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A9.6 AQA assembly language and bitwise operations

Computer architecture · A level · OCR H446 1.2.4, AQA 7517 4.7.3.5 · about 25 min

What this lesson is about

AQA's instruction set, compare and branch, masks and shifts, and the bit fields that drive BugBot's motors.

• The instruction set
• Selection
• Iteration
• Running AQA assembly in Python
• What the bitwise operations are for
• Bits that drive motors

Questions 6 marks in all

1. [1 mark]What is the difference between MOV R1, #50 and LDR R1, 50?

   1. AMOV puts the number 50 in R1; LDR loads the value stored at address 50
   2. BThey do the same thing
   3. CMOV loads from address 50; LDR puts the number 50 in R1
   4. DLDR moves R1 to address 50
   Answer: A. # means immediate; a plain number in LDR is a direct memory address.

2. [1 mark]What does this program print?

   r0 = 0b11001010
   print(format(r0 & 0b00001111, '08b'))
   print(format(r0 | 0b00000001, '08b'))
   print(format(r0 >> 3, '08b'))

   Answer:

   00001010
   11001011
   00011001

   AND keeps the bottom four bits, OR sets bit 0, and a right shift by 3 moves every bit three places down.

3. [1 mark]R0 holds 13. What does R1 hold after LSL R1, R0, #2?

   Answer: 52. Shifting left by 2 places multiplies by 4.

4. [1 mark]Which instruction toggles (flips) the lowest four bits of R2, leaving the others alone?

   1. AEOR R2, R2, #15
   2. BAND R2, R2, #15
   3. CORR R2, R2, #15
   4. DMVN R2, R2
   Answer: A. XOR with 1 flips a bit; XOR with 0 leaves it unchanged.

5. [1 mark]A program does CMP R0, #10 then BLT small. When does it branch?

   1. AWhen the value in R0 is less than 10
   2. BWhen 10 is less than the value in R0
   3. CWhen R0 holds 10
   4. DAlways
   Answer: A. CMP compares the register with operand2; BLT branches if the register was less.

6. [1 mark]Why does an if-else in AQA assembly need an unconditional B after the 'if' part?

   1. AOtherwise the processor falls through into the else part as well
   2. BB stops the program
   3. CBranch instructions must come in pairs
   4. DIt resets the flags
   Answer: A. Instructions run in order unless the PC is changed, so the else code must be skipped explicitly.

The task: decode the control bytes

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

commands = [161, 98, 3, 0]
modes = ["coast", "forward", "reverse", "brake"]

from bugbot import *
connect()
commands = [161, 98, 3, 0]
modes = ["coast", "forward", "reverse", "brake"]
for byte in commands:
    speed = byte >> 2
    mode = modes[byte & 0b11]
    print(f"{byte}: speed {speed} {mode}")
    if mode == "forward":
        forward(speed, distance=10)
    elif mode == "reverse":
        backward(speed, distance=10)
    elif mode == "brake":
        stop()