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WJEC A-Level Computer Science – Unit 4
Practice Examination
Total questions: 150 Question type: Multiple choice Topics: Computer architecture, assembly language, data transmission, data representation and data types, organisation and structure of data, databases and distributed systems, operating systems, software systems, data security and integrity processes, networking, network security, and web design (HTML/CSS/JavaScript).
Questions with Answers and Explanations
- The fetch–execute cycle consists of which three stages? A) Fetch, Encode, Store B) Fetch, Decode, Execute C) Fetch, Decode, Store D) Read, Decode, Execute Answer: B Explanation: The three stages of the cycle are fetch (retrieve instruction from memory), decode (interpret the instruction), and execute (carry out the instruction).
- During the fetch stage, which register holds the address of the next instruction to be fetched? A) Memory Data Register (MDR) B) Current Instruction Register (CIR) C) Program Counter (PC) D) Accumulator (ACC) Answer: C Explanation: The Program Counter (PC) keeps the address of the next instruction to be executed. Its contents are copied to the MAR during fetch.
- After an instruction is fetched from memory, it is temporarily stored in which register before decoding?
A) Memory Address Register (MAR) B) Accumulator (ACC) C) Current Instruction Register (CIR) D) Program Counter (PC) Answer: C Explanation: The contents of the MDR are copied to the CIR to ensure the current instruction is kept safe while the MDR is used during the execute stage.
- What is the role of the Control Unit (CU) during the decode stage? A) To perform arithmetic calculations B) To fetch the next instruction from memory C) To decode the instruction in the CIR by splitting it into operand and opcode D) To store the result of the operation in the accumulator Answer: C Explanation: The CU decodes the instruction in the CIR, splitting it into operand and opcode to determine what type of instruction needs to be carried out.
- What is meant by the term ‘opcode’ in an instruction? A) The data to be processed B) The address of the next instruction C) The operation to be performed by the processor D) The memory location of the data Answer: C Explanation: Opcode stores the instruction that needs to be carried out by the processor. It specifies what operation (e.g., add, subtract, load) needs to be done with the data.
- What is the role of the Accumulator (ACC) in the execute stage? A) It holds the address of the next instruction B) It temporarily stores the result of arithmetic/logic operations C) It decodes the current instruction D) It stores the data fetched from memory Answer: B Explanation: During the execute stage, the result of the instruction is stored in the accumulator, a general-purpose register.
- What is pipelining in processor design? A) Storing multiple instructions in a single register
C) 16,777,
D) 4,294,967,
Answer: C Explanation: With 24 bits per pixel (8 bits each for red, green, blue), the number of possible colours is 2²⁴ = 16,777,216.
- A bitmap image has dimensions 1920 × 1080 pixels with a colour depth of 32 bits. What is the approximate file size in MB? A) 6.6 MB B) 7.9 MB C) 8.3 MB D) 66 MB Answer: B Explanation: 1920×1080 = 2,073,600 pixels × 32 bits = 66,355, bits ÷ 8 = 8,294,400 bytes ≈ 7.9 MB.
- Which form of image file uses lossless compression and supports transparency? A) JPEG B) GIF C) PNG D) BMP Answer: C Explanation: PNG (Portable Network Graphics) uses lossless compression and supports alpha transparency. JPEG is lossy, GIF uses lossless but is limited to 256 colours.
- What is the binary representation of the decimal number 25? A) 11001 B) 10011 C) 10101 D) 11100 Answer: A Explanation: 25 in binary is 16 + 8 + 1 = 11001₂.
- Convert the hexadecimal value 3F to decimal. A) 31 B) 39 C) 61 D) 63 Answer: D Explanation: 3F₁₆ = (3 × 16) + 15 = 48 + 15 = 63.
- Convert the binary number 10110110 to hexadecimal. A) 96 B) B C) 6B D) D Answer: B Explanation: 1011 = B, 0110 = 6, so 10110110₂ = B6₁₆.
- What is two’s complement representation used for? A) Representing floating-point numbers B) Representing positive integers only C) Representing signed integers D) Encrypting data in memory Answer: C Explanation: Two’s complement is the standard method for representing signed integers (both positive and negative) in computer systems.
- What is the decimal value of the 8-bit two’s complement binary number 11111101? A) – B) – C) 253 D) 125 Answer: B Explanation: The most significant bit is 1 (negative). Invert bits: 00000010, add 1: 00000011 = 3, so value = –3.
- What is the maximum value that can be stored in an 8-bit two’s complement integer? A) 127 B) 128 C) 255 D) 256 Answer: A Explanation: With 8 bits in two’s complement, the range is –128 to +127. The maximum positive value is 2⁷ – 1 = 127.
- A single processor can only execute … at a time. A) multiple instructions B) one instruction
D) 8 GB
Answer: C Explanation: 2³² addresses × 1 byte per address = 4,294,967, bytes = 4 GB.
- What is the purpose of the Memory Address Register (MAR) in the fetch stage? A) To store the data fetched from memory B) To hold the address of the next instruction to be fetched C) To decode the current instruction D) To store the result of an arithmetic operation Answer: B Explanation: The MAR holds the address of the next instruction to be executed. During fetch, the PC address is copied to the MAR.
- During the execute stage of an arithmetic instruction, where are any required data fetched from? A) The Accumulator only B) The Program Counter C) Main memory D) The Control Unit Answer: C Explanation: For an arithmetic instruction, any required data are fetched from main memory before the calculation is executed by the ALU.
- What is the primary role of the Arithmetic Logic Unit (ALU)? A) To fetch instructions from memory B) To decode instructions C) To perform arithmetic and logical operations D) To manage input/output devices Answer: C Explanation: The ALU performs all arithmetic (addition, subtraction, etc.) and logical (AND, OR, NOT, etc.) operations required by instructions.
- What is the significance of pipelining in terms of the fetch– execute cycle? A) It slows down the cycle to improve reliability B) It allows multiple instructions to be processed at different stages simultaneously C) It removes the need for the decode stage
D) It stores the entire program in the accumulator Answer: B Explanation: Pipelining allows multiple instructions to be overlapped in execution, with different instructions at different stages (fetch, decode, execute) simultaneously.
- Which of the following is a type of assembly language instruction? A) SELECT * FROM table B) MOV AL, 42 C) class MyClass D) Answer: B Explanation: MOV is a common assembly language mnemonic that moves data between registers or memory locations. SELECT is SQL, class is object-oriented, is HTML.
- What is meant by the term ‘low-level programming language’? A) A language that is close to machine code and specific to processor architecture B) A language that is easy for humans to read but slow to execute C) A language designed for database queries D) A language that runs in a web browser Answer: A Explanation: Low-level languages (such as assembly language) are close to machine code and are specific to a particular processor architecture. They offer fine control over hardware but are more difficult for humans to read and write.
- What is an assembler? A) A program that converts high-level language code into machine code B) A program that converts assembly language into machine code C) A program that executes source code line by line D) A program that links multiple object files together Answer: B Explanation: An assembler translates assembly language (mnemonic instructions) into machine code (binary) that the processor can execute directly.
- Which of the following is a valid assembly language instruction format?
C) AX (Accumulator eXtended) D) MAR (Memory Address Register) Answer: C Explanation: AX (Accumulator eXtended) is a general-purpose register in x86 architecture. PC, IR, and MAR are special-purpose registers.
- What is the purpose of the instruction ‘CMP’ (compare) in assembly language? A) To move data between memory locations B) To compare two operands and set status flags C) To jump to a different part of the program D) To call a subroutine Answer: B Explanation: The CMP instruction subtracts one operand from another without storing the result, but updates the processor’s status flags (e.g., zero, carry, overflow) which can be used for conditional jumps.
- What does the instruction ‘JNZ’ (Jump if Not Zero) do? A) It always jumps to a specified memory address B) It jumps to a specified address if the zero flag is not set (i.e., result not zero) C) It calls a subroutine if the result is zero D) It compares two values and jumps if equal Answer: B Explanation: JNZ is a conditional jump instruction that jumps to a target address if the zero flag is not set (meaning the previous comparison or operation produced a non-zero result).
- Which of the following is an example of a valid assembly language command? A) PRINT "Hello" B) SUB CX, 5 C) FOR I = 1 TO 10 D) SELECT * FROM students Answer: B Explanation: SUB CX, 5 is a valid assembly instruction that subtracts 5 from the CX register. PRINT, FOR, and SELECT are from higher-level languages.
- What is a data packet in computer networking? A) A single bit of data transmitted over a network
B) A formatted unit of data consisting of a header, payload, and trailer C) The physical cable used to connect devices D) The software used to manage network traffic Answer: B Explanation: A data packet is a formatted unit of data containing a header (with addressing information), a payload (the actual data), and sometimes a trailer (error detection).
- Why is data broken into packets before transmission? A) To make the data more secure B) To allow efficient transmission and error recovery if a packet is lost C) To reduce the number of devices on the network D) To increase the bandwidth available Answer: B Explanation: Splitting data into packets makes transmission more efficient, allows multiple packets to be sent simultaneously over different routes, and if a packet is lost, only that packet needs to be resent rather than the entire file.
- Which protocol is responsible for ensuring reliable, error-checked delivery of packets between applications? A) IP (Internet Protocol) B) HTTP (Hypertext Transfer Protocol) C) TCP (Transmission Control Protocol) D) UDP (User Datagram Protocol) Answer: C Explanation: TCP provides reliable, connection-oriented delivery with error checking, sequencing, and retransmission of lost packets. IP handles addressing and routing.
- What is the role of the TCP/IP model’s Application Layer? A) To route data across networks B) To interact directly with network applications (e.g., web browsers, email clients) C) To ensure error-free delivery between devices D) To handle the physical transmission of bits Answer: B Explanation: The Application Layer is where users interact with the network using applications such as web browsers and email clients.
- Which layer of the TCP/IP model is responsible for ensuring end-to-end communication between devices, including flow control and error checking?
- What is the function of a router in a network? A) To connect devices within the same local network only B) To forward data packets between different networks based on IP addresses C) To provide power to networked devices D) To store data for long-term access Answer: B Explanation: Routers operate at the Internet Layer, forwarding packets between different networks based on IP addresses.
- What is meant by packet switching? A) Dividing data into packets that are sent independently and reassembled at the destination B) Switching packets from one application to another on the same device C) Encrypting each packet individually D) Combining multiple packets into a single transmission Answer: A Explanation: Packet switching is a method where data is broken into packets, each of which may take different routes to the destination and is reassembled there.
- What is the role of the Hypertext Transfer Protocol (HTTP)? A) To transfer files between computers B) To send email messages C) To transport HTML documents, images, and other resources over the internet D) To secure network communications through encryption Answer: C Explanation: HTTP is used for transferring hypertext (web pages) and other resources over the internet.
- Which protocol adds a layer of security to HTTP by using encryption (TLS/SSL)? A) SMTP B) FTP C) HTTPS D) POP Answer: C Explanation: HTTPS (HTTP Secure) uses TLS/SSL encryption to secure data transmitted between a web browser and a web server.
- Which protocol is used for sending email from a client to an email server? A) POP B) IMAP C) SMTP D) FTP Answer: C Explanation: SMTP (Simple Mail Transfer Protocol) is used for sending email across networks.
- Which email protocol is used for retrieving emails from a server and typically stores emails on the server after downloading a copy to the client? A) POP B) SMTP C) HTTP D) FTP Answer: A Explanation: POP3 (Post Office Protocol version 3) downloads emails to the client and normally deletes them from the server, though some configurations keep a copy.
- Which email protocol allows users to manage emails stored on the server without downloading them, supporting server-based folders? A) POP B) IMAP C) SMTP D) DNS Answer: B Explanation: IMAP (Internet Message Access Protocol) keeps emails on the server, allowing users to access and manage them from multiple devices.
- What is FTP (File Transfer Protocol) primarily used for? A) Sending email B) Transferring files between a client and a server C) Browsing web pages D) Resolving domain names to IP addresses Answer: B Explanation: FTP is used for transferring files between a client and a server over a network.
D) All of the above Answer: D Explanation: Star, bus, and mesh are all examples of network topologies describing how devices are interconnected.
- In a star network topology, how are devices connected? A) Each device is connected to a central hub or switch B) Each device is connected to every other device C) Devices are connected along a single backbone cable D) Devices are connected in a closed loop Answer: A Explanation: In a star topology, all devices are connected directly to a central hub, switch, or router.
- What is a key disadvantage of a bus network topology? A) It is expensive to set up B) If the central hub fails, the entire network fails C) If the main cable fails, the entire network fails D) It requires more cable than other topologies Answer: C Explanation: In a bus topology, devices are connected along a single backbone cable. If the main cable breaks, the entire network fails.
- In which network topology are devices connected in a closed loop where each device is connected to exactly two others? A) Star topology B) Bus topology C) Mesh topology D) Ring topology Answer: D Explanation: In a ring topology, each device is connected to exactly two others, forming a closed loop.
- What is a key advantage of a fully connected mesh network topology? A) Low cost B) High redundancy and reliability – multiple paths between devices C) Easy to set up and manage D) Requires very little cabling Answer: B Explanation: In a fully connected mesh topology, each device is
connected to every other device, providing multiple paths for data and high fault tolerance.
- Which network protocol is designed for wireless local area networks (WLANs)? A) Ethernet B) TCP C) Wi-Fi (IEEE 802.11) D) Bluetooth Answer: C Explanation: Wi-Fi (IEEE 802.11 standards) is the family of protocols designed for wireless local area networks.
- What is a relational database? A) A database that organises data into tables with relationships between them B) A database that stores data in a single unstructured file C) A database that only stores numerical data D) A database that does not allow duplicate records Answer: A Explanation: A relational database organises data into one or more tables (relations) that are connected by relationships.
- In a database table, what do rows and columns represent? A) Rows represent fields; columns represent records B) Rows represent records (tuples); columns represent fields (attributes) C) Both represent primary keys D) Rows represent database indexes; columns represent queries Answer: B Explanation: In a relational database table (relation), rows are records (tuples) and columns are fields (attributes).
- What is a primary key in a database table? A) A key that can be null B) A unique identifier for each record in a table C) A key used to encrypt data in the database D) A key that references another table Answer: B Explanation: A primary key is a unique identifier for each record in a table. Every table in a relational database has a primary key.
A) INSERT INTO
B) SELECT
C) UPDATE
D) CREATE TABLE
Answer: A Explanation: INSERT INTO adds new rows (records) to a table.
- How would you modify existing records in a table using SQL? A) SELECT B) INSERT C) UPDATE D) ALTER Answer: C Explanation: UPDATE is used to modify existing records in a table.
- To delete records from a table, which SQL keyword would you use? A) REMOVE B) DELETE C) DROP D) ERASE Answer: B Explanation: DELETE removes rows from a table based on a condition.
- What is a composite key in a database? A) A key that consists of multiple columns to uniquely identify a record B) A key that is encrypted for security C) A key that references multiple tables D) A key that is composed of numbers and letters Answer: A Explanation: A composite key is a primary key composed of multiple columns (fields) used to identify a unique record.
- Which of the following is an advantage of using a distributed system? A) Data is stored in a single location B) Scalability – the system can be expanded by adding more machines C) Reduced complexity compared to centralised systems D) Guaranteed zero latency Answer: B Explanation: Advantages of distributed systems include scalability
(adding more machines), reliability (if one fails, others continue), and resource sharing.
- What is a disadvantage of distributed systems? A) All data is stored in one place, risking single point of failure B) Complexity in ensuring correct communication between components C) Inability to share resources across the network D) Lack of any security measures Answer: B Explanation: Disadvantages include complexity, security issues due to open network communication, and performance issues due to network latency.
- What is the role of a Database Management System (DBMS)? A) To physically store data on disk B) To interact with the database for creating, retrieving, updating, and managing data C) To design web pages D) To compile source code Answer: B Explanation: A DBMS is software that interacts with the database for creating, retrieving, updating, and managing data.
- What is a data warehouse? A) A single location where all data is stored in raw format B) An integration of diverse data from various sources to make it accessible for querying and analysis C) A backup copy of a database stored offsite D) A real-time transaction processing system Answer: B Explanation: Data warehousing integrates diverse data from a variety of sources to make it accessible for querying and analysis.
- What is ‘Big Data’? A) Data that is stored on a large hard drive B) Data sets that are too large or complex for traditional data-processing software to handle C) Data that has been compressed for storage D) Data that contains only numerical values Answer: B
