Computer Science 0478 | Paper 1 Mock 01 | Computer Systems

(a) A character set is a collection of characters where each character is assigned a unique binary code. [1] It defines the mapping between text characters (letters, digits, punctuation and symbols) and their binary representations, allowing computers to store, transmit and display text consistently. [1]

(b) The key difference is capacity: ASCII uses 7 bits per character, allowing a maximum of 128 different characters (covering the English alphabet, digits and basic symbols). Unicode can use up to 32 bits per character, supporting over a million unique characters. [1] This means Unicode can represent virtually every written language in the world, including Chinese, Arabic, Hindi and emoji, while ASCII is limited to basic Latin characters.

(a) The circuit passes input A through a NOT gate, then feeds the result (NOT A) and input B into an AND gate to produce output X. The truth table is completed by evaluating each row:

Row 1: NOT 0 = 1; 1 AND 0 = 0. [1]
Row 2: NOT 0 = 1; 1 AND 1 = 1. [1]
Row 3: NOT 1 = 0; 0 AND 0 = 0. [1]
Row 4: NOT 1 = 0; 0 AND 1 = 0. [1]

(b) X = (NOT A) AND B [1+1]

This reads: the output X is 1 only when A is 0 (NOT A = 1) and B is 1.

(c) A real-world example: a security light that turns on (X = 1) when it is dark outside (A = 0, meaning NOT A = 1) AND motion is detected (B = 1). During daytime (A = 1) or when there is no motion (B = 0), the light stays off. [2]

(d) Truth table for X = A OR (NOT B):

Row 1: NOT 0 = 1; 0 OR 1 = 1. [1]
Row 2: NOT 1 = 0; 0 OR 0 = 0. [1]
Row 3: NOT 0 = 1; 1 OR 1 = 1.
Row 4: NOT 1 = 0; 1 OR 0 = 1. [1]

(e) Two uses of logic gates in everyday electronic devices:

1. Burglar alarm systems use logic gates to combine signals from multiple sensors (door, window, motion) and determine whether to trigger the alarm. [1]
2. Calculator circuits use combinations of logic gates to perform arithmetic and logical operations on binary numbers. [1]

(a) The process of analogue-to-digital conversion for a sound recording:

1. A microphone converts sound waves in the air into a continuous analogue electrical signal whose voltage varies with the amplitude and frequency of the sound. [1]
2. The sampler measures the amplitude of this analogue signal at regular intervals determined by the sample rate (for example, 44,100 times per second at CD quality). [1]
3. Each measured amplitude value is rounded (quantised) to the nearest available level determined by the bit depth. [1]
4. The rounded values are encoded as binary numbers and stored as a sequence of digital data in a sound file. [1]

(b) Quantisation is the process of rounding each measured amplitude to the nearest available level, because there are only a finite number of levels determined by the bit depth (for example, 256 levels with 8-bit depth). [1] The difference between the actual amplitude and the nearest available level is called quantisation error. With a low bit depth, fewer levels are available, so the rounding error is larger, producing audible noise or distortion in the recording. [1]

(c) The reverse process (digital-to-analogue conversion):

1. A digital-to-analogue converter (DAC) reads the binary values from the digital file and converts each sample back into an analogue voltage level. [1]
2. The resulting analogue electrical signal is sent to a speaker, which vibrates a diaphragm to recreate sound waves that travel through the air and can be heard. [1]

(a) Firmware is software that is permanently (or semi-permanently) stored in hardware, typically in ROM or flash memory. It controls the fundamental low-level operations of a device and is not routinely changed by the user. [1] Regular software (application or system software) is stored on secondary storage, loaded into RAM when needed, and can be easily installed, updated, or removed by the user at any time. [1]

(b) The BIOS/UEFI in a computer is a common example of firmware. It runs every time the computer is switched on, before the operating system loads. [1]

(c) Firmware is stored in ROM (Read-Only Memory) because ROM is non-volatile, meaning it retains its contents when the power is turned off. [1] This is essential because the firmware must be available immediately when the device is switched on, before the operating system or any other software has loaded from secondary storage. If it were in RAM (which is volatile), it would be lost every time the power was cut. [1]

(d) During the boot process, the BIOS/UEFI performs the following role:

1. It carries out a Power-On Self-Test (POST) to check that essential hardware components (CPU, RAM, storage devices) are present and functioning correctly. [1]
2. It identifies the boot device (hard drive, SSD, USB, or network) that contains the operating system, following a pre-configured boot order. [1]
3. It loads the operating system's boot loader from the boot device into RAM and transfers control to it, which then continues loading the full operating system. [1]

(a) To convert denary 347 to hexadecimal, use repeated division by 16:

1. 347 / 16 = 21 remainder 11 (B in hex).
2. 21 / 16 = 1 remainder 5.
3. 1 / 16 = 0 remainder 1.

Read the remainders from bottom to top: 347 in hexadecimal = 15B. [3]

(b) Convert 15B hex back to denary to verify:

• 1 x 256 (16\(^2\)) = 256
• 5 x 16 (16\(^1\)) = 80
• B (11) x 1 (16\(^0\)) = 11

256 + 80 + 11 = 347. Verified. [2]

(c) A programmer would need this conversion when defining colours for web design (e.g. converting RGB values from denary to hex colour codes) or when reading hexadecimal memory addresses during debugging and needing to understand the denary value stored at that location. [1]

(d) A single byte can store values from 0 to 255. Since 347 > 255, it cannot fit in 1 byte. [1] Two bytes (16 bits) can store values from 0 to 65535, which is more than enough for 347. Therefore 2 bytes are needed. [1]

(a) When the motion sensor detects movement, the following sequence occurs:

1. The PIR (passive infrared) motion sensor detects a change in infrared radiation in its field of view, which happens when a warm body (a person or animal) moves through the monitored area. [1]
2. The sensor sends a signal to the microprocessor indicating that motion has been detected. [1]
3. The microprocessor processes this signal. If the security system is armed (i.e. in active monitoring mode), it simultaneously sends output signals to activate three responses: trigger the alarm siren to sound loudly, start the camera recording video of the area, and send an alert notification (e.g. text message or app notification) to the property owner or security monitoring service. [1]
4. These three responses happen simultaneously to maximise security effectiveness: the siren deters the intruder, the camera captures evidence, and the alert ensures someone is notified even if they are not on-site. [1]

(a)

(b) The best approach for most offices is a combination of both: wired connections for desktop computers and servers that need consistent speed and maximum security [1], and wireless access points for laptops, tablets and mobile devices so that staff can work flexibly in meeting rooms and open-plan areas. [1]

(a) Database term definitions: [3]

1. (i) Record: a record is a complete set of related data about one entity. In a database table, a record corresponds to one row. For example, one student's complete entry (ID, name, date of birth, year group) is one record. [1]
2. (ii) Field: a field is a single item of data within a record. In a database table, a field corresponds to one column. For example, "FirstName" is one field that stores just the student's first name. [1]
3. (iii) Primary key: a primary key is a field (or combination of fields) that uniquely identifies each record in a table. No two records can have the same primary key value, ensuring every record can be distinguished from all others. [1]

(b) The most suitable primary key is StudentID. [1] This is because it is guaranteed to be unique for every student. Other fields such as FirstName or DateOfBirth could be shared by multiple students (two students could have the same name or birthday), but each student is assigned a distinct StudentID. [1]

(c) Appropriate data types for each field: [4]

(a) Purpose of the ALU [2]

The ALU (Arithmetic Logic Unit) performs arithmetic operations such as addition, subtraction, multiplication, and division on numerical data. [1]

It also performs logical operations such as comparisons (AND, OR, NOT) and determining whether values are equal, greater than, or less than each other. These logical comparisons are essential for decision-making in programs (e.g. IF statements). [1]

(b) Purpose of the Control Unit [2]

The Control Unit (CU) controls and coordinates the activities of all other components within the CPU and the wider computer system. [1]

It fetches instructions from memory, decodes them to determine what operation is required, and sends the appropriate control signals to other components (ALU, registers, memory, I/O devices) to execute each instruction in the correct sequence. [1]

(c) Register purposes [4]

(i) Program Counter (PC): Holds the memory address of the next instruction to be fetched and executed. After each fetch, it is incremented (usually by 1) so it points to the following instruction. [1]

(ii) Memory Address Register (MAR): Holds the address of the memory location that is about to be read from or written to. This address is placed on the address bus. [1]

(iii) Memory Data Register (MDR): Holds the data that has been fetched from memory (during a read operation) or the data that is about to be written to memory (during a write operation). It acts as a buffer between the CPU and main memory. [1]

(iv) Accumulator (ACC): Holds the results of calculations and logical operations performed by the ALU. It is the primary working register for arithmetic and serves as a temporary store for intermediate results. [1]