The CPU or Central Processing Unit performs the following tasks:

1) Fetch instruction: - It reads an instruction from memory
2) Interpret instruction: - The instruction is translated to decide what action is to be performed
3) Fetch data: - In order to execute an instruction some data may need to be read from memory or an Input Output device
4) Process data: - In order to execute an instruction some kind of logical or arithmetic operation may be performed on the data
5) Write data: - Once the instruction is executed the result may be written to memory or an Input Output device

In order to perform the above tasks the CPU consists of two major components:

1) Arithmetic and Logic Unit (ALU)
2) Control Unit (CU)

Any form of computation is done in the ALU, while the CU controls the operation of the ALU, as well as the movement of data into and out of the CPU.


The ALU also requires some sort of memory to store data before and after computation. CPU memory is in the form of a set of locations called REGISTERS.

CPU registers may be divided into two categories:

1) User-visible registers: These registers can be accessed by programmers through machine language. These registers can be of the following categories: -
a) General Purpose – used for general purpose
b) Data – Used to hold data only
c) Address – Used to hold addresses only
d) Conditional codes – used to store flags

2) Control and status registers: These registers are used by the CU to control operations of the CPU, and by operating system programs to control program execution.