Classic mask-programmed ROM chips are joined circuits that physically encode the information to be saved, and therefore it is inaccessible to modify their contents after fabrication. Several methods of non-volatile solid-state memory allow some degree of modification −
Programmable read-only memory (PROM) − It is a one-time programmable ROM (OTP) and can be written to or programmed through a unique device known as a PROM programmer. This device uses high voltages to permanently damage or generate internal connections (fuses or anti-fuses) inside the chip.
Erasable Programmable read-only memory (EPROM) − It can be erased by hazard to powerful ultraviolet light (generally for 10 minutes or higher), then rewritten with a process that again requires application of higher than typical voltage. Repeated disclosure to UV light will ultimately wear out an EPROM, but the strength of most EPROM chips exceeds thousands of cycles of erasing and reprogramming.
EPROM chip packages can be recognized by the outstanding quartz “window” which enables UV light to enter. After programming, the window is generally wrapped with a label to avoid accidental deletion. Various EPROM chips are factory-erased since they are packaged, and contains no window, these are effectively PROM.
Electrically Erasable Programmable read-only memory (EEPROM) − It is placed on the same semiconductor structure to EPROM, but enable its whole content to be electrically erased, thus rewritten electrically, so that they require not to be eliminated from the computer (or camera, MP3 player, etc.)
Electrically alterable read-only memory (EAROM) − It is a type of EEPROM that can be changed one bit at a time. Writing is a very easy process and needed a higher voltage (generally around 12V) that is used for the reading approach.
EAROMs are designed for applications that need occasional and only incomplete rewriting. EAROM can be used as non-volatile storage for demanding system setup data, in some applications, EAROM has been replaced by CMOS RAM provided by central power and backed up with a lithium battery.
Flash memory − It is a current type of EEPROM designed in 1984. Flash memory can be erased and rewritten quicker than ordinary EEPROM, and the recent design feature has very high strength (exceeding 1, 00,000 cycles).
Recent NAND flash creates effective use of silicon chip space, resulting in single ICs, with size as high as 32GB as of 2007, this feature forward with strength and physical persistence, has granted NAND flash to restore magnetic in various applications including USB flash drives.
Flash memory is known as Flash ROM or Flash EEPROM when used as a restoration for earlier ROM types, but not in applications that take advantage of its capability to be changed quickly and repeatedly.