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Computer System
Lesson 3 - LMC
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2. Copy LMC Resources Go to Computer Student Resources Computing Year 9
Computer Science
Copy “Algorithms”
Paste it your Computer Science / Computing folder in your area

3. The Little man computer
Mr A Jalloh

4. Instructions
When a computer is instructed to run a program it is directed to the start address for these data and instructions.
The CPU fetches the first instruction from this start location and decodes it to find out what to do next.

5. The contents of memory location 1101
Program Instructions
A program instruction has two parts –
Operator – This is the instruction part and ,
Operand – This the data part 1 1 1
Operator
ADD to the accumulator
Operand
The contents of memory location 1101

6. Encoding Instructions
The two part to the instructions are further encoded to binary.
Opcode - a binary value which represents the instruction
Data – Could be null or will contain the data needed for the instruction in binary.
When a instruction is passed to a micro-processor the first thing which happens is the opcode is decoded.
This means the micro-processor will look the binary code up in a table.
Once the instruction is found it will execute it.

7. What is the Little Man Computer?
The Little Man Computer (LMC) is a simulator which copies the basic features of a modern computer.
It is based on the Von Neumann architecture featuring a central processing unit consisting of
An Arithmetic Logic unit 
Registers,
A control unit containing an instruction register and program counter,
Input and Output mechanisms and 
Memory to store both data and instructions.

8. What is the Little Man Computer?
The LMC is based on the concept of a little man (shut in a small room or inside a computer) acting like the control unit of a CPU i.e. 
Fetching instructions from memory,
Decoding and
Executing the instructions, and managing the input and output.
The LMC can be programmed using either "Machine Code" or "Assembly Language". 

9. Which LMC?
There are many implementations of the Little Man Computer (LMC).
We will use the excellent web based one that can be found here: LMC/

10. Parts of the LMC Assembly instructions RAM with 100 memory locations
Output
CPU with 4 registers
Input

11. Parts of the CPU CPU register Function Accumulator Program Counter
This stores data that is being used in ALU to perform calculation. It can perform simple addition and subtraction.
Accumulator
This contains the memory address of the next instruction to be loaded. This automatically ticks to the next memory address when an instruction is loaded. It can be altered during the running of the program depending on the state of the accumulator.
Program Counter
An Instruction Register to hold the top digit of the instruction read from memory.
Instruction Register
An Address Register to hold the bottom two digits of the instruction read from memory.
Address Register
This registers allows the user to input numerical data to the LMC.
Input
Output
This shows the data to output to the user.

12. How the LMC works
The LMC will start to load the instruction from the memory address in the program counter.
When the LMC first loads up this will set at zero.
This memory location needs to be an instruction and will be dealt with as such.
When the data is loaded the program counter is incremented to the next memory location.

13. LMC Instruction set
The LMC processor understands 10 basic commands (plus 1 instruction to label data).
The LMC only understands these instructions in a numerical form.
This can be difficult for us to program in so there is a set of mnemonics we can use instead.
This is known as assembly language.
This will be converted into the LMC code before the program can run.

14. LMC Instruction set ADD 1xx SUB SUBTRACT 2xx STA STORE 3xx LDA LOAD
Mnemonic code
Instruction
Numeric code
Description
ADD
1xx
SUB
SUBTRACT
2xx
STA
STORE
3xx
LDA
LOAD
5xx
INP
INPUT
901
OUT
OUTPUT
902
Add the value stored in mailbox xx to whatever value is currently on the accumulator (calculator).
Subtract the value stored in mailbox xx from whatever value is currently on the accumulator (calculator).
Store the contents of the accumulator in mailbox xx (destructive).
Load the value from mailbox xx (non-destructive) and enter it in the accumulator (destructive).
Go to the INBOX, fetch the value from the user, and put it in the accumulator (calculator)
Copy the value from the accumulator (calculator) to the OUTBOX.

15. LMC Instruction set
Mnemonic code
Instruction
Numeric code
Description
BRA
BRANCH (unconditional)
6xx
BRZ
BRANCH IF ZERO (conditional)
7xx
BRP
BRANCH IF POSITIVE (conditional)
8xx
HLT
HALT
DAT
DATA
Set the program counter to the given address (value xx). That is, value xx will be the next instruction executed.
If the accumulator (calculator) contains the value 000, set the program counter to the value xx. Otherwise, do nothing.
If the accumulator (calculator) is 0 or positive, set the program counter to the value xx. Otherwise, do nothing.
Stop working.
DAT can is used in conjunction with labels to declare variables.
For example, DAT 984 will store the value 984 into a mailbox at the address of the DAT instruction.

16. Examples - input & output
This program simply asks the user for an input and then outputs what was input.
INP
OUT
HLT
The program has been assembled into RAM and you can see the numeric codes for the instructions in the first three memory locations.
Try the example above and save a copy on Notepad as “Input and Output

17. Activity – LMC Instructions
Open the following files
1-LMC Instruction Activities and
2 - LMC Code Activities
Complete all task

18. Using memory (Variable)
INP
STA FIRST
STA SECOND
LDA FIRST
OUT
LDA SECOND
HLT
FIRST DAT 0
SECOND DAT 0
Try the example on the left and save a copy on Notepad as “Load Variable”
This program asks the user to input a number.
This is stored in a memory location defined by the DAT label.
A second number is asked for and stored.
These numbers are then loaded and output in order.

19. Using memory – Adding Two numbers together
INP
STA FIRST
STA SECOND
LDA FIRST
ADD SECOND
STA ANSWER
OUT
HLT
FIRST DAT 0
SECOND DAT 0
ANSWER DAT 0
Try the example on the left and save a copy on Notepad as “Adding two Variables”
This program asks the user to input a number.
This is stored in a memory location defined by the DAT label.
A second number is asked for and stored.
These numbers are then add together and the result is save in a variable called ANSWER
The variable ANSWER is displayed the result

20. LMC Activity Using memory
This program asks the user to input a number.
This is stored in a memory location defined by the DAT label.
A second number is asked for and stored.
These numbers are then add together
After the addition one is subtracted and display the result
NOTE: the value 1 in a variable called ONE
Guess what this program does
Try the example on the left and save a copy on Notepad as “Adding and Subtracting Variable”
INP
STA FIRST
ADD FIRST
STA SECOND
SUB ONE
OUT
HLT
FIRST DAT 0
SECOND DAT 0
ONE DAT 1

21. Set o Instruction Move (number of step ) Forward
Move (number of step ) Backward
Move (number of step ) Right
Move (number of step ) Left

22. Activity – LMC Instructions
Open the following files
3- LMC Matching and
4 - LMC CPU Worksheet
Complete all task

23. Branches and loops in LMC
BRA – Branch Always
This is the same a forever loop in Scratch. It is an unconditional loop.
Try the example on the left and save a copy on Notepad as “Branch Always”.
Add comments explaining what the code does
This example will keep outputting the value which is stored in A forever.
The program will not stop.
LOOPTOP LDA A
OUT
BRA LOOPTOP
A DAT 5
ONE DAT 1

24. Branches and loops in LMC
BRZ – Branch if Zero
This allows us to branch our program if the value in the accumulator is zero.
This can be used to create counting loops similar to the repeat loop in Scratch or a for loop in Python.
LOOPTOP LDA A
SUB ONE
STA A
OUT
BRZ END
BRA LOOPTOP
END HLT
A DAT 5
ONE DAT 1
Try the example on the left and save a copy on Notepad “Branch if Zero”.
Add a comment describing what it does
This example uses a BRA to create the loop and
A BRZ to break the loop.

25. Branches and loops in LMC
BRP – Branch if Positive
This also allows us to create a branch in our program, depending on whether the contents of the accumulator are positive or not.
These can also be used to create loops.
LOOPTOP LDA A
SUB ONE
STA A
OUT
BRP LOOPTOP
HLT
A DAT 5
ONE DAT 1
Try the example on the left and save a copy on Notepad as “Branch if Positive”.
Add a comment describing what it does
This example uses a BRP to both create and terminate the loop.

26. Activity – LMC Instructions
Open the following files
6 - While_Loops_in_LMC and
7 - CPU_LMC_Extension Worksheet
Complete all task

27. Bigger
INP
STA FIRST
STA SECOND
SUB FIRST
BRP FIRSTBIG
LDA SECOND
OUT
HLT
FIRSTBIG BRZ SAME
LDA FIRST
SAME LDA ZERO
FIRST DAT 0
SECOND DAT 0
ZERO DAT 0
This program uses two branch commands to alter the path of the program.
There is no greater than or less than command so we simply subtract the second number from the first.
If it is positive then the first number must have been bigger so we branch if positive.
The two numbers could be the same however so we need to check to see if the result is zero. We branch if it is.
The biggest number is output or zero if they are both the same.

28. Points to note
The instruction set is very limited so you often need to come up with a different way to perform things like multiplication or comparing two numbers.
The LMC does not store decimals.
The LMC does not have a loop structure but you can use a Branch Always command to redirect the code to an earlier command.

29. How to Write a Little man Computer program
Writing an LMC program can be quite a challenge. As the instruction set is very limited we often need to perform what seems to us to be a very simple task in an even simpler way.
Using a Flow chart to help write the program is very helpful.
When the flow chart is created we can simply look at each shape on the chart and think what instructions would we need to have for that shape.
These will often be no more that a couple of lines of LMC code.

30. How to Write a Little man Computer program
In flow charts there are 4 symbols that we commonly use.
Symbol
Meaning
LMC instuctions
Start / Stop
Start has no instruction but Stop is HLT.
Input
&
Output
OUT is the output command. It may need to be
This could be a DAT command where we see variables initialised (e.g. counter = 0).
addition and subtraction commands fit into this.
A process such as X = X + Y would need to be done in the correct order. So we would Load X, Add Y and then store the result as X. This would be LDA X, ADD Y, STA X
Process
There are only two instructions that can have two alternatives. Branch if Positive and Branch if Zero.
If the test is true then the program can branch to another part of the program. If not the program carries on.
Decision

31. Using memory (Variable)
INP
STA FIRST
STA SECOND
LDA FIRST
OUT
LDA SECOND
HLT
FIRST DAT 0
SECOND DAT 0
This program asks the user to input a number.
This is stored in a memory location defined by the DAT label.
A second number is asked for and stored.
These numbers are then loaded and output in order.

32. Using memory (Variable)
Start
FIRST = 0
First thing - create a flow chart to show what needs to be done.
Be as detailed as you can be.
SECOND= 0
INPUT FIRST
INPUT
SECOND
LOAD FIRST
OUTPUT
FIRST
LOAD SECOND
OUTPUT
SECOND
End

33. Using memory (Variable)
Start
FIRST = 0
Note any values you need to remember.
These will be the variables.
In LMC code they will become the DAT commands.
Note if they have a start value.
SECOND= 0
INPUT FIRST
INPUT
SECOND
LOAD FIRST
OUTPUT
FIRST
LOAD SECOND
OUTPUT
SECOND
End

34. Using memory (Variable)
Start
FIRST = 0
We start
We have 2 variables
FIRST DAT 0
SECOND DAT 0
NOTE: The variable is at the top of the Program in the Flowchart.
SECOND= 0
INPUT FIRST
INPUT
SECOND
LOAD FIRST
OUTPUT
FIRST
LOAD SECOND
OUTPUT
SECOND
End

35. Using memory (Variable)
Start
FIRST = 0
Now start at the top and write down the commands for the instructions for the flow chart.
Assigning values can be ignored so the first command in Input number
The LMC command is INP
If we need to store that we need to follow this with a store command and save it to memory using the DAT label we created
SECOND= 0
INP
STA FIRST
STA SECOND
INPUT FIRST
INPUT
SECOND
LOAD FIRST
OUTPUT
FIRST
LOAD SECOND
OUTPUT
SECOND
End

36. Using memory (Variable)
Start
FIRST = 0
INP
STA FIRST
STA SECOND
LDA FIRST
OUT
Now we use the LDA command to load the value stored in FIRST Variable.
We use the OUT command to output the value stored in FIRST Variable.
SECOND= 0
INPUT FIRST
INPUT
SECOND
LOAD FIRST
OUTPUT
FIRST
LOAD SECOND
OUTPUT
SECOND
End

37. Using memory (Variable)
Start
FIRST = 0
INP
STA FIRST
STA SECOND
LDA FIRST
OUT
LDA SECOND
We do the same for the Second value
We use the LDA command to load the value stored in SECOND Variable.
We use the OUT command to output the value stored in SECOND Variable.
SECOND= 0
INPUT FIRST
INPUT
SECOND
LOAD FIRST
OUTPUT
FIRST
LOAD SECOND
OUTPUT
SECOND
End

38. Using memory (Variable)
Start
FIRST = 0
Once we Output both FIRST and SECOND Value,
We use HLT to end the program
INP
STA FIRST
STA SECOND
LDA FIRST
OUT
LDA SECOND
HLT
SECOND= 0
INPUT FIRST
INPUT
SECOND
LOAD FIRST
OUTPUT
FIRST
LOAD SECOND
OUTPUT
SECOND
End

39. Activity – LMC Instructions
Open the following files
5 - More LMC Task and
Complete all task
For each task create a flowchart and write LMC codes
Save a copy of your LMC code in Notepad

40. Example - The Problem We want a program to calculate averages.
We want to be able to keep entering values until we enter a zero.
The average is then calculated and displayed.

41. PLANNING AND DESIGN
First thing - create a flow chart to show what needs to be done.
Be as detailed as you can be.

42. Note any values you need to remember.
These will be the variables.
In LMC code they will become the DAT commands.
Note if they have a start value.

43. We have 4 variables
total DAT 0
count DAT 0
result DAT 0
num DAT

44. If we need to add on or subtract a specific value we need to be able to store that too.
We need to be able to add 1 do we can do this by having
one DAT 1

45. Assigning values can be ignored so the first command in Input number
Now start at the top and write down the commands for the instructions for the flow chart.
Assigning values can be ignored so the first command in Input number
The LMC command is INP
If we need to store that we need to follow this with a store command and save it to memory using the DAT label we created.
INP
STA num

46. Next we see if the user has entered a zero.
We can use Branch Zero to do this.
If the accumulator is zero we will jump to another section of the code.
We need to give this section a label. I will call this section CALCULATE.
I will need to do that code later.
INP
STA num
BRZ CALCULATE

47. INP STA num BRZ CALCULATE LDA total ADD num STA total
The next section of code happens if num does NOT equal zero.
Now I need to add the num to the total.
I will load the total and then add the num.
LDA total
ADD num
This then needs to be saved back as the total.
STA total
INP
STA num
BRZ CALCULATE
LDA total
ADD num
STA total

48. Now I need to add one to the count.
INP
STA num
BRZ CALCULATE
LDA total
ADD num
STA total
LDA count
ADD one
STA count
Now I need to add one to the count.
So i need to load count and then add one.
The result needs to be saved as count

49. LOOPTOP INP STA num BRZ CALCULATE LDA total ADD num STA total
LDA count
ADD one
STA count
BRA LOOPTOP
The code now loops back to the Input command.
We can use a Branch always command to do this.
We need to label where we want the BRA command to jump to.
I will call it LOOPTOP.
I need to add this label to the INP command and use it in the BRA command.

50. Now we need to go back to out CALCULATE code.
LOOPTOP INP
STA num
BRZ CALCULATE
LDA total
ADD num
STA total
LDA count
ADD one
STA count
BRA LOOPTOP
Now we need to go back to out CALCULATE code.
This code performs a division.
LMC does not have a divide command.

51. The result will store this count.
LOOPTOP INP
STA num
BRZ CALCULATE
LDA total
ADD num
STA total
LDA count
ADD one
STA count
BRA LOOPTOP
We can perform a divide by repeatedly subtracting the count from the total until we get to zero.
Keeping a count of how many times we successfully subtract the count will be the same as dividing.
The result will store this count.

52. This code will run in a loop.
LOOPTOP INP
STA num
BRZ CALCULATE
LDA total
ADD num
STA total
LDA count
ADD one
STA count
BRA LOOPTOP
This code will run in a loop.
We need to load total and then subtract the count.
We then need to see if the count is below zero.
If it is not we will add one to the result and then loop around.

53. BRA LOOPTOP CALCULATE LDA total
LOOPTOP INP
STA num
BRZ CALCULATE
LDA total
ADD num
STA total
LDA count
ADD one
STA count
BRA LOOPTOP CALCULATE LDA total
SUB count
BRP DIVIDE
So the first command is to load the total and subtract the count.
LDA Total
SUB count
Then we Branch if the result is zero or higher, so we need BRP in order to add one to the result.
I will give it the label DIVIDE and deal with that later.

54. BRA LOOPTOP CALCULATE LDA total
LOOPTOP INP
STA num
BRZ CALCULATE
LDA total
ADD num
STA total
LDA count
ADD one
STA count
BRA LOOPTOP CALCULATE LDA total
SUB count
BRP DIVIDE
LDA RESULT OUT
HLT
If the value in the accumulator is negative then the BRP does not run.
We now need to load the result and output it to the user.
Once we do that the program is done.

55. BRA LOOPTOP CALCULATE LDA total
LOOPTOP INP
STA num
BRZ CALCULATE
LDA total
ADD num
STA total
LDA count
ADD one
STA count
BRA LOOPTOP CALCULATE LDA total
SUB count
BRP DIVIDE
LDA RESULT OUT
HLT
DIVIDE LDA result
STA result
BRA CALCULATE
Now we need to go back to what happens if the total - count is positive.
Remember we jumped to a label called DIVIDE.
We need to add one to the result and then start the loop again.
We can use Branch always to jump back to the top of our loop.
The loop already has a label, so we can use that.

56. All that remains is to add the DAT commands to the end of our program.
LOOPTOP INP
STA num
BRZ CALCULATE
LDA total
ADD num
STA total
LDA count
ADD one
STA count
BRA LOOPTOP
CALCULATE LDA total
SUB count
BRP DIVIDE
LDA RESULT
OUT
HLT
DIVIDE LDA result
STA result
BRA CALCULATE
total DAT 0
count DAT 0
num DAT
result DAT 0
one DAT 1
All that remains is to add the DAT commands to the end of our program.

57. Plenary Open Realsmart and Click on Computer Science
Click on Unit 1 - Computer System RAFL
Under Lesson 3 - LMC
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Explain what you have learned today
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