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Memory can be characterized as volatile or nonvolatile. The term “volatile” means that the memory loses the stored data if the electric power to the memory chip is lost. Nonvolatile memory can maintain stored information even when electric power to the chip is lost.
Volatile memory is used when the stored information is needed only while the program is running. One example of such information is values of program variables. There are several types of volatile memories. The most commonly used today is Random Access Memory (RAM). The term “random access” refers to the fact that individual memory locations need not be accessed sequentially. RAM can be cahracterized as either static or dynamic. Static RAM uses flip-flops to store information. Dynamic RAM uses capacitors instead. SRAM is faster than DRAM but has lower storage density, and costs more per bit of storage.
Nonvolatile memory is used for information that needs to be saved permanently, which may include programs and fixed data. There are several kinds of nonvolatile memory in common use today. These include read only memory (ROM), one-time programmable memory (OTP), erasable programmable read only memory (EPROM), electrically erasable programmable read only memory (EEPROM), and flash memory. EEPROM data can be modified byte by byte. Flash memory data can be modified only block by block. There is a trend to replace traditional EEPROM with flash memory because it can be programmed much faster.