1. The “Little Man Computer” Version
How the CPU Works
The “Little Man Computer” Version

2. Download the tutorial
You can download your own copy of the Little Man Computer simulation from the University of Hertfordshire
Start the download now
Save it to your hard drive
We will use it shortly
The link:
You can access this link directly from my website

3. The Little Man Computer

4. LMC Instruction Codes LOAD instruction – op code 1 Note:
The Little Man goes to the mailbox specified
Reads the three-digit number in the mailbox
Puts that into the Calculator
Note:
The number in the mailbox is unchanged
The number in the calculator is replaced by the new number

5. LMC Instruction Codes STORE instruction – op code 2 Note:
The Little Man goes to the Calculator and reads the number there
He writes the number down on a slip of paper and puts it into the mailbox specified in the instructions
Note:
The number in the Calculator does not change
The number in the Mailbox is replaced with the new value

6. LMC Instruction Codes ADD instruction – op code 3 Note:
The Little Man walks over to the mailbox address specified in the instructions
He reads the number and adds this number to the number already in the calculator.
Note:
The number in the mailbox is unchanged

7. LMC Instruction Codes SUBTRACTION – op code 4 Note:
Same as the ADD instruction, except the number is subtracted
For this model we will assume that the Little Man handles negative numbers correctly, though we will not deal with it here.
Note:
The number in the mailbox is left unchanged

8. LMC Instruction Codes
INPUT (or READ) – op code 5, “address” 00 (the 00 is ignored for this special instruction code)
The Little Man gets a slip of paper in from the in-basket
He types this number into the calculator
Each INPUT instruction handles only 1 slip of paper
The Little Man will ignore the address portion of this code

9. LMC Instruction Codes
OUTPUT (or PRINT) – op code 6, “address” 00 (the 00 is ignored for this special instruction code)
The Little Man walks to the calculator and writes down the number on a slip of paper
This slip is put in to the out basket
Each OUTPUT instruction places 1 piece of paper in the out basket
The Little Man will ignore the address portion of this code

10. LMC Instruction Codes COFFEE BREAK (or HALT) – op code 7
The Little Man takes a break.
He will ignore the address portion of the instruction

11. A Simple Program A few items
We need to store the instructions somewhere
We need a method to tell the Little Man where to find the particular instructions that he is supposed to perform at any given time

12. A Simple Program Assumed Solutions
Assume that the instructions are stored in the mailbox starting at mailbox number 00
The Little Man will perform instructions by looking at the value in the instruction location counter and execute the instruction found in the mailbox whose address has that value

13. A Simple Program Assumed Solutions
Each time the LM completes and instruction, he will increment the Instruction Location Counter and do the instruction specified by the counter

14. A Simple Program
Look at a program where the USER uses the Little Man Computer to add two numbers.
The User will place two numbers in the in basket
The sum of the two will appear as a result in the out basket

15. A Simple Program
INPUT 500
The first step is to give the LM access to the data
This instruction has the LM read the first number from the in basket in to the calculator

16. A Simple Program
STORE
It is not possible for the LM to simply read another number into the calculator.
It would destroy the first number
We must save the first number somewhere
Mailbox 99 is chosen so that we don’t have any conflicts with other data or programs
Discuss this

17. A Simple Program INPUT 500 ADD 399
The LM reads the second number into the calculator
ADD 399
We do not need to save the 2nd number in the example, but we could have.
The 1st number from mailbox 99 is added to the 2nd number in the calculator

18. A Simple Program OUTPUT 600 COFFEE BREAK 700 Note:
The LM puts the result in to the out basket
COFFEE BREAK 700
The program terminates
Note:
These instructions are stored sequentially starting from mailbox 00.
This is so we could run the program again

19. A Simple Program Code Summary 00 500 INPUT 01 299 STORE DATA
Mailbox Instruction
Step code description
INPUT
STORE DATA
INPUT 2ND #
ADD 1ST # TO it
OUTPUT RESULT
STOP
99 DATA

20. Extended Code Set Example of Branching
45 LDA is assumed to have a value
46 BRZ Branch if the value is 0
47 BR Exit loop; Jump to NextStatement
48 … This is where the task is located
59 BR End of Task; loop test again
60 Next code statement

21. Run the CPU Simulator Now you try it.
Do the sample program included with the simulation
Write your own program (and show me that they work)
Write a program to add 3 numbers
Write a program that correctly calculates x + y + z – a – b (Ex – 4 – 5 = -3)
For the bold and intelligent ones:
Write the last program only using 2 mailboxes!

22. How the CPU Works
A More Complex View

23. Overview
We saw in the “Little Man Computer” a simplified version of how a CPU works.
Memory, the Control Unit, the Accumulator (calculator) were all located in the same area
Decimal system used for communication of instructions
Memory was limited to 100 “mailboxes”

24. CPU Components 3 Major Parts
Arithmetic/Logic Unit (ALU)
Control Unit (CU)
Memory
The ALU & CU are known as the Central Processing Unit

25. Block Diagram of System
Memory
CPU
ALU
I/O
Control Unit Program Counter

26. Direct Comparison of LMC & CPU
Control Unit
ALU
I/O Interface
Memory
Program Counter

27. Arithmetic/ Logic Unit (ALU)
Memory
CPU
ALU
I/O
Control Unit Program Counter
Arithmetic/ Logic Unit (ALU)
The component where data is held temporarily
Calculations occur here
It knows how to perform operations such as ADD, SUB, LOAD, STORE, SHIFT.
It knows the commands that make up the machine language of the CPU
It is the calculator

28. Control Unit Program Counter
Memory
CPU
ALU
I/O
Control Unit Program Counter
Control Unit
Interprets and controls the execution of instructions
This control corresponds to the “fetch-execute” cycle
The fetch cycle gets the instructions
The execute cycle does the work specified in the instruction
Most of the actions of the fetch-execute cycle are the movement of data or addresses from one part of the cpu to another
Note

29. Control Unit Program Counter
Memory
CPU
ALU
I/O
Control Unit Program Counter
Registers
“A Register is a single, permanent storage location within the CPU used for a PARTICULAR, defined purpose.”
“A register is used to hold a binary value temporarily for storage, for manipulation, and/or for simple calculations.”
Registers have special addresses

30. The End…