Computer Science 9618/11 October/November 2024: question paper and mark scheme

Mark scheme

1(a) 1 mark for: 1 • A tebibyte = 1024 gibibytes / 1 048 576 kibibytes / 240 bytes whereas a gigabyte = 1000 megabytes / 1 000 000 kilobytes / 109 bytes • Tebi is binary prefix giga is denary prefix 1(b)(i) 1 mark for: 1 C67 1(b)(ii) 1 mark for: 1 −1641 1(b)(iii) 1 mark for: 1 573 1(c) 1 mark for working 1 mark for answer. 2 Working Using the two’s complement and adding: 23 in binary = 00010111 so minus 23 is 11101001 0100 1010 + 1110 1001 (1)0011 0011 Direct subtraction: 0100 1010 – 0001 0111 0011 0011 Answer: 0011 0011 1(d) 1 mark from: 1 • The answer cannot be represented in the number of bits available • The answer is larger than the maximum positive number that can be stored in the register • The answer is smaller than the most negative number that can be stored in the register

Mark scheme

2(a) 1 mark for limitation and 2 marks for corresponding explanation 3 e.g. Data redundancy // data duplication • Separate linked tables are used • Data items are stored once so reduces data duplication / data redundancy Data inconsistency / Poor data integrity • Data changed once in one place will automatically update elsewhere • Linked data cannot be entered differently in two tables • Referential integrity can be enforced The data structure used depends on the application • Changes to the data structure are managed by the DBMS • … and queries are not dependent on the structure of the data • … and changes to the data do not require programs to be re-written 2(b)(i) 1 mark for each correct relation 3 2(b)(ii) 1 mark for each bullet point 5 • Create table REPAIR_PART with opening and closing brackets, all statements inside the brackets • PartID and RepairNumber as varchar (or equivalent) • Quantity as integer • At least 1 appropriate constraint • Dual primary key For example: CREATE TABLE REPAIR_PART( PartID VARCHAR(20) NOT NULL, RepairNumber VARCHAR(4) NOT NULL, Quantity INT NOT NULL, PRIMARY KEY (PartID, RepairNumber) ); 2(b)(iii) 1 mark for each bullet point 3 • Select SUM of AmountDue • From the correct table and one correct condition • Second correct condition SELECT SUM(AmountDue) FROM INVOICE WHERE SupplierID = "JK675" AND Paid = FALSE; 2(c) 1 mark for each correct definition 3 Database Term Definition All duplicate entries of data between tables are Referential Integrity consistent // all foreign keys are matched to an appropriate primary key A field that could be a primary key but is not // an Candidate Key attribute or smallest set of attributes in a table where no tuple has the same value A row / record in a table // one instance of an entity in Tuple a table

Mark scheme

3(a) 1 mark for system clock and 1 mark for Control Unit 2 System clock • To synchronise operations • ... by creating and transmitting timing signals on the control bus Control Unit • Sends/receives control signals along control bus • Reads an instruction from the contents of the memory location whose address is stored in PC • Coordinates/synchronises the activity of other components in the CPU • Manages the execution of instructions • Controls communication between the components in the CPU 3(b)(i) 1 mark for the feature, 1 mark max for the matching reason 2 e.g. Feature: clock speed • Higher clock speed means that more F-E cycles are executed per second // Higher clock speed results in more throughput Feature: bus width • Larger bus width means that more data transferred at the same time 3(b)(ii) 1 mark for correct port. 2 marks for explanation 3 Port: USB / Universal Serial Bus Explanation: • A voltage change occurs when the drive is plugged in • The computer detects this voltage change • The code of the device is transferred to computer • … the OS finds the code of the device in the list of devices • … and loads the appropriate device driver 3(c) 1 mark for each bullet point (max 2) 2 e.g. • DRAM requires constant refresh cycles unlike SRAM • DRAM has lower access speed than SRAM 3(d)(i) 1 mark for each bullet point (max 4) 4 • The disc is spun at high speed • A laser is shone onto the disc to read / write • … using optical head to move it into position • … it follows the spiral track from the centre outwards • When writing the laser burns pits to represent the data • When reading the laser reflects from pits and lands • The reflection from a pit and a land is different • ... the differences are interpreted as 1 or 0 3(d)(ii) 1 mark for each bullet point (max 3) 3 • The computer and optical disc reader/writer send and receive at different speeds • A buffer allows temporary storage of the data • … so that the computer can transfer data to the buffer at the higher speed • ... and is not held up waiting for data to transfer // so the computer can carry on with other tasks • ... so the optical disc reader/writer is not overloaded • … and so that data is transferred to optical disc reader/writer from the buffer at the slower rate

Mark scheme

4(a)(i) 1 mark for each bullet point (max 2) 2 e.g. • Secondary storage space is divided into file allocation units • Space on secondary storage is allocated to particular files • Maintains/creates directory structures • Specifies the logical method of file storage e.g. FAT or NTFS • Provides file naming conventions • Controls user access // implements access rights //implements password protection // Makes file sharing possible • Controls access to the file by other software • Specifies tasks that can be performed on a file e.g. open, close, delete, copy, create, move, etc • Allows searching for a file 4(a)(ii) 1 mark for each bullet point (max 2) 2 • To allow data to be retrieved / restored when lost • To automatically make a duplicate copy of data… • …so the user does not have to remember to backup data • To make regular duplicate copies of data 4(b)(i) 1 mark for each bullet point (max 3) 3 e.g. • Less of the teacher’s storage space is used • ... so more student work can be stored • Transmission time is reduced • … so the teacher does not have to wait as long for it to arrive • Bandwidth usage is reduced • … so other transmissions are not adversely affected • Less data allowance is used on the teacher’s email system 4(b)(ii) 1 mark for each bullet point (max 3) 3 • Run-Length Encoding // RLE • Repeated sequences of the same characters are replaced by • ... a single copy of the character • ... and a count of the number of characters 4(c) 1 mark for each bullet point (max 3) 3 e.g. • It saves development time for the student • … by using pre-written code • There is less testing to do // the routines are more likely to work correctly • … because they have already been tested and are error-free • It results in a more robust program • Subroutines can be used that they do not have the skill to write • … which enables a more complex program to be written 4(d)(i) 1 mark for correct name, 1 mark for matching description 2 • Expand/collapse code blocks… • … sections of source code that are part of the same block can be expanded to see the content or collapsed so that the overall code is seen • Auto-indentation // auto-formatting... • ... automatically indents/formats code as the user types so that the structure is clear // aids readability 4(d)(ii) 1 mark for correct name, 1 mark for matching description 2 • Breakpoints … • … stops the code running on a set line to view the current status/variable contents/program flow • Report window // variable watch window … • … shows the values in variables/data structures and how they change when each line is run

Mark scheme

5(a) 1 mark for all 3 appropriate sensors 1 Scenario Type of sensor A door is open. Pressure sensor // Infra-red The external daylight-level is below a set Light sensor amount. A person is detected within 2 metres. Infra-red 5(b) 1 mark for each correct expression 2 X = E AND A AND C Y = E AND (B OR C OR D) 5(c) 1 mark for each bullet point (max 3) for a correct justification 3 No marks for the identification of the system Monitoring system • There is no use of feedback // The light and the alarm are just warnings • The output of the floodlight or the audio alarm does not affect the input of data from the sensors • The system does not have any actuators

Mark scheme

6 1 mark for each bullet point (max 4) 4 e.g. • It uses image recognition • The pixels of each image from the camera are stored • … and matched to the expected shape/size/colour of registration number • It uses optical character recognition • The pixels within the registration number area identified are analysed • … and compared to expected characters/letters/numbers

Mark scheme

7(a) 1 mark for each bullet point (max 2) 2 • The user can test/trial the program before purchasing to see whether it meets needs • There is no initial cost • Users who try the software may spread the word so the developer gets more custom • The developer can get feedback to improve the program 7(b) 1 mark for each bullet point (max 2) 2 • The owner/developer can earn a fee • Comprehensive support is usually offered so problems can be easily solved • The software is usually well tested so few bugs

Mark scheme

8(a) 1 mark for each bullet point (max 2) 2 • To store the value of flags/bits • … that can be changed/set/cleared after arithmetic / logical operations • To allow flags to be checked • … to change instruction sequence 8(b) 1 mark for each bullet point (max 2) 2 • Special purpose registers have a specified role in the machine whereas general purpose registers can be used for all purposes defined by the programmer • Special purpose registers hold the state of the program’s execution while general purpose registers hold the program’s data during operations