Erasable Programmable Read Only Memory. An EPROM is a memory chip that primarily holds the operating program or boot program for a processor. It is often in a ceramic or plastic package with a quartz glass window to allow the chip to be erased with ultraviolet light. To prevent erasure, it is usually covered with a sticker (often with the program version number listed on it) or a square of black electrical tape. A special programmer is used to write the EPROM. EPROMs are also available in a plastic package without the window; these are only programmable once, but are cheaper for mass production. A variant, the EEPROM (Electrically Erasable Programmable Read Only Memory) can be erased electically with a special programming sequence, and is lacking the window. Modern microcontrollers use flash memory for program storage, and often have the flash as part of the processor.