An EPROM, or erasable programmable read-only memory, is a type of computer memory chip that retains its data when its power supply is switched off. In other words, it is non-volatile. It is an array of floating gate transistors individually programmed by an electronic device that supplies higher voltages than those normally used in electronic circuits. Once programmed, an EPROM can be erased only by exposing it to strong ultraviolet light. EPROMs are easily recognisable by the transparent window in the top of the package, through which the silicon chip can be seen, and which admits UV light during erasing.

As the glass window is expensive to make, OTP (one-time programmable) chips were introduced; the only difference is that the EPROM chip is packed in an opaque package, so it can not be erased after programming. OTP versions are manufactured for both EPROMs themselves and EPROM-based microcontrollers. However, OTP EPROMs are being increasingly replaced by flash memory chips, which offer similar price of silicon and reprogrammability in a windowless package.
This 8749 Microcontroller stores its program in EPROM.A programmed EPROM retains its data for about ten to twenty years and can be read an unlimited number of times. The erasing window must be kept covered to prevent accidental erasure by sunlight. Old PC BIOS chips were often EPROMs, and the erasing window was often covered with a label containing the BIOS publisher’s name, the BIOS revision, and a copyright notice.

Some microcontrollers, often those from before the era of Flash Memory, use EPROM to store their program. This is useful for development, as using one-time programmable devices would be horribly wasteful for debugging.

The EPROM was invented by engineer Dov Frohman.

EPROMs come in several sizes both in physical packaging as well and storage capacity

EPROM Type	Size - bits	Size - bytes	Length (hex)	Last address (hex)
2716, 27C16	16 Kibit	2 KiB	800	007FF
2732, 27C32	32 Kibit	4 KiB	1000	00FFF
2764, 27C64	64 Kibit	8 KiB	2000	01FFF
27128, 27C128	128 Kibit	16 KiB	4000	03FFF
27256, 27C256	256 Kibit	32 KiB	8000	07FFF
27512, 27C512	512 Kibit	64 KiB	10000	0FFFF
27C010, 27C100	1 Mibit	128 KiB	20000	1FFFF
27C020	2 Mibit	256 KiB	40000	3FFFF
27C040	4 Mibit	512 KiB	80000	7FFFF

 

NOTE: 27x series EPROMs containing a C in the name are CMOS based, without the C are NMOS