AQA A-Level Computer Science Paper 2, June 2025: Question 14

✅ Correct Purpose (1 Mark)

Any of the following answers achieves full credit:

• Keep going forward / wait / loop until a touch sensor (or at least one of sensors X, Y, or Z) is activated.
• Keep going forward / wait / loop while no touch sensors are activated.
• Check if at least one of touch sensors X, Y, or Z is activated.

📐 Bit Tracing

1. Operand is decimal #7 = binary 00000111₂ .
2. Performing AND R1, R0, #7 masks out bits B₇–B₃, isolating only bits B₂ (X), B₁ (Y), and B₀ (Z).
3. If none of these sensors are active, the result in R1 is 0.
4. BEQ loop1 branches back while R1 == 0 . Once any touch sensor reads 1, the branch fails and execution continues.

🧠 Exam Technique

Remember that Section 1 already turned the motors on to drive forward. Because this section does not change the outputs, the physical robot continues doing what it was doing—driving forward—while waiting for the input.

❌ Common Errors

• Literal description: Writing "Performs an AND with 7 and branches if zero" scores 0 marks. Examiners demand the physical effect on the robot.
• Omission of sensors: Saying "Wait until a sensor is pressed" without specifying touch sensor or X, Y, Z is insufficient.

✅ Correct Purpose (1 Mark)

• Stop the robot moving (forward).
• Stop both wheel motors / turn off left and right motors.

📐 Bit Tracing

1. Decimal #63 = binary 00111111₂ .
2. Bitwise AND with 00111111₂ forces bits B₇ and B₆ to 0 .
3. Bits B₅ through B₀ remain completely unchanged.
4. Because B₇ (left wheel) and B₆ (right wheel) are set to 0, both drive motors stop.

💡 Key Knowledge: Bit Clearing Mask

To turn off specific bits while preserving all other bits, apply a bitwise AND using a mask that contains 0 at the target bit positions and 1 everywhere else.

❌ Common Errors

Claiming this turns off all robot components or resets all sensors. Inputs cannot be cleared by writing to memory location 100 (the preamble states only B₇–B₅ change upon writing), and bit B₅ is already 0.

✅ Correct Purpose (2 Marks)

1 mark: Count how many touch sensors (sensors X, Y, and Z) are activated.

1 mark: End the program / stop execution if the number of active touch sensors is not 2 (i.e. is 0, 1, or 3) [OR: continue to activate arm only if exactly 2 are active].

(Both points stated together award 2 marks directly).

📐 Step-by-Step Register Trace

1. R2 is initialised to 0 (counter of activated sensors).
2. R3 is initialised to 0 (loop iteration counter).
3. AND R1, R0, #1 isolates the lowest bit (B₀ initially). If set, increment R2 .
4. LSR R0, R0, #1 shifts all bits right by 1 position (B₁ moves into B₀, B₂ into B₁, etc.).
5. The loop executes exactly 3 times ( CMP R3, #3 ), testing B₀, B₁, and B₂.
6. Finally: CMP R2, #2 followed by BEQ active . If R2 == 2 , it jumps to active . Otherwise, it hits HALT .

🧠 Exam Technique: Identifying Nested Logic

Whenever you see a combination of a counter register ( ADD R2, R2, #1 ), a loop counter ( CMP R3, #3 ), and LSR , recognise this standard assembly idiom: tallying the number of set bits (Hamming weight) across a subfield.

❌ Common Errors & Examiner Warnings

• Not Enough (NE): Saying "Stop the robot" instead of "End/halt the program" scores no mark for the second point, because the motors were already halted in Section 3!
• Vagueness: Stating "Checks if touch sensors are on" without mentioning that it specifically checks for exactly 2 active sensors misses the second mark.

✅ Correct Purpose (1 Mark)

• Turn on the robot accessory arm.
• Move / start the robot accessory arm motor.

📐 Bit Tracing

1. Decimal #32 = binary 00100000₂ .
2. Bit 5 (B₅) controls the accessory arm motor.
3. ORR R0, R0, #32 performs a bitwise logical OR, forcing bit B₅ to 1 while leaving every other bit completely untouched.
4. Writing the value back ( STR R0, 100 ) energises the accessory arm.

💡 Key Knowledge: Bit Setting Mask

To turn on specific hardware outputs without disturbing the rest of the register state, use a bitwise ORR with a mask containing a 1 at the target bit position and 0 everywhere else.

❌ Common Errors

Confusing bit numbering: 2⁵ = 32 corresponds to bit B₅ (the 6th bit from the right, starting at index 0). Misidentifying B₅ as the ultrasonic sensor (B₄) or right motor (B₆) results in zero marks.

✅ Correct Purpose (1 Mark)

Any of the following descriptions achieves the mark:

• Keep the arm moving until the light sensor is activated.
• Stop the arm when the light sensor is activated.
• Wait until the light sensor is activated, then stop the arm (and end program).

📐 Bit Tracing

1. AND R1, R0, #8 : Decimal #8 = 00001000₂ (isolates B₃ = Light sensor).
2. BEQ loop3 : Loops continuously while B₃ is 0 (light sensor inactive).
3. Once light sensor triggers (B₃ becomes 1), loop exits.
4. AND R0, R0, #223 : Decimal #223 = 11011111₂ (255 − 32). This clears bit B₅ to 0, switching off the arm motor.
5. HALT terminates the program.

🧠 Exam Technique: Holistic Event Summary

Notice the two-part structure of this section: a condition wait (polling) followed by an action (shutoff). High-scoring responses link cause and effect: "When [sensor input happens], do [actuator action]."

❌ Common Errors

• Omitting the light sensor entirely and just saying "Stop the arm after a delay".
• Stating that the robot stops moving forward (the wheels were already stopped in Section 3).