Originally Posted by [http://www.tmo.com/faq/glossary.htm#eprom"
EPROM - Let's start with the last three letters. ROM means Read-Only Memory. This is a permanent storage place for computer programs. Computers can read their operating program from this device. It cannot be written over (as opposed to RAM, which we are sure you have heard of). It is made with the computer program already etched in right at the factory. PROM means Programmable ROM. A PROM is a device like a ROM, except that it leaves the factory blank and can be filled with a program once, and only once. An EPROM is an Erasable PROM. It can be erased after being written, but only with an intense ultra-violet light source. That is typically why you see thick labels on them - so they won't accidentally get erased due to prolonged exposure to sunlight or flourescent lights. Why don't auto manufacturers use ROMs or PROMs instead of EPROMS? We suspect that at the beginning of a model year, many things in the ECU computer program are still in flux. After all, the smog testing for the entire model line may not have been completed yet, and the smog rules themselves might have changed since they designed the car. In order to accomodate any last minute quick fixes, the manufacturers use EPROMs so they won't get stuck with a bunch of ECUs that have the wrong program in them.