Much the way that a PC motherboard has its system
BIOS--low-level software that runs the chipset
and other key components of the motherboard--hardware RAID controllers do too. The
controller BIOS is the code that operates the controller, managing the RAID array.
Over time, the manufacturer of the RAID controller may change the BIOS code; this is
usually done to correct minor problems with the controller card, but sometimes occurs to
enable new features. It is also occasionally done to enable support for hardware that did
not exist at the time the controller shipped. Like PC BIOSes, RAID controller card BIOSes
can usually be upgraded through a flash process similar to that used for motherboards. A new BIOS is
typically downloaded from the web site of the manufacturer and written to the EEPROM
(flash chip) on the RAID controller. A special software program, called a flash
program for unsurprising reasons, is used to ensure that the BIOS upgrade works
properly.
Warning: To avoid
potential problems, always follow the manufacturers' instructions carefully when
attempting to flash any BIOS. Doing the flash procedure incorrectly can render
the controller card inoperative. Always use the right BIOS image for your card, and make
sure the data on the array has been backed up before you begin.
It's not necessarily a good idea to update the BIOS every time the manufacturer puts
out new code. Check the release notes for the new version to find out what has changed; if
no important changes have been made that affect how you will use the controller, you may
not need to bother.
For software RAID there is no "controller BIOS", since the operating system
is running the array. The analog for the controller BIOS upgrade here is the operating
system patch, which most PC users know all too well. 
It's rare for problems with
RAID in an operating system to occur that necessitate such a software patch, but it's
still worth keeping current on whatever operating system you are using, since patches
usually correct scores (or even hundreds) of bugs and problems.
Next: RAID Interfaces
