Question

what is rom ? and there characterstics​

Answer 1

ROM is a chip level memory, frequently used for fixed data storage.

Several variations of ROM include:

Mask level ROM- Data provided by a customer is converted to a wafer fab mask, so that the silicon that emerges is pre-programmed with the data. Higher up-front cost to create the mask, but for high volumes this is cheaper than a chip that requires later programming. An example where this is used are game cartridges.
EPROM (Electrically Programmable ROM) - Data is burned into the blank silicon and cannot be changed once written.
EEPROM (Electrically Eraseable Programmable ROM, pronounced “E-squared PROM”) - An EPROM that can be blanked out and rewritten. The chip has a clear window for exposure of the silicon to UV light, required to erase it.
Much of the market for EEPROM has moved to flash memory, which is simpler and cheaper.