BIOS Chipology

Remember An essential part of studying the BIOS is studying the chips (as in integrated circuits made from silicon "chips") on which it is stored and delivered to the PC.

l Read Only Memory (ROM): Although not solely a BIOS chip, ROM chips are permanently loaded with instructions during the manufacturing processes. The instructions written to a ROM chip, which cannot be changed under any circumstance, are called firmware. No longer a common vehicle for the system BIOS, on earlier PCs, the BIOS was stored on a ROM chip.

A ROM BIOS chip mounted on a PC motherboard.

l Programmable Read Only Memory (PROM): A PROM is essentially a blank ROM chip that can be programmed with data or instructions. A PROM burner (also called a PROM programmer), a special device used to write to the PROM, enables you to store any data you want. The PROM burner induces high voltage (12 volts compared to the 5 volts used for normal PROM operations) to load the data to the chip. The higher voltage burns a memory location to turn its preexisting binary 1 into a 0, if needed. This process is irreversible so what you burn is what you get (WYBIWYG). After you burn that zero into the PROM, there's no going back. For that reason, you may hear PROM memory referred to as One Time Programmable Memory.

l Erasable Programmable Read Only Memory (EPROM): An EPROM (pronounced "e-prom," which isn't a dance attended over the Internet) is a variation of the original PROM with the added feature of data that can be erased so that the chip can be reprogrammed. Unlike the PROM, you can reuse the EPROM instead of discarding it when its contents are no longer valid. The EPROM has a small quartz crystal window on the top of the chip through which ultraviolet (UV) rays can access the chip's circuitry. The UV light causes a chemical reaction that erases the EPROM by turning the 0's back into 1's again. To prevent accidental erasure of the EPROM chip, a label tape is normally placed over the quartz crystal window. Figure 5-3 shows an EPROM chip. The downside of an EPROM is that it must be removed from the system to be reprogrammed, which is not always possible.

An EPROM chip.

l Electronically Erasable Programmable Read Only Memory (EEPROM): An EEPROM (pronounced "e-e-prom") is the common BIOS chip on newer systems. An EEPROM chip can be reprogrammed like the EPROM, but unlike the EPROM it doesn't need to be removed from the motherboard. An EEPROM can be updated through specialized software that is usually downloaded from the BIOS or chip manufacturer's Web site. This process is known as flashing, which is why this chip is also commonly called flash ROM . Because they're easy to upgrade, EEPROM chips are also used in a variety of other things, such as cars, modems, cameras, and telephones.