1. central heating system
eReader
Lego® Mindstorms®
robot
Starter
car
Where can a microprocessor be found?
smart glasses
Raspberry Pi®
smartphone
tablet
fitness band
digital TV
washing machine
camera
games console
central heating system

2. Assembly Language
4.2.3

3. Learning Objectives 4.2.3 Understand the function of assembly code and
be able to interpret a block of assembly code using a given set of commands
Teacher: Make sure there are paper or on-line accessible copies of assembly language mnemonics

4. Assembly Language What is it? What does it look like?
How is it connected to what I know?

5. Assembly Language: What is it?
Can you name some?
Easy or difficult for humans to read or understand?
High level languages
Python, Java, JavaScript, Scratch, Lego Mindstorms
Humans can easily read the words/symbols
Assembly languages
Use mnemonics (abbreviations) in place of words
Lets programmers manipulate data directly in registers
Machine code
Binary digits
0s and 1s, all strung together
Difficult for humans to interpret
Easy or difficult for humans to read or understand?
What is a register? Where are registers?
What does it look like?
Easy or difficult for humans to read or understand?
More on why we use assembly languages in next lesson.

6. Assembly Language: What does it look like?
Use mnemonics (abbreviations) instead of words
Programmers manipulate data directly in registers
What is MOV a mnemonic for?
SUB?
MUL?
ADD?
What could R3 be the name for?
R1?
R2?
MOV R3, #5
SUB R1, R1, #2
MUL R0, R1, R3
ADD R1, R1, R3
MOVE
SUBTRACT
MULTIPLY
ADD
Register 3
Register 1
Register 2
Mnemonic
What could the # symbol mean?
You can already read assembly code
That means the denary value 5, as opposed to R5.
More in a minute on how these instructions are put together.

7. Assembly Language: Why two parts?
Operation code
‘Opcode’
Operation to perform
Operands
Items to act upon or use
Opcode
Operands
Means
Possible Machine Code
MOV
R3, #5
R3 = 5
SUB
R1, R1, #2
R1 = R1 - 2
MUL
R0, R1, R3
R0 = R1 * R3
ADD
R1, R1, R3
R1 = R1 + R3

8. Assembly Language: How is it connected to what I already know?
MOV R3, #5
SUB R1, R1, #2
MUL R0, R1, R3
ADD R1, R1, R3
Each assembly language line is exactly one instruction in memory
The fetch-decode-execute cycle deals with exactly one instruction at a time
The fetch moves instruction and data from memory to registers
The arithmetic logic unit performs the operation
Remember this diagram?
Remember the jobs of the different parts?

9. Specification
Page 42-44
You do need to know how to read these instructions.
You do not need to memorise them.
The tables will be reproduced for you in the exam paper, like formulas.

10. Example: 11 + 22 The assembly code The assembly mnemonics
A trace table
Line R3 R4
Line R3 R4 2 11 3 33
Can you fill it in?
A blank cell means that no change has taken place.
Teacher Guide

11. Your Turn – Simple Example
5 min
Your Turn – Simple Example
A trace table
Line R2 R3 R4
Line R2 R3 R4 7 3 8 4 9 12 10 6 11 18
The assembly code
What arithmetic expression is this?
3 * = 18
The assembly mnemonics

12. Branching – Changing flow control
Jumping around blocks of code not to be executed
Like an ‘if’ statement
Jumping to blocks of code that need executing
Like calling a ‘subprogram’
The destination line has a label, like ‘LABEL1:’ or ‘TOP:’

13. Labels: Destination for next instruction
5 min
Your Turn - Branching
Complete the trace table
What did you find?
The assembly code
Line R3 R4 13 11 14 33 15
Line R3 R4
Labels: Destination for next instruction
B always branches, so this code loops forever.

14. Conditional Branching
The CMP should be followed by a BRANCH
To check the condition, an extra mnemonic is added to the B for Branch
BEQ indicates a ‘Branch If Equal’ condition
BGT indicates a ‘Branch If Greater’ condition
Comments are bounded by /* and */
Example: /* This is a comment */

15. Conditional Branching – Selection (IF)
5 min
Conditional Branching – Selection (IF)
Complete the trace table
Line R2 R3 19 20 5 21 10 22 23 9 27
Line R2 R3
The assembly code
What is the final value in R2? 9
This is the equivalent pseudo-code:
Teacher: Make sure there are paper or on-line accessible copies of assembly language mnemonics

16. Conditional Branching – Repetition (LOOP)
5 min
Conditional Branching – Repetition (LOOP)
Line R1 R2 38 3 39 40 41 2 42 43 5 1 6 44
Line R1 R2
The assembly code
Complete the trace table
What is the final value in R2? 6
Why have we put in numbers of lines that didn’t change register contents?
This is the equivalent
pseudo-code:
So we can see flow of the program.
Teacher: Make sure there are paper or on-line accessible copies of assembly language mnemonics

17. Plenary ADD R1,R2,R3 What are these shortened words, like ADD called?
Mnemonic
Which is the opcode?
ADD
Which are the operands?
R1,R2,R3
What does this instruction do?
Adds R2 and R3, then puts it into R1
Translate this instruction to pseudo-code.
R1 = R2 + R3
ADD R1,R2,R3

18. The assembly mnemonics
Comments are bounded by /* and */
Example: /* This is a comment */