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

(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 four steps of the fetch stage are:

1. Step 1: The address of the next instruction is copied from the Program Counter (PC) into the Memory Address Register (MAR). The PC holds the address; the MAR is the register that feeds the address bus. [1]
2. Step 2: The address now held in the MAR is placed onto the address bus and sent to main memory. Simultaneously, a read control signal is sent along the control bus, telling memory to provide the contents at that address. [1]
3. Step 3: The instruction stored at the specified memory address is fetched from memory and travels along the data bus into the Memory Data Register (MDR). The MDR now holds the instruction ready for decoding. [1]
4. Step 4: The Program Counter is incremented by 1 (PC = PC + 1), so it now points to the address of the next instruction to be fetched in the following cycle. [1]

(b) During the decode stage, the Control Unit takes the instruction from the MDR and interprets (decodes) it. [1] It identifies the type of operation to be performed (e.g. arithmetic, data transfer, comparison) and determines which data or memory addresses are involved, then prepares the appropriate control signals to carry out the instruction during the execute stage. [1]

(c) Two examples of instructions that may be executed:

1. The ALU performs an arithmetic calculation, such as adding two numbers together, and stores the result in the Accumulator (ACC). [1]
2. Data held in the MDR is written to a specified memory address via the data bus, storing a result or value back into main memory. [1]

(d) The three system buses play distinct roles during the cycle:

• Address bus: carries the memory address from the MAR to main memory, specifying which location to read from or write to. [1]
• Data bus: carries the actual data or instructions between the CPU and memory in both directions (bidirectional). [1]
• Control bus: carries control signals such as read, write and clock signals that coordinate the operations of the CPU, memory and I/O devices. [1]

(a) Converting each denary number to 8-bit two's complement binary:

45:

45 = 32 + 8 + 4 + 1 = 00101101. [2]

-45:

1. Start with the positive representation: 00101101
2. Flip all bits (one's complement): 11010010
3. Add 1: 11010010 + 1 = 11010011

The result is 11010011. [2] This "flip and add 1" method is the standard algorithm for finding the two's complement (negative) of a binary number.

-100:

1. First convert 100 to binary: 100 = 64 + 32 + 4 = 01100100
2. Flip all bits: 10011011
3. Add 1: 10011011 + 1 = 10011100

The result is 10011100. [2]

In two's complement, the most significant bit (MSB) indicates the sign: 0 for positive, 1 for negative. The range of an 8-bit two's complement number is -128 to +127.

(a) Binary addition of 01101011 + 00110101:

Work from right to left, adding corresponding bits plus any carry from the previous column.

  Carry:  0 1 1 1 0 1 1 0
          0 1 1 0 1 0 1 1
        + 0 0 1 1 0 1 0 1
        ------------------
          1 0 1 0 0 0 0 0

Detailed column work (right to left):

1. Column 1 (units): 1 + 1 = 10. Write 0, carry 1.
2. Column 2: 1 + 0 + 1 (carry) = 10. Write 0, carry 1.
3. Column 3: 0 + 1 + 1 (carry) = 10. Write 0, carry 1.
4. Column 4: 1 + 0 + 1 (carry) = 10. Write 0, carry 1.
5. Column 5: 0 + 1 + 0 (carry) = 1. Write 1, carry 0.
6. Column 6: 1 + 1 + 0 = 10. Write 0, carry 1.
7. Column 7: 1 + 0 + 1 (carry) = 10. Write 0, carry 1.
8. Column 8: 0 + 0 + 1 (carry) = 1. Write 1, carry 0.

Result: 10100000. [3]

(b) Overflow occurs when the result of an arithmetic operation exceeds the maximum value that can be stored in the available number of bits. [1] In an 8-bit register, the largest unsigned value is 255 (11111111). If a calculation produces a value above 255, there is a carry out of the most significant bit position, meaning the result cannot fit in 8 bits. [1]

(c) Overflow has not occurred. [1] The result 10100000 fits entirely within 8 bits, and there was no carry generated beyond the 8th bit position during the addition. Both operands were within the 8-bit range, and their sum (107 + 53 = 160) is still within the unsigned 8-bit range of 0 to 255. [1]

(d) 11111111 in denary: 128 + 64 + 32 + 16 + 8 + 4 + 2 + 1 = 255. [1]

(e) The maximum value in a 16-bit unsigned register is calculated as \(2^{16} - 1\). [1] This equals 65535. [1] The formula works because 16 bits can represent \(2^{16}\) = 65536 different values (from 0 to 65535), so the largest is 65535.

(a) The automated central heating system maintains a set temperature through a continuous feedback cycle involving three components:

1. A temperature sensor continuously measures the current temperature of the room and sends this reading as a signal to the microprocessor. [1]
2. The sensor sends the temperature reading as a digital signal (after analogue-to-digital conversion if the sensor is analogue) to the microprocessor. [1]
3. The microprocessor compares the current temperature reading with the set point (the desired temperature chosen by the user). [1]
4. If the current temperature is below the set point, the microprocessor sends a control signal to the actuator to switch on the boiler/heater, producing heat to raise the room temperature. [1]
5. If the current temperature is at or above the set point, the microprocessor sends a signal to the actuator to switch off the boiler/heater, stopping unnecessary heating. [1]
6. This process repeats continuously, with the sensor providing updated readings to the microprocessor so the system can react to changing conditions (e.g. a window being opened). [1]

(b) This is described as a feedback loop because the output of the system (the heater turning on or off) directly affects the condition being measured (the room temperature). [1] The sensor then feeds this changed condition back to the microprocessor, which adjusts its output accordingly. This creates a continuous cycle of monitoring and adjustment - the system's own actions change its input, which in turn changes its next action. [1]

(c) Two advantages of an automated system over manual control:

• The system maintains a constant, comfortable temperature without the user needing to intervene - it reacts faster than a person could and works even when no one is home. [1]
• It saves energy by automatically turning off the heating the moment the desired temperature is reached, rather than overshooting because a person forgot to turn it off. [1]

(d) If the temperature sensor failed, the microprocessor would stop receiving accurate temperature readings and could not compare the room temperature with the set point. [1] The feedback loop would be broken: the heater might remain permanently on (causing the room to overheat, wasting energy and potentially creating a fire hazard) or permanently off (leaving the room cold), because the system has no way to know when to switch states. [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) Network hardware device functions:

(a)

(b) Logic errors are typically the hardest to find. [1] The program runs without any error messages, so the programmer receives no automatic indication of where the problem is. They must trace through the code manually or use debugging tools to identify the exact point where the output deviates from the expected behaviour. [1]

(a)

Red = 255, Green = 0, Blue = 0. [1] This pixel is pure red, because the red channel is at maximum intensity while both green and blue are at zero. [1]

(b) A white pixel has all three channels at maximum intensity:

• Red = 11111111 (255), Green = 11111111 (255), Blue = 11111111 (255) [2]

White is the combination of all three colour channels at full intensity.

(c) Red = 0, Green = 255, Blue = 0 represents pure green, because only the green channel is at maximum while the other two are off. [1]

(d) 24-bit colour can produce over 16 million distinct colours, which closely matches the number of different colours the human eye can perceive. For most display purposes, this is more than sufficient for a photorealistic image. [1]

(e) The number of different colours representable with 24-bit colour depth:

\( 2^{24} = 16\,777\,216 \) different colours. [1]

(f) 24-bit colour requires three times as much storage per pixel as 8-bit colour (24 bits versus 8 bits), resulting in significantly larger file sizes for the same image resolution. [1]