PROM (Programmable Read Only Memory) can be written to once, typically at the factory. EPROMs (Erasable Programmable Read Only Memory) and EEPROMs (Electrically Erasable Programmable Read Only Memory) may be “flashed,” or erased and written to multiple times.
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The capacitors used by DRAM leak charge, and must be continually refreshed to maintain integrity, typically every few to a few hundred milliseconds, depending on the type of DRAM. Refreshing reads and writes the bits back to memory. SRAM does not require refreshing, and maintains integrity as long as power is supplied
The term “flashing” derives from the use of EPROMs: flashing ultraviolet light on a small window on the chip erased the EPROM. The window was usually covered with foil to avoid accidental erasure due to exposure to light. EEPROMs are the modern type of ROM, electrically erasable via the use of flashing programs.
Flash Memory - Flash memory (such as USB thumb drives) is a specific type of EEPROM, used for small portable disk drives. The difference is any byte of an EEPROM may be written, while flash drives are written by (larger) sectors. This makes flash memory faster than EEPROMs, but still slower than magnetic disks.