three benefits of fibre-optic cable over copper cable. 1 … … 2 … … 3 … … [3]

5 9608/12/O/N/18 © UCLES 2018 [Turn over (ii) Give two drawbacks of fibre-optic cable over copper cable. 1 … … 2 … … [2]

6 9608/12/O/N/18 © UCLES 2018 3 The following table shows assembly language instructions for a processor which has one general purpose register, the Accumulator (ACC) and an Index Register (IX). Instruction Explanation Op code Operand LDD <address> Direct addressing. Load the contents of the location at the given address to ACC. LDX <address> Indexed addressing. Form the address from <address> + the contents of the Index Register. Copy the contents of this calculated address to ACC. LDR #n Immediate addressing. Load the number n to IX. STO <address> Store contents of ACC at the given address. ADD <address> Add the contents of the given address to ACC. INC <register> Add 1 to the contents of the register (ACC or IX). DEC <register> Subtract 1 from the contents of the register (ACC or IX). CMP <address> Compare contents of ACC with contents of <address>. JPE <address> Following compare instruction, jump to <address> if the compare was True. JPN <address> Following compare instruction, jump to <address> if the compare was False. JMP <address> Jump to the given address. OUT Output to the screen the character whose ASCII value is stored in ACC. END Return control to the operating system. (a) (i) State what is meant by absolute addressing and symbolic addressing. Absolute addressing … … Symbolic addressing … … [2] (ii) Give an example of an ADD instruction using both absolute addressing and symbolic addressing. Absolute addressing … Symbolic addressing … [2]

7 9608/12/O/N/18 © UCLES 2018 [Turn over (b) (i) State what is meant by indexed addressing and immediate addressing. Indexed addressing … … Immediate addressing … … [2] (ii) Give an example of an instruction that uses: Indexed addressing … Immediate addressing … [2] (c) The current contents of a general purpose register (X) are: X 1 1 0 0 0 0 0 1 (i) The contents of X represent an unsigned binary integer. Convert the value in X into denary. … [1] (ii) The contents of X represent an unsigned binary integer. Convert the value in X into hexadecimal. … [1] (iii) The contents of X represent a two’s complement binary integer. Convert the value in X into denary. … [1]

8 9608/12/O/N/18 © UCLES 2018 (d) The current contents of the main memory, Index Register (IX) and selected values from the ASCII character set are: Address Instruction ASCII code table (selected codes only) 40 LDD 100 ASCII code Character 41 CMP 104 300 / 42 JPE 54 301 * 43 LDX 100 302 - 44 CMP 105 303 + 45 JPN 47 304 ^ 46 OUT 305 = 47 LDD 100 48 DEC ACC 49 STO 100 50 INC IX 51 JMP 41 52 53 54 END … 100 2 101 302 102 303 103 303 104 0 105 303 IX 1 This is a copy of the instruction set. Instruction Explanation Op code Operand LDD <address> Direct addressing. Load the contents of the location at the given address to ACC. LDX <address> Indexed addressing. Form the address from <address> + the contents of the Index Register. Copy the contents of this calculated address to ACC. LDR #n Immediate addressing. Load the number n to IX. STO <address> Store contents of ACC at the given address. ADD <address> Add the contents of the given address to ACC. INC <register> Add 1 to the contents of the register (ACC or IX). DEC <register> Subtract 1 from the contents of the register (ACC or IX). CMP <address> Compare contents of ACC with contents of <address>. JPE <address> Following a compare instruction, jump to <address> if the compare was True. JPN <address> Following a compare instruction, jump to <address> if the compare was False. JMP <address> Jump to the given address. OUT Output to the screen the character whose ASCII value is stored in ACC. END Return control to the operating system.

9 9608/12/O/N/18 © UCLES 2018 [Turn over Complete the trace table for the given assembly language program. Instruction address ACC Memory address IX OUTPUT 100 101 102 103 104 105 2 302 303 303 0 303 1 40 [7]

10 9608/12/O/N/18 © UCLES 2018 4 A student has written the steps of the fetch stage of the fetch-execute (FE) cycle in register transfer notation. The student has made some errors. Line 1 MDR [PC] Line 2 PC PC + 1 Line 3 MDR [MAR] Line 4 CIR PC (a) Identify the line numbers of three errors that the student has made. Write the correct notation for each error. Line number of error Correct notation [3] (b) One stage of the FE cycle includes checking for interrupts. (i) Give three different events that can generate an interrupt. 1 … 2 … 3 … [3] (ii) Explain how interrupts are handled during the fetch-execute cycle. … … … … … … … … … … [5]

11 9608/12/O/N/18 © UCLES 2018 [Turn over (c) The processor uses buses in the FE cycle. The diagram shows three buses and two descriptions. Draw one line from each bus to its appropriate description. Bus Description Control bus Address bus Data bus Unidirectional (one direction) Bidirectional (two directions) [2] 5 This question presents three scenarios. For each scenario, tick (✓) one box to show whether you think the person’s behaviour is ethical or unethical. Justify your choice. (a) Wendy is a software engineer who is developing a program for her company. Her friend, Noah, is developing a program for a different company. Wendy looks at the code that Noah is writing to get ideas for her own program. Ethical Unethical Justification … … … … [2]

12 9608/12/O/N/18 © UCLES 2018 (b) Amit is fixing some bugs in the computer system of a large multinational company. He is asked to sign a confidentiality agreement. He sees some confidential information which contains the names of other multinational companies that have broken the law. He copies this information and releases it on the Internet. Ethical Unethical Justification … … … … [2] (c) Farah is providing a company with an estimate for the cost of writing a program. The company she works for is in financial difficulty so she increases the estimate by 10%. Ethical Unethical Justification … … … … [2] 6 Kim is using her laptop computer to write a program in a high-level language. (a) Kim needs to make sure the program is secure against unauthorised access. She has already set up a username and password on her laptop. Identify two additional electronic measures that Kim can use to keep the program secure. 1 … 2 … [2] (b) Kim will use library routines in her program. (i) Describe what is meant by a library routine. … … … … [2]

13 9608/12/O/N/18 © UCLES 2018 (ii) Describe one benefit and one drawback of using library routines. Benefit … … … … Drawback … … … … [4] (c) Kim develops her program and makes it ready for use. To do this, she uses first an interpreter and then a compiler. Explain why Kim needs to use both an interpreter and a compiler. Interpreter … … … … Compiler … … … … [4]

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16 9608/12/O/N/18 © UCLES 2018 Permission to reproduce items where third-party owned material protected by copyright is included has been sought and cleared where possible. Every reasonable effort has been made by the publisher (UCLES) to trace copyright holders, but if any items requiring clearance have unwittingly been included, the publisher will be pleased to make amends at the earliest possible opportunity. To avoid the issue of disclosure of answer-related information to candidates, all copyright acknowledgements are reproduced online in the Cambridge International Examinations Copyright Acknowledgements Booklet. This is produced for each series of examinations and is freely available to download at www.cie.org.uk after the live examination series. Cambridge International Examinations is part of the Cambridge Assessment Group. Cambridge Assessment is the brand name of University of Cambridge Local Examinations Syndicate (UCLES), which is itself a department of the University of Cambridge. BLANK PAGE

This document consists of 11 printed pages. © UCLES 2018 [Turn over Cambridge Assessment International Education Cambridge International Advanced Subsidiary and Advanced Level COMPUTER SCIENCE 9608/12 Paper 1 Written Paper October/November 2018 MARK SCHEME Maximum Mark: 75 Published This mark scheme is published as an aid to teachers and candidates, to indicate the requirements of the examination. It shows the basis on which Examiners were instructed to award marks. It does not indicate the details of the discussions that took place at an Examiners’ meeting before marking began, which would have considered the acceptability of alternative answers. Mark schemes should be read in conjunction with the question paper and the Principal Examiner Report for Teachers. Cambridge International will not enter into discussions about these mark schemes. Cambridge International is publishing the mark schemes for the October/November 2018 series for most Cambridge IGCSE™, Cambridge International A and AS Level components and some Cambridge O Level components.

9608/12 Cambridge International AS/A Level – Mark Scheme PUBLISHED October/November 2018 © UCLES 2018 Page 2 of 11 Generic Marking Principles These general marking principles must be applied by all examiners when marking candidate answers. They should be applied alongside the specific content of the mark scheme or generic level descriptors for a question. Each question paper and mark scheme will also comply with these marking principles. GENERIC MARKING PRINCIPLE 1: Marks must be awarded in line with: • the specific content of the mark scheme or the generic level descriptors for the question • the specific skills defined in the mark scheme or in the generic level descriptors for the question • the standard of response required by a candidate as exemplified by the standardisation scripts. GENERIC MARKING PRINCIPLE 2: Marks awarded are always whole marks (not half marks, or other fractions). GENERIC MARKING PRINCIPLE 3: Marks must be awarded positively: • marks are awarded for correct/valid answers, as defined in the mark scheme. However, credit is given for valid answers which go beyond the scope of the syllabus and mark scheme, referring to your Team Leader as appropriate • marks are awarded when candidates clearly demonstrate what they know and can do • marks are not deducted for errors • marks are not deducted for omissions • answers should only be judged on the quality of spelling, punctuation and grammar when these features are specifically assessed by the question as indicated by the mark scheme. The meaning, however, should be unambiguous. GENERIC MARKING PRINCIPLE 4: Rules must be applied consistently e.g. in situations where candidates have not followed instructions or in the application of generic level descriptors. GENERIC MARKING PRINCIPLE 5: Marks should be awarded using the full range of marks defined in the mark scheme for the question (however; the use of the full mark range may be limited according to the quality of the candidate responses seen). GENERIC MARKING PRINCIPLE 6: Marks awarded are based solely on the requirements as defined in the mark scheme. Marks should not be awarded with grade thresholds or grade descriptors in mind.

9608/12 Cambridge International AS/A Level – Mark Scheme PUBLISHED October/November 2018 © UCLES 2018 Page 3 of 11 Question Answer Marks 1(a)(i) 2 1 1(a)(ii) 1 mark per bullet point • Number of pixels: 6 × 6 // 36 • Number of bits: Number of pixels (36) × 2 « • « = 72 bits // 9 bytes 3 1(b)(i) 1 mark per bullet point • Number of pixels: 1000 × 1000 // 1 000 000 • Number of bytes: Number of pixels (1 000 000) × 2 // 2 000 000 // Number of bits: Number of pixels (1 000 000) × 16 // 16 000 000 • Conversion to megabytes • 2 (MB) // 1.91 (MB) 4 1(b)(ii) 1 mark per method correctly linked to its description max 3 1 mark for each compression type correctly linked to its method(s). max 2 Description Method Compression type 5 1(c) 1 mark per bullet point. Max 2 marks for each reason. • Smaller file size • Can be transferred quicker/downloaded quicker • Enlarges without pixilation • Needs to be used on different screens / devices / resolutions 4 Question Answer Marks 2(a) 1 mark for 1 correct answer, 2 marks for all 3 correct answers 1 Gopal types into the web browser 2 B (Web browser sends URL to Domain name Service (DNS)) 3 DNS looks up URL in a table 4 A (DNS finds corresponding IP address) 5 C (DNS returns IP address to web browser) 2

9608/12 Cambridge International AS/A Level – Mark Scheme PUBLISHED October/November 2018 © UCLES 2018 Page 4 of 11 Question Answer Marks 2(b) 1 mark per bullet point to max 2 • Gives each device on a network an identifier // IP address used to locate a device on a network • Each address is unique within the network • Allows a device/gateway/node to send data to the correct destination / a specific device/gateway/node 2 2(c)(i) 1 mark per bullet point to max 3 • Less interference in signal • Signal does not degrade as fast // Needs less signal boosting • More difficult to hack // more secure • Greater bandwidth // Faster transmission speeds possible 3 2(c)(ii) 1 mark per bullet point to max 2 • (Initial) installation cost is higher // Cable / hardware is more expensive to buy (per metre) • Specialists / trained personnel needed to install / maintain • Difficult to terminate // Electronics at both ends are more complex • Fibre-optic cables can break when bent • Only transmits data in one direction • If a fibre-optic cable connection fails, many more services can be affected 2 Question Answer Marks 3(a)(i) 1 mark per bullet point Absolute addressing: • The operand is a numeric address // The numeric address is given // referring directly to a memory location Symbolic addressing: • The operand is a word/symbol // A word/symbol represents the memory location/address 2 3(a)(ii) 1 mark per example Absolute addressing: For example, ADD 230 Symbolic addressing: For example, ADD num1 2 3(b)(i) 1 mark per bullet point Indexed addressing: • The address to be used is formed by: operand + the contents of the Index Register (IX) Immediate addressing: • The operand is not an address // the operand is the actual value to be loaded 2

9608/12 Cambridge International AS/A Level – Mark Scheme PUBLISHED October/November 2018 © UCLES 2018 Page 5 of 11 Question Answer Marks 3(b)(ii) 1 mark per example Indexed: For example, LDX 20 Immediate: For example, ADD #20 2 3(c)(i) 193 1 3(c)(ii) C1 1 3(c)(iii) –63 1 3(d) 1 mark per bullet point • Loading 2, comparing with 104 (instructions 40 and 41) • Loading 302 (instruction 43) • Comparing, and branching to 47 (instructions 44, 45) • Loading, decrementing accumulator and storing (instructions 47, 48 and 49) • Incrementing Index Register (instruction 50) • Loading 303, comparing and outputting + (instructions 43–46) • Loading, decrementing accumulator and storing, incrementing Index Register and end (instructions 47–51, 41, 42 and 54) 7

9608/12 Cambridge International AS/A Level – Mark Scheme PUBLISHED October/November 2018 © UCLES 2018 Page 6 of 11 Question Answer Marks Instruction address ACC Memory address IX OUTPUT 100 101 102 103 104 105 2 302 303 303 0 303 1 40 2 41 43 302 44 45 47 2 48 1 49 1 50 2 51 41 42 43 303 44 45 46 + 47 1 48 0 49 0 50 3 51 41 54

9608/12 Cambridge International AS/A Level – Mark Scheme PUBLISHED October/November 2018 © UCLES 2018 Page 7 of 11 Question Answer Marks 4(a) 1 mark per correct line, max 3 Line number of error Correct notation 1 MAR ← [PC] 3 MDR ← [ [MAR] ] 4 CIR ← [MDR] 2 PC ← [PC] + 1 3 4(b)(i) 1 mark for each event to max 3 For example: • Hardware fault // Example of hardware fault • I/O request // Example of I/O request • Program/software error // Example of software error • End of a time-slice 3 4(b)(ii) 1 mark per bullet point to max 5 • At the end of each fetch–execute cycle the processor checks for interrupt(s) • Check if an interrupt flag is set // Check if bit set in interrupt register • Processor identifies source of interrupt • Processor checks priority of interrupt • If interrupt priority is high enough // Lower priority interrupts are disabled • Processor saves current contents of registers • Processor calls interrupt handler / Interrupt Service Routine (ISR) • Address of ISR is loaded into Program Counter (PC) • When servicing of interrupt complete, processor restores registers • Lower priority interrupts are re-enabled • Processor continues with next F–E cycle 5

9608/12 Cambridge International AS/A Level – Mark Scheme PUBLISHED October/November 2018 © UCLES 2018 Page 8 of 11 Question Answer Marks 4(c) 1 mark for 1 correct connection 2 marks for all 3 correct connections 2 Question Answer Marks 5(a) 1 mark per bullet point for each justification, to max 2 Either Unethical • Noah’s work may be confidential • Wendy shouldn't claim someone else’s ideas / work as her own • She is bringing the profession into disrepute • Reference to IEEE standards in context Or Ethical • The code could be open source • Wendy may have permission from Noah • Wendy isn’t copying the code, just getting ideas • Reference to IEEE standards in context 2

9608/12 Cambridge International AS/A Level – Mark Scheme PUBLISHED October/November 2018 © UCLES 2018 Page 9 of 11 Question Answer Marks 5(b) 1 mark per bullet point for each justification, to max 2 Either Unethical • Amit has a responsibility to his company • He should have taken it to the police rather than putting it on the Internet • He has a signed agreement to say he will not give anything away • Reference to IEEE standards in context Or Ethical • Amit is acting in the public interest • Amit may not have actually signed the confidentiality agreement • If acting illegally, the multinational company should be brought to justice • Reference to IEEE standards in context 2 5(c) 1 mark per bullet point for each justification, to max 2 Either Ethical • It might save people’s jobs • Farah is acting in the best interest of her company • Reference to IEEE standards in context Or Unethical • Farah has a responsibility to act in the best interest of her client • It could give her company a bad reputation • Reference to IEEE standards in context 2 Question Answer Marks 6(a) 1 mark for each method to max 2 • Biometric authentication // by example • Two-step authentication // by example • Firewall / proxy • Encryption • Different access rights for different users • Password protect file (using a different password) • Anti-malware 2 6(b)(i) 1 mark per bullet to max 2 • Pre-existing / pre-compiled / pre-written modules / code « • « can be linked into her program (without amendment) • To perform common / complex tasks 2

9608/12 Cambridge International AS/A Level – Mark Scheme PUBLISHED October/November 2018 © UCLES 2018 Page 10 of 11 Question Answer Marks 6(b)(ii) 1 mark per bullet point. Max 2 for one benefit, max 2 for one drawback Benefit: • Less code needs to be written • « saves time / saves re-inventing the wheel • Pre-tested // Used by many people • ... reduces time testing // can be fairly sure that the function will perform as it should • Can be written in a different programming language • « making use of special features of that language • Can be complex algorithms (e.g. mathematical/graphics functions) • ... she does not need to work out how to write it // «that she may not know how to code • Simplifies the program • « since just the name of the function included in the source code Drawback: • Compatibility issues • « may not work with the other code/may require changing program for it to work • Not guaranteed thorough testing • « may be unknown or unexpected bugs / virus • Library routine may not meet exact needs • « may give unexpected results // « may need editing • If library routine is changed • « there may be unexpected results / errors 4

9608/12 Cambridge International AS/A Level – Mark Scheme PUBLISHED October/November 2018 © UCLES 2018 Page 11 of 11 Question Answer Marks 6(c) 1 mark per bullet point. Max 3 marks for interpreter, max 3 marks for compiler Interpreter: • Used during development • Debugging is easier • « Because errors are reported as they are found // No need to wait until the end of the process for the error report • « Because errors can be corrected as they are found Compiler: • Compiler used when development complete // compiler used when program ready for distribution • Produces an executable file (.exe) • « After compilation the compiler does not need to be present for the program to run • « The program can be given to others without access to (source) code • « Final program does not need to be re-compiled each time it is run • Cross-compilation, the program can be compiled to run on different platforms 4