1. Objectives
Describe common CPU components and their function: ALU Arithmetic Logic Unit), CU (Control Unit), Cache
Explain the function of the CPU as fetch and execute instructions stored in memory

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

3. The CPU

4. 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?

5. 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
It processes data by performing the fetch-decode- execute cycle.

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

7. Registers in the CPU
Program Counter
Control Unit
ALU
Accumulator

8. Registers in the CPU
Program Counter: A special purpose register that holds the address of the next instruction to be executed
The Accumulator: Holds data, such as numbers during arithmetic calculations
Program Counter
Accumulator

9. 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

10. 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

11. 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

12. 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)

13. 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)

14. 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)

15. 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)

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

17. 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