Main memory is taking an important role in the working of computer. We have seen that computer works on Von-Neuman stored program principle. We have to keep the information in main memory and CPU access the information from main memory.
The main memory is made up of semiconductor device and by nature it is volatile. For permanent storage of information we need some non volatile memory. The memory devices need to store information permanently are termed as external memory. While working, the information will be transferred from external memory to main memory.
The devices need to store information permanently are either magnetic or optical devices.
