Chapter 6: The BIOS and the Boot Process^119
As shown in Figure 6-5, the EPROM chi phas a quartz crystal window on the face of
the chip. This erasing window allows ultraviolet (UV) light to access the chip’s interior
circuitry. The UV light causes a chemical reaction that turns the 0s back into 1s, thereby
erasing the EPROM. There is normally a label or piece of dark tape placed over the eras-
ing window to prevent accidental erasures.Electronically Erasable Programmable Read-Only
Memory (EEPROM)
Newer PCs feature a newer type of ROM BIOS chip. The EEPROM (pronounced
“e-e-prom” and meaning Electronically Erasable Programmable Read-Only Memory)
can be reprogrammed like the EPROM, but the EEPROM does not need to be removed
from the motherboard to be reprogrammed. An EEPROM is reprogrammed or updated
with specialized software usually supplied by the BIOS or chip manufacturer. The
process that updates an EEPROM under software control is calledflashing, which is why
an EEPROM is also referred to asflash ROM.
Because they are easily upgraded, EEPROM chips are used in a variety of applica-
tions, such as cars, modems, cameras, and telephones. Flashing allows you to easily apply
bug fixes or add new features to your system that may not have been available at the time
yoursystemwasmanufactured,suchasbootingtoaCD-ROMdrive.ImprovingtheBIOS
can also add new routines that improve your system’s boot or overall performance.Figure 6-4. A ROM chip on a computer motherboard