Worksheet

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

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'))

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

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

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

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

The task: decode the control bytes

commands holds four motor control bytes, each a whole number from 0 to 255 laid out as in the table above. For each byte, use bitwise operators (>> and &) to split it into its speed (0 to 63) and its mode, and: - print a line in the form 161: speed 40 forward: the byte in denary, the speed, then the mode as coast, forward, reverse or brake; - then act on it: forward drives forward 10 cm at that speed, reverse drives backward 10 cm at that speed, brake calls stop(), and coast does nothing. Do not convert the byte to a string of bits.

# 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"]

Plan your program here, then type it in and press Run.

Do it on the robot
www.bugbotlab.com/learn/a9-6-aqa-assembly-and-bitwise-operations/
The simulator checks it and tells you when it passes. Nothing to install, no account.

Challenges

1. Write AQA assembly that sets bit 0 of the value at address 60 to 1 and stores it back, leaving the other bits alone.
2. Write AQA assembly that multiplies the value in R0 by 10 using only shifts and one ADD. (Hint: 10 = 8 + 2.)
3. Write AQA assembly that counts how many of the bits in R0 are 1.