Control Unit (CU) The control unit is the component of the processor that directs a coordinates most of operations in the computer .It interprets each instruction issued by a program and then initiates the appropriate action to carry out the instructions. Arithmetic Logic Unit (ALU) The Arithmetic and Logic Unit (ALU) another component of the processor performs arithmetic, comparison, and logical operation. Arithmetic operations include basic calculation such as addition, subtraction, multiplication and division. Logical operation include conditions along with logical operator such as AND, OR, and NOT. Primary Storage or Main Memory (RAM) : Primary storage, presently known as main memory or simply RAM (Random Access Memory), is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them as required. Any data actively operated on is also stored there in uniform manner. RAM varies in size like 128 MB, 256 MB, 512 MB to 2 GB. RAM can not store data and instruction permanently. When we switch off the computer, all data and instructions from RAM are washed or vanished. Therefore it is called volatile memory. ROM stands for Read only Memory. It consists of those instructions that prepare the computer for use. ROM instructions are automatically loaded into the Main Memory. These instructions cab only be read but cannot be changed or deleted. It is not possible to write new instructions into the ROM. It stores data and instructions permanently. When we switch off the computer, the instructions stored in the ROM are not lost. Therefore it is called a non-volatile memory.