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

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) The following floating-point number stored is not normalised. Calculate the denary value for the floating-point number. Show your working. Mantissa Exponent 0 0 0 0 1 1 0 0 0 0 0 0 0 1 0 1 Working ..................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Denary value ............................................................................................................................ [3] (b) (i) Normalise the floating-point number given in part (a). Write your answer in the following boxes. Mantissa Exponent [2] (ii) Describe one problem that can occur when floating-point numbers are not normalised. ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [2]

Mark scheme: Question Answer Marks 1(a) Exponent = 5 (conversion of exponent to denary) 3 0.00011 or 0.09375 or 3/32 (value of mantissa) //moving of binary point 3 (answer) 1(b)(i) Mantissa = 011000000000 2 Exponent = 0010 1(b)(ii) Any two from 2 Precision lost Redundant leading zeros in the mantissa Bits lost off right hand end / least significant end Multiple representations of a single number

2 Data types can be classified as composite or non-composite. A record is declared of type box using the following pseudocode. TYPE size = (small, medium, large) TYPE box DECLARE volume : size DECLARE price : REAL DECLARE colour : STRING ENDTYPE DECLARE myBox : ARRAY [1:6] OF box (a) (i) Identify one composite and three non-composite data types used in the pseudocode. Composite data type ......................................................................................................... Non-composite data type 1 ............................................................................................... Non-composite data type 2 ............................................................................................... Non-composite data type 3 ............................................................................................... [4] (ii) Identify the data type in the pseudocode that is enumerated. ..................................................................................................................................... [1] (b) A box is red, with medium volume and a price of $10.99. Write pseudocode to store the details of this box in the first element of the array. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3]

Mark scheme: 2(a)(i) Composite 4 box Non-composite size / enumerated REAL STRING 2(a)(ii) size 1 2(b) myBox[1].volume ← medium 3 myBox[1].price ← 10.99 myBox[1].colour ← "red"

3 The use of the TCP/IP protocol suite is essential for successful communication over the Internet. (a) (i) Describe the TCP/IP protocol suite. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [5] (ii) A group of over 100 students has produced a movie. The size of the movie file is very large. The students would like to use peer-to-peer file sharing to share this file with friends and family. Identify the most appropriate TCP/IP protocol for sharing this file over the Internet and describe the way this protocol works. Protocol ............................................................................................................................. Description ........................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [5] (b) (i) Files shared over the Internet are sent using packet switching or circuit switching methods. Identify and describe the most suitable method for the large movie file from part (a)(ii). Method .............................................................................................................................. Description ........................................................................................................................ ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... [4] (ii) State one benefit and one drawback of the method you identified in part (b)(i). Benefit ............................................................................................................................... ........................................................................................................................................... Drawback .......................................................................................................................... ........................................................................................................................................... [2]

Mark scheme: 3(a)(i) Any five from 5 A layered model / stack … with 4 layers Uses a set of protocols for transmission of data .. transport control protocol with internet protocol Named layers of Application Layer, Transport Layer, Internet/Network Layer, Data Link Layer two layers correct … … all four layers correct … in the correct order 3(a)(ii) One mark for protocol, four marks for description 5 Protocol: BitTorrent Description: Any four from BitTorrent client software made available to friends and family’s computers a complete copy of the torrent/file to be shared is available on at least one computer the torrent/file is split into small pieces… … rare pieces are given priority for downloading the torrent descriptor file is made available a computer/user joins (the swarm) by using the BitTorrent software to load the torrent descriptor file the computer/user can now download a piece of the torrent/file once a computer has a piece/ the whole file, it can become a seed (and upload to other members of the swarm) pieces of the torrent/file are both downloaded and uploaded (by each member of the of the swarm) a server called a tracker keeps records of all the computers in the swarm the tracker shares their IP addresses allowing connection to each other 3(b)(i) One mark for method 4 Method: Packet switching Three marks for description, any three from Description: file is divided into packets … and necessary data added to each packet, e.g. header which are sent independently of each other and do not need to take the same route packets are reassembled at the destination missing / corrupt packets can be resent 3(b)(ii) One mark for benefit, one mark for drawback 2 Benefit: packets can be rerouted if there are problems // packets can take the least congested route // transmission errors can be detected // missing / corrupt packets can be resent Drawback: packets can be dropped / delayed

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 0 0 0 0 1 1 0 0 1 0 1 0 0 1 1 0 1 1 0 0 1 0 1 0 1 0 1 1 1 0 0 1 1 1 1 1 1 (a) Write the Boolean expressions for the truth table as sum-of-products. X = ............................................................................................................................................ ................................................................................................................................................... Y = ............................................................................................................................................ ................................................................................................................................................... [4] (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). [3] (ii) Write the simplified sum-of-products expressions for this output from part (c)(i). ..................................................................................................................................... [3] (d) Identify the common logic circuit given by the truth table in part (a). Give the use of each output. Logic circuit ............................................................................................................................... Use of X .................................................................................................................................... Use of Y .................................................................................................................................... [3]

Mark scheme: 4(a) For each expression, 2 marks all products correct no incorrect products seen, 4 1 mark 2 or 3 products correct, max 4 X = A. B. C + A. B. C + A. B. C + A. B. C Y = A. B. C + A. B. C + A. B. C + A. B. C 4(b) One mark for each correct K-map max 2 2 4(c)(i) One mark for OUTPUT X no loops 3 OUTPUT Y one mark vertical loop correct one mark horizontal loops correct 4(c)(ii) One mark for each correct product and no incorrect products max 3 3 A.B + B.C + A.C 4(d) Logic circuit: Full Adder 3 X: Sum Y: Carry

5 Complete these statements about a virtual machine. A virtual machine is ................................................ that emulates a ................................................ computer system. A virtual machine allows multiple ................................................ operating systems to run on one computer using a ................................................ operating system. [4]

Mark scheme: 5 Software / a program 4 Physical / different Guest Host

6 Anita is studying computer science and she is confused about some of the computer security terminology as some of the words are similar. Anita wants to know the similarities (features that are the same) and differences (features that are different) between some of the terms. (a) Give the similarities and differences between a public key and a private key. Similarities ................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Differences ................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [4] (b) Give the similarities and differences between a digital certificate and a digital signature. Similarities ................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Differences ................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [4] (c) Give the similarities and differences between phishing and pharming. Similarities ................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... Differences ................................................................................................................................ ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... [4]

Mark scheme: 6(a) Three marks similarities, three marks differences max 4 4 Similarities: any three from Both used in asymmetric … encryption … as a pair of keys is required … one is used to encrypt the data/message and the other is used to decrypt the data/message Both hashing algorithms Differences: any three from Private key only known to owner of the key pair …The public key can be distributed to anyone When messages are sent to the owner of a public key, they are encrypted with the owners public key …so they can only be decrypted by the owner’s private key Message digests are encrypted with the private key of the sender to form a digital signature … messages are encrypted with the public key of the receiver 6(b) Three marks similarities, three marks differences max 4 4 Similarities: any three from Both used for authentication Both are unique to the owner/subject Include / use owner’s public key include / make use of hash algorithm Differences: any three from Certificate obtained from issuing authority … signature created from a message Certificate provides authentication of owner …Signature used to authenticate messages that are sent by the owner Certificate remains unchanged whilst it is valid …new signature created for every message Only certificate provides extra information Only signature makes use of a private key 6(c) Three marks similarities, three marks differences max 4 4 Similarities: any three from Both malware / malicious software Both collect personal / sensitive data …via fake websites …the data are then used illegally Differences: any three from Phishing uses (malicious) emails … … that direct users to fake websites Pharming misdirects browser to a bogus website … by modifying entries on a DNS server … or by being installed on your computer

7 A company has a number of lorries that deliver items around the country. The items in each lorry are its load. Each lorry has a monitoring system that provides information to the driver about the state of the load and other data from each trip. • Data is stored in three memory locations with addresses 801 to 803. • Location 801 contains the distance travelled in kilometres for the current trip, stored as a binary integer. • Location 802 contains the quantity of fuel used in litres for the current trip, stored as a fixed-point binary number with six places before the binary point and two places after the binary point. • The four most significant bits of location 803 are flags used to identify problems with the load, for example it is too heavy. A flag is set to 1 if there is a problem, or 0 if not. The problems are: • Bit 7 – load too heavy • Bit 6 – load too high • Bit 5 – load unstable • Bit 4 – load not secured (risk of the load falling off) • Bits 0 to 3 are not used (a) The current contents of addresses 801 to 803 are: Most significant bit Least significant bit 801 0 1 1 0 1 1 0 0 802 0 0 1 0 1 0 0 1 803 0 0 1 0 0 0 0 0 State the information that the current contents of addresses 801 to 803 will provide to the driver. ................................................................................................................................................... ................................................................................................................................................... ................................................................................................................................................... ............................................................................................................................................. [3] (b) A lorry has a load that is too heavy and is not secured. It has travelled 120 kilometres and used 35.25 litres of fuel. Complete the contents of the addresses to record this information. 801 802 803 [3] (c) The following table shows the instructions for the lorry load monitoring system in assembly language. There is one general purpose register, the Accumulator (ACC). Table 7.1 Instruction Explanation Label Op code Operand LDM #n Load the number n to ACC Load the contents of the location at the LDD <address> given address to ACC Store the contents of ACC at the given STO <address> address Bitwise AND operation of the contents of AND #n ACC with the operand CMP #n Compare the contents of ACC with number n Following a compare instruction, jump to JPE <address> <address> or <label> if the compare was True JMP <address> Jump to the given 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 801 and 802 to zero, and set all four most significant bits of the contents of address 803 to one. Use the instruction set from Table 7.1. ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ........................................................................................................................................... ..................................................................................................................................... [3] (ii) A program written in assembly language, continuously checks the flags. If a flag is set, the program jumps to the error-handling routine at the specified label. For example, if the load is too heavy, the program jumps to the error-handling routine with the label TOOHEAVY. 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 CHECKLOAD: LDD 803 AND &F0 STO TEMP AND &80 CMP &80 JPE TOOHEAVY LDD TEMP AND &40 CMP JPE TOOHIGH LDD TEMP AND CMP &20 JPE UNSTABLE LDD AND &10 CMP &10 JPE NOTSECURED JMP TEMP: [4]

Mark scheme: 7(a) 108 kilometres 3 10.25 litres Load unstable 7(b) One mark for 801 correct 3 One mark for 802 correct One mark for 803 correct 801 0 1 1 1 1 0 0 0 802 1 0 0 0 1 1 0 1 803 1 0 0 1 0 0 0 0 7(c)(i) Loading 0 and storing in 801 and 802 3 Loading 240 …and storing in 803 LDM #0/B00000000/&(0)0 STO 801 STO 802 LDM #240/B11110000/&F0 STO 803 7(c)(ii) &40 / #64 / B01000000 4 &20 / #32 / B00100000 TEMP CHECKLOAD