An
EPROM (rarely EROM), or
erasable programmable read-only memory, is a type of memory
chip that retains its data when its power supply is switched off. Computer memory that can retrieve stored data after a power supply has been turned off and back on is called
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 digital circuits. Once programmed, an EPROM can be erased by exposing it to strong
ultraviolet light source (such as from a mercury-vapor light). EPROMs are easily recognizable by the transparent
fused quartz window in the top of the package, through which the
silicon chip is visible, and which permits exposure to
ultraviolet light during erasing.