CareerGears

Instructions which are critical to the operation of a computer are stored permanently on Read only Memory. (ROM) chip installed by the manufacturer inside the computer. This ROM chip is also called firm ware, retains instructions in a permanently accessible nonvolatile form. When the power in the computer is turned off, the instructions stored in ROM are not lost.
It is necessary and also convenient to have instructions stored in ROM. The more instructions in ROM, the fewer diskettes you may have to handle. Until recently the process of manufacturing ROM chips and recording data on them was more expensive than the process of producing RAM chips. As a result the manufacturers tended to record in ROM only those instructions that were crucial to the operation of the computer.
Today, due to improvements in the manufacturing process of ROM chips have lowered the cost to the point where manufacturers are beginning to include additional software instructions.
In addition to ROM, three additional categories of non volatile memory are used in some computer systems. They are PROMs, EPROMs, and EPROMs. PROM stands for programmable read only memory.
It works similar to that of ROM. PROM chips are custom made for the user by the manufacturer. The user determines what data and instructions are to be recorded on them. The data on PROM is permanent and cannot be erased.
Erasable programmable read only memory (EPROM), developed as an improvement over PROM. The data on the EPROM can be read with the help of a special device that uses ultra violet light. The data or instructions on the EPROM are erasable and new data can be entered in its place. EPROM functions exactly same as PROM.
Electronically erasable programmable read only memory (EEPROM) avoids the inconvenience of having to take chips out of the computer to change data and instructions. Changes can be made electrically under software control. These are used in point of sale terminals to records price related data for products. The prices recorded on them can be easily updated as needed. The only disadvantage of EEPROM is, the regular ROM chips.