Central Processing Unit (CPU):

The CPU is the physical device that controls computer operations. It is considered as the 'brain' of the computer system. The CPU comprises three components: The control unit, the arithmetic/logic unit, and the main memory (main storage) All these are composed of microelectronic devices, virtually always based on silicon semiconductor technology.  

The main memory is the largest component of CPU, and is divided into individual computer words, each of which may be accessed individually by its address. All present day computers are based upon semiconductor technology, the ability to place many thousands of electronic components Within a limited area of silicon, which has been doped with carefully controlled amounts of other elements. The main memory holds program instructions and data required as the programs operate. 

The arithmetic/logic unit contains a set of registers or accumulators, individual words of very high speed storage into which words from the main storage are copied, prior to instructions being applied to them. Attached to the registers is the circuitry which carries out the few hundreds of basic operations available to the computer. In essence, when an individual instruction needs to be carried out on some word in the main storage, that word is copied into a register, the appropriate circuitry activated, and the register's contents copied back to the main location. 

The control unit extracts program instructions one at a time in the required sequence from their locations in the main storage, decodes them, and issues instructions to the main storage to access the required word, and to the arithmetic/logic unit to activate the required instruction circuitry.