Main memory is fast and limited in capacity. The CPU can only directly access information in main memory. Main memory cannot retain information when the computer is switched of. Main memory consists of a series of numbered locations called bytes , each byte being eight bits . The number associated with a byte is the address of the byte.
Secondary memory is slow and virtually unlimited in capacity.
It retains information when the computer is switched off. Information on external memory can only be accessed by the CPU if it is first transferred to main memory. The internal representation of information in the computer and on external memory is in terms of the Binary system using only the basic symbols 0 and 1. Programs to be executed by the computer are placed in main memory and the CPU fetches each instruction in turn from memory and executes it. Each type of CPU has its own machine language, the set of instructions it can obey.