Computer Science (9-1) 0984 | Paper 1 Mock 01 | Computer Systems

(a) The three number systems commonly used in computing are:

Binary uses only two digits because computer hardware operates on two electrical states (on/off). Denary is the everyday human counting system. Hexadecimal is a shorthand for binary: each hex digit maps to exactly four binary bits, making long binary strings easier to read.

(b) Converting 157 to binary using the place-value method:

157 = 128 + 16 + 8 + 4 + 1, so the binary representation is 10011101. [2]

(c) Converting 11010110 to hexadecimal:

Split the 8-bit binary number into two groups of 4 bits:

• 1101 = 8 + 4 + 0 + 1 = 13 = D
• 0110 = 0 + 4 + 2 + 0 = 6 = 6

Therefore 11010110 in hexadecimal is D6. [1]

(a) How an automated guided vehicle (AGV) navigates a warehouse:

1. The AGV follows predefined paths marked on the warehouse floor using magnetic strips, painted lines, or embedded wires that emit signals. [1]
2. Sensors on the underside of the AGV detect these markers, and a microprocessor continuously adjusts the drive motors' speed and direction to keep the vehicle precisely on the correct path. [1]
3. Alternatively, some modern AGVs use LIDAR sensors or cameras to create and store a digital map of the warehouse layout, navigating using stored route data and real-time position calculations rather than physical floor markings. [1]

(b) Two sensors for collision avoidance:

• Ultrasonic proximity sensor: Emits high-frequency sound waves and measures the time for the echo to return from nearby objects. [1] If the echo indicates an object (a person, another AGV, or fallen stock) is within a dangerously close distance, the AGV automatically slows down or stops completely to prevent a collision. [1]
• Bumper contact sensor: A pressure-sensitive strip mounted around the front edge of the AGV. [1] If it makes physical contact with an unexpected obstacle (as a last line of defence), the AGV immediately stops its motors and may reverse slightly or send a signal to the warehouse management system requesting human assistance. [1]

(a) An embedded system is a dedicated computer system built into a larger device to perform a specific, fixed function. [1] Unlike a general-purpose computer, it is designed for one task only. It typically contains a microprocessor, a small amount of memory and input/output interfaces, and runs a fixed program stored in ROM that does not change during normal operation. [1]

(b) Devices containing embedded systems include:

1. Washing machine: the embedded system controls the wash cycle by reading sensor inputs (water temperature, water level) and controlling outputs (motor speed, inlet valve, heater element) to follow the programme selected by the user. [2]
2. Microwave oven: the embedded system monitors the timer and power settings selected by the user, controls the magnetron and turntable motor, and stops heating when the set time has elapsed.
3. Car engine management system: the embedded system reads data from sensors (oxygen sensor, temperature sensor, throttle position) and adjusts fuel injection timing and air-fuel mixture for efficient engine performance.

(Any three valid device-plus-task pairs earn the marks.)

(a)

The pattern follows the formula: maximum value = \(2^n - 1\) and number of different values = \(2^n\), where n is the register size in bits. [6]

(b) The minimum register size is 16-bit. [1] An 8-bit register can store a maximum of 255, which is far less than 50000. A 16-bit register can store up to 65535, which is greater than 50000, so it is the smallest register from the table that can hold this value. [1]

(a) Explain what the black dots on the diagram represent. [2]

The black dots are sample points where the amplitude (loudness) of the analogue sound wave has been measured and recorded as a digital value. [1] Each dot captures the height of the wave at one specific moment in time. The analogue-to-digital converter (ADC) takes these measurements at regular intervals determined by the sampling rate. [1]

(b) Explain why the digitised version is not a perfect copy. [2]

The digital recording only captures the wave's amplitude at the discrete sample points, so any changes in the sound between samples are missed entirely. The faster the wave changes, the more detail is lost between samples. [1] Additionally, the amplitude value at each sample is rounded to the nearest level allowed by the bit depth (this is called quantisation). If the true amplitude falls between two available levels, the recorded value is an approximation, introducing a small error called quantisation noise. [1]

(c) Describe two ways the accuracy of the digital recording could be improved. [2]

Way 1: Increase the sampling rate (take more samples per second). This captures the wave at more frequent intervals, so less detail is lost between sample points. The digitised waveform follows the original shape much more closely. [1]

Way 2: Increase the bit depth (use more bits per sample). This provides more amplitude levels to choose from, so each sample value is a closer approximation to the true amplitude. The gap between consecutive levels becomes smaller, reducing quantisation error. [1]

(a) How a concept keyboard works:

Each key on the concept keyboard is a large button with an image or label representing a specific item or action (such as "Burger", "Large", or "Complete Order"). [1] When the operator presses a button, it sends a signal to the computer system to add the corresponding item to the order or perform the associated action. [1]

Unlike a standard keyboard where each key represents a single character, each key on a concept keyboard triggers a predefined command specific to the application it was designed for.

(b) Two advantages of using a concept keyboard in a fast-food restaurant:

1. Staff need very little training, as they only need to press clearly labelled picture buttons rather than learn a standard keyboard layout or memorise product codes. [1]
2. Orders can be taken more quickly, since each button press directly selects a product, reducing the number of steps compared to typing item names or codes. [1]

(c) Two disadvantages of using a concept keyboard:

1. The keyboard can only be used for the specific application it was designed for, unlike a standard keyboard which is general-purpose and can be used across many different tasks. [1]

(a) The completed table:

A microphone is an input device because it sends data into the computer. A projector is an output device because it receives data from the computer and presents it visually. A scanner is an input device because it captures external data (a printed page) and digitises it for the computer to use.

(a) A FOR loop repeats a block of code a predetermined number of times. It uses a counter variable that is automatically initialised, incremented and checked by the loop structure itself. [1] A WHILE loop repeats a block of code as long as a specified condition remains true. The number of repetitions is not known in advance; the loop continues until the condition becomes false. [1]

(b) A FOR loop is more appropriate when the number of iterations is known in advance. For example, printing exam results for exactly 30 students is best handled by a FOR loop. [1] The loop counter automatically counts from 1 to 30, making the code simpler and avoiding the need for a separate counter variable and manual condition check. [1]

(c) Selection (IF-THEN-ELSE) allows the program to choose between different paths of execution based on whether a condition evaluates to true or false. [1] For example: IF a student's mark is 50 or above THEN display "Pass" ELSE display "Fail". The condition is evaluated, and only one of the two branches executes. [1]

(d) Nested selection is when one IF statement is placed inside another, allowing the program to test multiple conditions in a structured sequence. [1] For example, a ticketing system might first check if the customer is under 16 (child), and if so, check whether they are under 5 (free ticket) or aged 5-15 (child-rate ticket). Each subsequent condition refines the decision further. [1]

(e) A WHILE loop checks its condition before the loop body executes. If the condition is initially false, the body never executes at all. [1] A REPEAT-UNTIL loop executes the body first and then checks the condition afterwards. This guarantees the body always executes at least once, regardless of the condition. [1]

(f)

REPEAT
    INPUT Password
UNTIL Password = "abc123"

The REPEAT-UNTIL structure is ideal here because the user must be asked for input at least once before the password can be checked. [1] The loop continues prompting until the correct password is entered. [1]

(g)

INPUT Mark
IF Mark >= 70 THEN
    OUTPUT "Distinction"
ELSE
    IF Mark >= 50 THEN
        OUTPUT "Merit"
    ELSE
        IF Mark >= 40 THEN
            OUTPUT "Pass"
        ELSE
            OUTPUT "Fail"
        ENDIF
    ENDIF
ENDIF

The nested IF structure tests conditions from highest to lowest. Once a condition is met, the corresponding grade is output and no further conditions are tested. [1] [1] [1] [1]

(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]