Cambridge A Level Computer Science 9608 — 2020 Oct/Nov Paper 3 · Variant 2

1 In a particular computer system, real numbers are stored using floating-point representation, with: • 12 bits for the mantissa • 4 bits for the exponent • two’s complement form for both mantissa and exponent. (a) Calculate the denary value for the following floating-point number. Show your working. Mantissa Exponent 0 1 0 1 0 0 0 0 0 0 0 0 0 1 1 0 Working ..................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Denary value ............................................................................................................................ [3] (b) A new operating system has been installed that has changed the way the floating-point numbers are used. The order of the exponent and the mantissa are reversed. (i) Calculate the new denary value for the following floating-point number that has the same bit pattern as the number in part (a). Show your working. Exponent Mantissa 0 1 0 1 0 0 0 0 0 0 0 0 0 1 1 0 Working ............................................................................................................................. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... Denary value ..................................................................................................................... [3] (ii) Identify two problems that can occur due to the change in the representation of the floating-point number. Problem 1 .......................................................................................................................... ........................................................................................................................................... Problem 2 .......................................................................................................................... ........................................................................................................................................... [2]

Mark scheme: Question Answer Marks 1(a) Exponent = 6 (conversion of exponent to denary) 3 0.101 or 0.625 or 5/8 (value of mantissa) // moving of binary point 40 (answer) 1(b)(i) Exponent = 5 (conversion of exponent to denary) 3 0.00000000110 or 3/1024 (value of mantissa) // moving of binary point 0.09375 or 3/32 (answer) 1(b)(ii) Any two from 2 The number calculated will change The same bit pattern is for a different number Software may crash (if not updated)

2 Data types can be classified as composite or non-composite. (a) Draw one line from each data type to its correct classification. Data type Classification Pointer Record Composite Set Class Non-composite Integer [2] (b) A user-defined data type, timeOfDay, is declared using the following pseudocode. TYPE timeOfDay = (morning, afternoon, evening, night) (i) Identify the type of user-defined data type declared and state its classification. Type .................................................................................................................................. Classification ..................................................................................................................... [2] (ii) Write pseudocode to declare the variable session of type timeOfDay. Assign the value afternoon to the variable session. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2]

Mark scheme: 2(a) 2 marks for all 5 single lines correct 2 1 mark for 4 lines correct otherwise zero Data type Classification Pointer Record Composite Set Class Non-composite Integer 2(b)(i) Type Enumerated 2 Classification Non-composite 2(b)(ii) DECLARE session : timeOfDay 2 session ← afternoon

3 The use of protocols is essential for successful communication between computers. (a) Define the term communication protocol. ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] (b) Identify two protocols that are used in the transfer of emails and state the purpose of each protocol. Protocol 1 ................................................................................................................................. Purpose .................................................................................................................................... ................................................................................................................................................... Protocol 2 ................................................................................................................................. Purpose .................................................................................................................................... ................................................................................................................................................... [4] (c) Manav and Miora want to have a video conversation over the Internet using a dedicated connection. (i) Identify and describe the switching method used to implement this connection. Method .............................................................................................................................. Description ........................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [3] (ii) State one benefit and one drawback of the method you identified in part (c)(i). Benefit ............................................................................................................................... ........................................................................................................................................... Drawback .......................................................................................................................... ........................................................................................................................................... [2]

Mark scheme: 3(a) Set of rules 2 For (successful) transmission (and receipt) of data 3(b) One mark for protocol, one mark for purpose must match protocol 4 Any two pairs from Protocol: POP3 Purpose: downloading email Protocol: SMTP Purpose: sending/transferring email Protocol: IMAP Purpose: downloading email // storing/organising emails on an email server Protocol: HTTP/HTTPS Purpose: accessing email using a browser 3(c)(i) One mark for method, max two marks for description 3 Method: Circuit switching Description: Set up for the duration of the conversation Set up before communication starts Maintained throughout the transmission All data travels down the same route Dropped at the end of the transmission Complete bandwidth used 3(c)(ii) One mark for benefit, one mark for limitation 2 Benefit: Manav and Miora can see other in real time // better synchronisation // full bandwidth available Drawback: Bandwidth / channel not available to other users // extra time required to set up circuit at start of conversation // alternative route not available without restarting the conversation // less secure as easier to intercept data if only one channel used // failure single route used means failure of transmission

4 The following truth table represents a logic circuit with three inputs and two outputs. INPUT OUTPUT A B C X Y 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1 0 1 1 0 0 0 0 1 0 1 0 1 1 1 0 0 0 1 1 1 1 1 (a) Write the Boolean expressions for the truth table as sum-of-products. X = ............................................................................................................................................ Y = ............................................................................................................................................ [3] (b) Complete the Karnaugh Maps (K-maps) for the truth table. OUTPUT X OUTPUT Y AB AB 00 01 11 10 00 01 11 10 0 0 C C 1 1 [2] (c) The K-maps can be used to simplify one of the expressions in part (a). (i) Draw loop(s) around appropriate group(s) of 1s to produce an optimal sum-of-products for the single output table that can be simplified in part (b). [2] (ii) Write the simplified sum-of-products expressions for this output from part (c)(i). ..................................................................................................................................... [2]

Mark scheme: 4(a) For X 1 mark for all products correct 3 For Y 2 marks for 3 products correct, no other products seen X = A.B.C + A.B.C Y = A.B.C + A.B.C + A.B.C 4(b) One mark for each correct K-map max 2 2 OUTPUT X OUTPUT Y AB AB 00 01 11 10 00 01 11 10 0 1 0 0 0 0 0 0 0 C 1 0 0 1 0 0 1 1 1 4(c)(i) One mark for OUTPUT X no loops 2 One mark for OUTPUT Y all loops correct and no others max 2 OUTPUT X OUTPUT Y AB AB 00 01 11 10 00 01 11 10 0 1 0 0 0 0 0 0 0 C 1 0 0 1 0 0 1 1 1 4(c)(ii) One mark for each correct product 2 A.C + B.C

5 Complete these statements about flip-flops. A flip-flop is a .................................................... . It has .................................................... stable states. A flip-flop is used for .................................................... . There are different types of flip-flop, for example .................................................... and .................................................... . [5]

Mark scheme: 5 (logic) Circuit // bi-stable 5 Two Memory // data storage // registers // storing one bit of data JK/SR/D/T SR/JK/T/D

6 Mahmoud is developing a new application, MyApp, that needs to work with three different operating systems (OS1, OS2 and OS3). He has decided to use virtual machine software to test MyApp with these three different operating systems. The diagram shows MyApp running on three virtual machines. MyApp MyApp MyApp OS1 OS2 OS3 Virtual machine software OS4 Hardware (a) Describe the roles of the four operating systems (OS1, OS2, OS3 and OS4) shown in the diagram. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] (b) Describe the role of the virtual machine software in the testing of MyApp. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] (c) Explain one benefit and one drawback of this approach to testing MyApp. Benefit ...................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Drawback .................................................................................................................................. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [4]

Mark scheme: 6(a) Max three, one mark for role, one mark for expansion 3 OS1, OS2 and OS3 are guest operating systems … secondary to the one installed on the hardware OS4 is the host operating system … interacts directly with the machine hardware MyApp needs to run on all three guest operating systems … … with identical results 6(b) Any three from 3 Create/delete/manage virtual machine Translate instructions used by guest operating system to that required by host operating system Hardware emulation Protecting each virtual machine … so instances of MyApp can be tested together 6(c) One mark for benefit and one mark for relevant explanation 4 One mark for drawback one mark for relevant explanation For example: Benefit: multiple operating systems can exist simultaneously … allowing for testing using the same hardware only one set of hardware required ... reduces cost of producing the app // no need to set up more than one computer Drawback: execution of extra code … so performance is degraded // more time taken to execute the app // cannot make judgements about response time etc

7 Sam wants to send confidential data to an organisation. He has already received the organisation’s digital certificate. The organisation has asked him to make sure that the message containing the confidential data is encrypted and is sent with a digital signature. (a) Explain the process the organisation followed to obtain its digital certificate. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] (b) Identify two items included in the organisation’s digital certificate that will be used when sending the message. Give a reason why each item is required. Item 1 ........................................................................................................................................ Reason ..................................................................................................................................... ................................................................................................................................................... Item 2 ........................................................................................................................................ Reason ..................................................................................................................................... ................................................................................................................................................... [4] (c) Identify two other items included in the organisation’s digital certificate. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [2] (d) Explain how the digital signature for Sam’s message is produced. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [4]

Mark scheme: 7(a) Any three from 3 Applied to an issuing certificate authority / CA … with some proof of identity … (for example) name of organisation / address of organisation etc … so their identity can be checked by an organisational registration authority / ORA … so that a digital certificate will only be issued to a trusted organisation 7(b) one mark for item, one mark for reason; must relate to item Max 4 4 Item: public key Reason: to encrypt / decrypt data Item: agreed encryption/hashing algorithm Reason: to produce hash total / message digest 7(c) Any two from 2 Serial number Name of subject/organisation Date valid from/to Signature to verify it came from the issuers Name of issuer Purpose of the public key Thumbprint algorithm Thumbprint/fingerprint for the hash CA digital signature 7(d) Any four from 4 Message is put through agreed hashing / encryption algorithm … to produce a hash total / message digest then the message digest / hash total is encrypted … with Sam’s private key … … this is now his digital signature

8 A car monitoring system provides information to the driver about the car’s performance and alerts the driver to possible problems. • Data about the car’s performance is stored in three memory locations with addresses 601 to 603. • Location 601 contains the distance travelled in kilometres for the current trip as a binary integer. • Location 602 contains the quantity of fuel used in litres for the current trip, as a fixed-point binary number with 5 places before the binary point and three places after the binary point. • The four least significant bits of location 603 are flags used to identify problems with the car, for example, the fuel is low. A flag is set to 1 if there is a problem, or 0 if not. These problems are: • Bit 0 - high engine temperature • Bit 1 - low oil pressure • Bit 2 - low battery • Bit 3 - low fuel • Bits 4 to 7 are not used (a) The current contents of addresses 601 to 603 are: Most significant Least significant 601 0 0 1 0 1 1 0 0 602 0 0 1 0 1 0 0 1 603 0 0 0 0 0 1 0 0 State the information that the current contents of addresses 601 to 603 will provide to the driver. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] (b) A car has low oil pressure and low fuel. It has travelled 80 kilometres and used 7.25 litres of fuel. Complete the contents of the addresses to record this information. 601 602 603 [3] (c) The following table shows the assembly language instructions for the car performance monitoring system. There is one general purpose register, the Accumulator (ACC). Table 8.1 Instruction Explanation Label Op code Operand LDM #n Load the number n to ACC Load the contents of the location at the given LDD <address> address to ACC STO <address> Store the contents of ACC at the given address Bitwise AND operation of the contents of ACC AND #n with the numeric operand CMP #n Compare the contents of ACC with the number n Following a compare instruction, jump to JPE <address> <address> or <label> if the compare was True JMP <address> Jump to <address> or <label> <label>: <op code> <operand> Labels an instruction Note: # denotes immediate addressing B denotes a binary number, for example B01001010 & denotes a hexadecimal number, for example &4A (i) Write assembly language instructions to set the contents of addresses 601 and 602 to zero, and set all four least significant bits of the contents of address 603 to one. Use the instruction set from Table 8.1. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] (ii) A program continuously checks the flags. If a flag is set, the program moves to the error-handling routine at the specified label. For example, if the engine temperature is high, the program jumps to the label for the error-handling routine HIGHTEMP. The error-handling routine instructions have not been provided. A programmer has written most of the instructions for the program in the following table. There are four missing operands. Complete the assembly language program by writing the four missing operands. Label Op code Operand CHECKFLAGS: LDD 603 AND &0F STO TEMP AND &01 CMP &01 JPE HIGHTEMP LDD TEMP AND &02 CMP JPE LOWOIL LDD TEMP AND CMP &04 JPE LOWBATT LDD AND &08 CMP &08 JPE LOWFUEL JMP TEMP: [4]

Mark scheme: 8(a) 44 kilometres 3 5.125 litres Low battery 8(b) One mark for 601 correct 3 One mark for 602 correct One mark for 603 correct 601 0 1 0 1 0 0 0 0 602 0 0 1 1 1 0 1 0 603 0 0 0 0 1 0 1 0 8(c)(i) 1 mark per bullet point 3 • Storing 0 in 601 and 602 • Loading the correct pattern for 603 • … Storing in 603 LDM &00 /#0 / B00000000 STO 601 STO 602 LDM B00001111 /#15 /&0F STO 603 8(c)(ii) &02 / #2 / B00000010 4 &04 / #4 / B00000100 TEMP CHECKFLAGS