Hybrid memories are developed that can be read and written as desired just like RAM, but maintain their contents without electrical power, just like ROM. EEPROM and flash are descendants of ROM devices. These are typically used to store code. The third hybrid, NVRAM, is a modified version of SRAM. NVRAM holds persistent data.
EEPROM: Electrically-erasable-and-programmable. In EEPROMs the erase operation is accomplished electrically, rather than exposure to ultraviolet light.
Flash memory devices are high density, low cost, nonvolatile, fast (to read, but not to write), and electrically reprogrammable. The technologies for flash and EEPROM are similar. The major difference is that flash devices can only erase sector by sector but not byte by byte. Typical sector sizes are in the range 256 bytes to 16KB. Despite this disadvantage, flash is much more popular than EEPROM due to its advantages.
NVRAM: Non-volatile RAM is the third member of Hybrid memory class. NVRAM has similar characteristics as hybrid memories discussed previously but it is physically different. An NVRAM is usually just an SRAM with a battery backup. When the power is turned on, the NVRAM operates just like any other SRAM. When the power is turned off, the NVRAM draws just enough power from the battery to retain its data. NVRAM application is limited due to its high cost.
