The main memory of the computer is also known as RAM , standing for Random Access Memory. It is constructed from integrated circuits and needs to have electrical power in order to maintain its information. This is broadly comparable with the speed at which the CPU will need to access data.
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.