Computer Science 9618/12 October/November 2023: question paper and mark scheme

Mark scheme

1(a) 1 mark for: 1 • infra-red / proximity (sensor) 1(b) 1 mark for each bullet point (max 2) 2 • Actuator generates a signal / causes an action / converts electrical energy into a mechanical force • ... to push an arm // to open a trap door // to pick up the chocolate bar with the incorrect weight 1(c)(i) 1 mark for each feature (max 2) 2 • Dedicated to a single task // limited number of functions • Built into a larger system // integrated into a larger system • An embedded system must contain a processor, memory and an I/O capability // Dedicated hardware 1(c)(ii) 1 mark for each bullet point (max 1) 1 • Difficult to change / update the firmware by the user // Difficult to upgrade devices to take advantage of new technology • Cannot be easily adapted for another task • Troubleshooting faults/upgrading is a specialist task • Devices are often thrown away rather than repaired as difficult to upgrade or repair

Mark scheme

2(a) 1 mark for each term (max 3) 3 Entity: • An object about which data can be stored Primary key: • The unique attribute / combination of attributes used to identify the record / tuple Referential integrity: • Makes sure that if data is changed in one place the change is reflected in all related records - cascading update/delete • Makes sure that data that does not exist cannot be referenced • Ensures that every foreign key has a corresponding primary key // A logical dependency of a foreign key on a primary key • Ensures that the data in the database is consistent / up to date • Prevents records from being added/deleted/modified incorrectly • Makes sure any queries return accurate and complete results 2(b) 1 mark for each bullet point (max 4) 4 Max 2 if no descriptions • Backup / recovery procedures • … automatically takes copies of the database and store off site on a regular basis / weekly, etc. • ... so that the data can be recovered if lost • Use of access rights • … some users are given different access permissions to different tables • ... read/write, read only, full access, etc. • Views • … different users are able to see different parts of the database • ... only see what users need to see // by example • Record and table locking • … prevents simultaneous access to data • ... so updates are not lost // data is not overwritten • Encryption • ... the data is turned into ciphertext • ... so it cannot be understood without a decryption key 2(c) 1 mark for each bullet point (max 4) 4 • Identify repeating groups of attributes ... • ... Subject and SubjectCode • Ensure each field is atomic • ... StudentName should be split into e.g. FirstName and LastName • Identify the primary key for the table

Mark scheme

3(a) 1 mark for: 1 Either • kibibyte = 1,024 bytes // 210 bytes and megabyte = 1000 kilobytes //1 000 000 bytes // 103 kilobytes // 106 bytes Or • kibi is binary prefix and mega is denary prefix 3(b)(i) 1111 0011 1100 1 3(b)(ii) 865 1 3(b)(iii) 470 1 3(c) 1 mark for a correct application 2 1 mark for a corresponding justification • An application that performs financial / banking calculations • ... because it is difficult to represent decimal values exactly in normal binary and financial transactions use only two decimal places and must be accurate, no accumulating errors Or • Electronic displays, e.g. calculators, digital clocks • ... because visual displays only need to show individual digits • ... because conversion between denary and BCD is easier Or • The storage of the date and time in the BIOS of a PC • ... because conversion with denary is easier

Mark scheme

4(a) 1 mark for each bullet point 3 • A NAND B • NOT(B XOR C) • Final NAND X = A NAND B NAND (NOT(B XOR C)) 4(b) 1 mark for each set of rows (shaded) 2 A B C X 0 0 0 0 0 0 1 0 0 1 0 1 0 1 1 0 1 0 0 1 1 0 1 1 1 1 0 1 1 1 1 1

Mark scheme

5(a) 1 mark for: 1 Either • Ensuring data can only be accessed by / disclosed to authorised persons Or • Ensuring data cannot be accessed by / disclosed to unauthorised persons 5(b) 1 mark for each bullet point (max 1) 1 • Ensuring the accuracy / completeness / consistency of data (during / after processing) • Ensuring the data is up to date 5(c) 1 mark for each bullet point. 4 Phishing email (max 2) • The email pretends to be from an official body • ... persuading individuals to disclose private information // by example such as bank details • ... or requesting authentication by redirecting to an unofficial/unauthorised website // inviting a user to click a link Spyware (max 2) • Malware downloaded without the user’s knowledge • ... which secretly records the user’s actions / keystrokes on the computer • ... and sends logs of the actions to a third party

Mark scheme

6(a) 1 mark for each bullet point (max 3) 3 Lossy compression (ticked) • Loss of quality will not be noticed • Needs to be viewed in real time so less bandwidth needed if file size smaller • Smaller file sizes will reduce buffering so the video will play more smoothly • Viewers may watch on different devices, so may not need high quality resolution Lossless compression (ticked) • Original recording may not have been made in high resolution • Could be streaming to high bandwidth devices • The reduction in the file size is sufficient for the receiving device • Viewers do not want any loss of quality 6(b) 1 mark for each bullet point (max 3) 3 Increase sampling resolution • … the number of bits used for each sample is increased • … there will be more values available to represent each sample // more amplitudes can be represented • ... each binary amplitude/note in the digital recording is closer to the analogue amplitude/note • … quantisation errors are reduced • … the digital soundwave is closer to the original analogue soundwave Decrease sampling resolution • … the number of bits used for each sample is decreased • … there will be fewer values available to represent each sample // fewer amplitudes can be stored • ... each binary amplitude/note in the digital recording is further from the analogue amplitude/note • … quantisation errors are increased • … the digital soundwave is less like the original analogue soundwave 6(c) 1 mark for answer, 1 mark for working 2 Answer: 2.5 mebibytes Working: (2048 × 1024 × 10) / (8 × 1024 × 1024)

Mark scheme

7(a) 1 mark for each bullet point (max 2) 2 • Only has four groups of digits // IPv6 has eight groups • Uses dotted notation instead of colons • Because it is a 32 bit / 4 byte address // IPv6 is 128 bits / 16 bytes 7(b)(i) 1 mark for server only connected to router 2 1 mark for two laptop computers connected only to router 7(b)(ii) 1 mark for each bullet point (max 2) 2 • The data from the sending laptop is transmitted to the router • The data has address of recipient • The router determines recipient’s destination address • ... by using a routing table • The router transmits data directly / only to recipient 7(b)(iii) 1 mark for each use (max 2) and 4 1 mark for corresponding expansion (max 2) • To improve the security of the LAN • … so that devices do not receive unintended data • … so that a compromised device does not expose the whole network • ... so not all devices can access all segments • To make the network management easier • … because faults can be isolated more efficiently • ... by appropriate example • To make the network easier to expand // For better control of network growth • … by allowing for greater range of IP addresses to be available • To improve network performance • To reduce network congestion • … by localising network communications // by dividing data between segments • … so that devices are not flooded with data • … because data sent between devices on the same subnet stays within the subnet 7(c) 1 mark for each bullet point (max 3) 3 • To monitor the communications channel • To send data only when there is no data being transmitted / the line is quiet / idle • To detect a collision and then stop transmissions of further data // transmit a jamming signal • To calculate a random wait time / back-off time • ... then retransmit the data after that random wait time • Increase random time if multiple collisions 7(d) 1 mark for each highlighted area 4 Type of IP address Description an IP address that is assigned to a device public IP address to allow direct access on the internet an IP address that is fixed / doesn’t static IP address change each time a device re-joins a LAN / network an IP address used for internal LAN private IP address communication only an IP address that may be refreshed / dynamic IP address changed each time a device re-joins a LAN / network

Mark scheme

8(a) 1 mark for each bullet point (max 1) 1 • To hide the complexities of the hardware from the user • To provide a platform for software to run • To provide a user interface 8(b) 1 mark for the name of the utility software 3 2 marks for the explanation • Defragmentation software • … because over time saving and deleting of small files fragments the disk • ... the software makes (individual) files contiguous • … so access time to the files is improved • … because head movement is reduced • Disk contents analysis/disk repair software • … to identify and mark bad sectors • ... to restore corrupted files • ... to recover lost data (due to hardware failure) • File compression • … to reduce the size of files • ... which saves storage and memory space // by example • ... and reduces transmission time // by example • Disk formatter • … to prepare a disk for use // set up the file system • ... to partition the disc • ... to delete all the data from the disc 8(c)(i) 1 mark for each bullet point (max 1) 1 • USB / Universal Serial Bus • HDMI 8(c)(ii) 1 mark for each component (max 3) 3 Buffers • A buffer temporarily holds data until it is ready to be transmitted to the device Address Bus • The address of the data to be written to the device (in RAM) is carried on the address bus Data Bus • All data to be written to the device / buffer is carried on the data bus

Mark scheme

9(a) 1 mark for each bullet point 4 • IN - Input and output of data • ADD - Arithmetic operations • JPE - Unconditional and conditional instructions • CMI - Compare instructions 9(b) 1 mark for each set of shaded rows 4 Instruction Memory address Output ACC IX address 100 101 110 111 0 0 51 65 10 0 11 49 12 49 13 51 14 15 16 49 17 65 18 1 19 65 20 13 65 14 15 21 A 22