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Erasable Programmable Read-Only Memory (EPROM) refers to a type of computer chip that can hold a small amount of data in resident memory. The main difference between EPROM chips and Programmable Read-Only Memory (PROM) chips is that the EPROM chips can be programmed more than one time, while PROM chips are not re-programmable. Primarily, the purposes of an EPROM chip are to provide programmers with a way to map addressable inputs to a set of pre-determined data outputs, similar to a lookup table, and to store small bits of data that reflect state changes in ongoing electrical processes.
An example of this chip's usage is in the way the operating system boots when a computer powers on. The electrical current sends a signal to the chip, which then starts the Basic Input/Output System (BIOS) firmware that controls the computer's internal hardware components. The chip stores the progressive changes in the state of the computer in memory until the operating system finishes its startup process. EPROM chips typically should not be reprogrammed without a computer manufacturer's explicit instructions to do so.
The four main types of EPROM chips are Flash Erasable Programmable Read-Only Memory (Flash EPROM), One-Time Programmable Read-Only Memory (OTPROM), Ultra-Violet Erasable Programmable Read-Only Memory (UV-EPROM), and Electrically Erasable Programmable Read-Only Memory (EEPROM). Each of the types has characteristics that make them distinct from one another; most computers use EEPROM chips because they are faster, cheaper, and smaller than earlier versions. When people use the term EPROM, they are usually referring to the ultra-violet EPROM version. The first commercial EPROM, designed in 1971 by Intel® Corporation, could only store 256 bytes of data, which is significantly less than the 8 megabytes or more that some newer EPROMs and EEPROMs can now hold.
Every EPROM chip contains a metal oxide semiconductor field-effect transistor (MOSFET) that controls its ability to conduct the flow of negatively charged electrons along the transistor's electrodes. The MOSFET allows programmers to change the chip's program through a series of electronic bursts that erase existing stored data and write new data to the chip. In order to erase and rewrite data, the process requires a special chip-programming interface that supplies the ultra-violet light frequency that actually permits data erasure and storage. EPROM chips that use ultra-violet light use protective housing devices to shield users from potentially dangerous or carcinogenic effects.