BLSS 2019 P1 Solution

BOON LAY SECONDARY SCHOOL PRELIMNARY EXAMINATION 2019 Name ANSWERS [ ] CCA Subject : COMPUTING Paper No : 1 Subject Code : 7155/01 Level : SECONDARY FOUR EXPRESS Date/Day : XX XXX 2019 / XXXDAY Time : XXXX - XXXX 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.

2 BLS/EYE/2019/4E/COMP/7155/01 This document consists of 15 printed pages. 1 (a) Denary numbers are stored as binary numbers inside a computer. (i) Convert the denary number 168 into 8-bit binary. Show your working. ………………………………………………………………………………………………….[1] 1010 1000 (ii) Convert the positive whole binary number 11010011 into a denary number. Show your working. ………………………………………………………………………………………………….[1] 211 (iii) Convert the hexadecimal number AC into its binary representation. Show your working. ………………………………………………………………………………………………….[1] 1010 1100 (b) Hexadecimal numbers are commonly used in file compression techniques and in Media Access Control (MAC) addresses. (i) Calculate the largest numerical value that can be stored by the following numbers. Show your working. A 4-character binary number (1111) : 15 ………………………………………………………………………………………………….[1] A 4-character hexadecimal number (FFFF) : 65535 ………………………………………………………………………………………………….[1]

3 BLS/EYE/2019/4E/COMP/7155/01 (ii) State and describe one other use of the hexadecimal number system. ……………………………………………………………………………………………………. ………………………………………………………………………………………………….[2] RGB colour scheme codes / Memory addresses 2 A computer network is a system of two or more computers that are connected together by a transmission medium for the exchange of data. (a) The diagram below shows how a home network is connected to the Metropolitan Area Network (MAN). (i) Complete the diagram by filling in the appropriate networking devices for the three parts labelled i., ii., and iii. Networking devices: Router, Modem, Bridge, Network Interface Card i. ………………………………………………………………………………………………… ii. ………………………………………………………………………………………………… iii. ………………………………………………………………………………………………[2] MAN i. Home network ii. iii. Server Home terminal Mobile devices iii.

4 BLS/EYE/2019/4E/COMP/7155/01 (ii) Explain the functions of the following networking devices: Router ……………………………………………………………………………………………………. …………………………………………………………………………………………………[2] Connects different networks, typically between internet and intranet. Determines which terminal to send information to through ip addresses. Information only meant for intended recipients. Network Interface Card (NIC) ……………………………………………………………………………………………………. …………………………………………………………………………………………………[2] Allows information exchange between a terminal and a network. Is uniquely identified by a MAC address. (b) (i) Compare the difference in purpose and functionality(how it works) between a client-server network and a peer-to-peer network. You may use an example to aid your answer. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. ………………………………………………………………………………………………….[3] Purpose: client-server hosts information centrally, larger number of nodes, p2p information is hosted on many individual nodes, generally smaller sharing number of nodes. [1] Functionality: client-server no need to have multiple copies of the same information. Highly scalable. Easy to add clients. possible to be local / non-local (VPN) P2p information is shared directly, no need for hosting. Usually local. (ii) With reference to the transfer of information to terminals on a network, explain one difference between a bus and a star network.

5 BLS/EYE/2019/4E/COMP/7155/01 ……………………………………………………………………………………………………. ………………………………………………………………………………………………….[2] information on a bus network is sent on the bus and is received by all terminals connected to the network. In a star network, information is only sent to the intended recipient(s). (iii) Explain what are parity bits and benefits of using parity bits in data communication. Include the working principle of either odd or even parity bits. ……………………………………………………………………………………………………. ………………………………………………………………………………………………….[4] Data that has been transmitted can be checked to determine if an error has been incurred during the course of transmission. Parity bit: a bit in the data packet is used to signify if there were supposed to be an odd or even number of 1s in the packet. This would have been pre-agreed upon. Even parity: it is set to 0 if the number of 1-bits in the packet is supposed to be an even number. If there is an odd number of 1-bits, the parity bit will be set to ‘1’ so that there will be an even number of 1-bits in the packet. The logic is similar for odd-parity. 3 Consider the following algorithm. 1 x = 0 2 y = 0 3 while True: 4 num = input("Key in an integer: ") 5 out = 0 6 for i in range (len(num)): 7 out += int(num[i])*i 8 if out % 5 == 0: 9 x += 1 10 else: 11 y += 1 12 print("X: {}, Y: {}".format(x,y)) 13 if (input("Try another number? (Y/N)").upper() != 'Y'): 14 break (a) (i) Complete the trace table for the algorithm. Use the data 1522, Y, 66733, Y, 23452, y, 35323, y, 1234, n, 8572, Y

6 BLS/EYE/2019/4E/COMP/7155/01 x y out 0 0 0 15 1 0 41 1 34 2 29 3 [5] (ii) State the purpose of the algorithm. Assume only valid inputs are keyed in. ……………………………………………………………………………………………………. ………………………………………………………………………………………………….[2] To find out the sum of digits in an input and to count the number of sums that are divisible by 5. (b) (i) Explain a benefit of including input validation checks in a computer program. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. …………………………………………………………………………………………………[1] To ensure that the input is valid so that the program can function as intended. (ii) Write a validation check for the user input in the program above using pseudo code. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. ……………………………………………………………………………………………………. …………………………………………………………………………………………………[3] check = True #declare tracking variable

7 BLS/EYE/2019/4E/COMP/7155/01 for digit in num: #valid method to check input if digit.isdigit() == False: check = False print(“Invalid input”) #valid output to inform user of invalid input. Accept methods for valid validation checks: presence, format, type. 4 State the function and describe one difference for each of the following (a) (i) Data and address buses Function: …..……………………………………………………………………………………. ……………………………………………………………………………………………………. Difference: ………………………………………………………………………………………. ………………………………………………………………………………………………….[3] Buses are used to transfer information between different parts of a computer. [1] Data buses are bi-directional and they allow for information transfer between the processor and memory/storage.[1] Address buses are uni-directional and only allow information transfer from the processor to memory. [1] (ii) Memory and