The Main Memory is also known as RAM which is used for to run all the applications in efficiently standing for Random Access Memory. It is constructed from integrated circuits and needs to have electrical power in order to maintain its information. Always remember when computer power is off then all the information is lost too. RAM is directly accessed by the CPU and access time to read or write any particular byte are independent of whereabouts in the memory that byte is and currently is approximately 50 nanoseconds . Main Memory is expensive compared to external memory so it has limited capacity. The capacity available for a given price is increasing all the time. The CPU will normally transfer data to and from the Main Memory in groups of two, four or eight bytes, even if the operation it is undertaking only requires a single byte.