1. GCSE OCR 1 The CPU Computer Science J276 Unit 1
System architecture, memory and storage 1

2. Objectives Understand the purpose of the CPU
Explain the role and operation of the following CPU registers used in Von Neumann architecture:
MAR (Memory Address Register),
MDR (Memory Data Register),
Program Counter,
Accumulator

3. What is this?

4. The CPU

5. Central Processing Unit (CPU)
The Central Processing Unit or CPU is arguably the most important component of a computer
What does it do?
If it was one organ in the human body what would it be?

6. Central Processing Unit (CPU)
You can think of the CPU as being like the brain in a human
It is responsible for all of a computer’s processing

7. The main components of a computer
MAIN MEMORY
SECONDARY
STORAGE
CENTRAL PROCESSING
UNIT (CPU)
INPUT
DEVICES
OUTPUT

8. Stored program concept
Before about 1943, early computers stored the data to be worked on in memory
The program was not stored – instructions were input one at a time using switches, or read in from paper tape and executed one at a time
In , mathematician von Neumann and his colleagues had the idea of storing the program instructions as well as the data in the same store
The stored-program computer was born!

9. Von Neumann architecture
Computer programs and the data the programs are using are stored in the same memory
Data then moves between the memory unit and the processor 1
Program 2 3 4 5
Data 6 7 8

10. General Purpose Registers
Registers in the CPU
Program Counter
Control Unit
ALU
General Purpose Registers

11. General Purpose Registers
Registers in the CPU
Program Counter: A special purpose register that holds the address of the next instruction to be executed
General Purpose Registers: Hold data, such as numbers during arithmetic calculations
Some architectures have just a single register called the Accumulator
Program Counter
General Purpose Registers

12. Program Counter 1 2 3 4 5
LDA 28 6
ADD #2 7
STO 10 8 9 10 11 12
The Program Counter points to the next instruction that needs to be executed
It is located in the Control Unit
The Program Counter is incremented after fetching an instruction
This moves it on to point to the next instruction to be executed
Program Counter 5

13. Accumulator
The accumulator is where arithmetic and logic results are temporarily stored much like the M+ function on a calculator
Without the accumulator, it would be necessary to write the result of each calculation to main memory
The accumulator speeds up the storage and retrieval of calculations

14. The Arithmetic Logic Unit
The ALU or Arithmetic Logic Unit performs:
Logical operations,
shift operations and
arithmetic operations
Accumulator 8
ALU
Main Memory
X = 3
Y = 5
ADD X, Y

15. MAR (Memory Address Register) MDR (Memory Data Register)
Memory registers
With the von Neumann architecture, both the instructions and the data are stored together in RAM
Two registers are required to implement this:
MAR (Memory Address Register)
MDR (Memory Data Register)

16. MAR (Memory Address Register)
The MAR holds the address of the current instruction being executed
The MAR knows the location of the data that needs to be accessed
It keeps track of where data is located
MAR (Memory Address Register)

17. MDR (Memory Data Register)
The MDR holds the actual data that is being stored in RAM
It acts as a buffer and holds data that is copied from RAM, ready for the CPU to process it
MDR (Memory Data Register)

18. MAR (Memory Address Register) MDR (Memory Data Register)
Working together
MAR (Memory Address Register)
MDR (Memory Data Register)
The two work together; The MAR knows where to look for data in RAM, the MDR keeps hold of that data until it’s ready to be used by the CPU
Memory
MAR (Memory Address Register)
MDR (Memory Data Register)

19. Control Unit
The control unit coordinates all of the activities taking place within the CPU
Much like the conductor of an orchestra

20. Processor registers
ALU: Temporarily stores arithmetic and logic results
PC: Points to the next instruction
MDR: Stores the actual data
CIR (Current Instruction Register): Holds the current instruction to be executed
MAR: Stores the address of the data

21. Fetch – Decode – Execute
The CPU operates by repeating three operations:
FETCH – causes the next instruction and any data involved to be fetched from main memory
DECODE – decodes the instruction
EXECUTE – carries out the instruction
Repeat…

22. Example – Step 1 (Add 8 + 4)
PC points to next instruction in location 5
Instruction is passed to the CIR and PC is incremented
Memory location 10 is passed to the MAR
The value in location 10 is passed from main memory to the MDR
8 is loaded into the accumulator 1 2 3 4 5
LDA 10 6
ADD #4 7
STO 11 8 9 10 11 12
Program Counter 5

23. Example – Step 2 6 PC now points to next instruction in location 6
Instruction is passed to the CIR and the PC is incremented
4 is added to 8 in the ALU and the result is stored in the accumulator 1 2 3 4 5
LDA 10 6
ADD #4 7
STO 11 8 9 10 11 12
Program Counter 6

24. Example – Step 3 7 PC points to the next instruction in location 7
Instruction is passed to the CIR
Memory location 11 is passed to the MAR
Total value 12 transferred from the accumulator into memory location 11 1 2 3 4 5
LDA 10 6
ADD #4 7
STO 11 8 9 10 11 12
Program Counter 7

25. Worksheet
Complete Tasks 1-3 on the worksheet