System Memory (continued...)

EPROM (Erasable Programmable Read Only Memory)
(Pronounced 'e-prahm')

An EPROM is a special kind of PROM chip that can be reprogrammed. Its information is stored as electrical charges deposited on the chip (1s and 0s). EPROMs are easy to spot on your computer's motherboard. They're still in the form of a DIP chip like ROMs and PROMs, but they'll have a metallic-like label on top of the chip (usually displaying the serial number, version, date and manufacturer's name). This is for good reason. The label is covering a small window.

An EPROM can be erased by removing it from the circuit, and shining an ultraviolet light through the window on top of the chip. It can then be reprogrammed using an EPROM burner. EPROMs are still considered non-volatile, they won't lose their information when the computer is turned off.

EEPROM
(Electronically Erasable Programmable Read Only Memory)
(Pronounced 'double e-prahm')

EEPROM doesn't have to be removed and sent back to the manufacturer to be reprogrammed. It can be electronically reprogrammed while in circuit using a software program designed specifically for that purpose. Also, EPROM has to be erased entirely and then reprogrammed. With EEPROM, a single byte can be erased and re-written. In fact, EEPROM is erased and written one byte at a time, which makes it quite slow as memory goes. Still, it allows manufacturers the ability to put configuration settings on an expansion card's EEPROM chip. By using software that came with a device, DMA channels, IRQs and I/O addresses can be assigned without the use of jumpers and DIP switches. The resource settings for software configurable devices can be changed without even opening the computer's case.

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