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At one time, engineers viewed the BIOS as a great sculpture or other great work of art, fixed forever in time, perfect and immutable. The BIOS code would be written in read-only memory chips that could not be altered, at least by mere mortals without electrical engineering degrees. But human frailties and rapidly changing technologies forced them to change their minds. They made mistakes in writing some BIOSs that only became apparent after they were shipped off to consumers in finished computers. Or, a few months after they had finished work on their BIOSs—and after computers using them found their ways into consumers' hands—someone decided it would be great if hard disks could store twice as much as the BIOS made allowances for. The only way to make up for the shortcoming of the permanent BIOS was to replace the memory chips, a task most people face with the same calm ease as disarming a tactical nuclear weapon.

Consequently, today most computers come with their BIOS code written in Flash ROM chips, which can be rewritten with relative safety and without even removing the lid from your computer system. If your BIOS needs an upgrade, either to fix an error or to accommodate a new technology, you can usually download a new BIOS from the Web site of the computer's manufacturer.

Updating a Flash BIOS usually involves two files: a binary file that contains the actual BIOS code, and a special program called a loader. The loader activates the motherboard circuitry required to write to the BIOS chips and then transfers the contents of the binary BIOS file to the chips. The loader's job is one of the most delicate in the operation of a computer because if it makes a mistake, your system may be rendered inoperable. It might not be able to boot up and rerun the loader. Consequently, for your security, some system-makers require you take elaborate precautions before you try making a BIOS upgrade. For example, the manufacturer may require you to use both the battery and AC power supplies of a notebook computer to make a BIOS update.

To prevent a BIOS upgrade from accidentally rendering your computer inoperable, many computers include boot block protection. This feature simply protects or prevents the block of the Flash ROM used for the essential bootup code—the code necessary to let your system read a floppy disk—from being erased during the upgrade. The protected code is sufficient to get your floppy going so that you can try upgrading your BIOS once again (or if the new BIOS itself is the problem, restore your old BIOS).

Because motherboard circuitry varies among different computer brands and models, the BIOS loader made for one system may not work with another. Unless you're advised otherwise by your computer's manufacturer or a BIOS-maker, you should assume that you need a matched pair of files—the BIOS code and loader—to make a BIOS upgrade. In general a loader will warn if it is not meant for your particular system, but don't depend on such a warning to avoid incompatibilities. A loader that makes a mistake can mean your computer must go back to the factory for new BIOS chips before it will boot again.

Along with fixing problems, upgrading a flash BIOS can sometimes cause them, too. BIOS-makers usually design their flash BIOS loaders to reset your computer's setup parameters to the factory defaults so your system gets a fresh start with its new BIOS. If you've customized your system before making the upgrade (for example, adjusting the energy conservation options of a notebook machine), you'll have to re-establish your settings after the upgrade. Sometimes you may find that the load process has altered the date set in CMOS. Operating your system without checking and resetting the date can cause problems with programs sensitive to the relationship between the current time and file date codes. For example, your backup system or disk maintenance program may not work properly after you've upgraded your BIOS if you don't properly restore your system clock.

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