Product specifications

PhoenixBIOS D686. This
BIOS chip is housed in a
PLCC package in a socket.
peripheral devices, a password for booting the system, or a hard disk password that limits access
to it and stays assigned even if the hard disk is moved to another computer.
Reprogramming
In modern PCs the BIOS is stored in rewritable memory, allowing the contents to be replaced or
"rewritten". This rewriting of the contents is sometimes termed flashing, based on the common use of a
kind of EEPROM known technically as "flash EEPROM" and colloquially as "flash memory". It can
be done by a special program, usually provided by the system's manufacturer, or at POST, with a
BIOS image in a hard drive or USB flash drive. A file containing such contents is sometimes termed
"a BIOS image". A BIOS might be reflashed in order to upgrade to a newer version to fix bugs or
provide improved performance or to support newer hardware, or a reflashing operation might be
needed to fix a damaged BIOS.
Overclocking
Some BIOS implementations allow overclocking, an action in which the CPU is adjusted to a higher
clock rate than its manufacturer rating for guaranteed capability. Overclocking may, however,
seriously compromise system reliability in insufficiently cooled computers and generally shorten
component lifespan. Overclocking, when incorrectly performed, may also cause components to
overheat so quickly that they mechanically destroy themselves.
Hardware
The original IBM PC BIOS (and cassette BASIC) was stored on mask-
programmed read-only memory (ROM) chips in sockets on the
motherboard. ROMs could be replaced, but not altered, by users. To
allow for updates, many compatible computers used re-programmable
memory devices such as EPROM and later flash memory devices.
According to Robert Braver, the president of the BIOS manufacturer
Micro Firmware, Flash BIOS chips became common around 1995
because the electrically erasable PROM (EEPROM) chips are cheaper
and easier to program than standard ultraviolet erasable PROM
(EPROM) chips. Flash chips are programmed (and re-programmed) in-
circuit, while EPROM chips need to be removed from the motherboard
for re-programming.
[16]
BIOS versions are upgraded to take advantage
of newer versions of hardware and to correct bugs in previous revisions
of BIOSes.
[17]
Beginning with the IBM AT, PCs supported a hardware clock settable through BIOS. It had a century
bit which allowed for manually changing the century when the year 2000 happened. Most BIOS
revisions created in 1995 and nearly all BIOS revisions in 1997 supported the year 2000 by setting the
century bit automatically when the clock rolled past midnight, December 31, 1999.
[18]
The first flash chips were attached to the ISA bus. Starting in 1997, the BIOS flash moved to the LPC
bus, a functional replacement for ISA, following a new standard implementation known as "firmware
hub" (FWH). In 2006, the first systems supporting a Serial Peripheral Interface (SPI) appeared, and the
BIOS flash memory moved again.