The BIOS (or Basic Input/Output System) is the code run by computers when they are initially turned on.  The BIOS is stored in flash memory (EEPROM) but can be updated with a special process from the manufacteur.  The BIOS is important in computer forensics for several reasons, include:

• Contains the system time, important for later correlations with other events
• Controls which media is used to boot the computer after the BIOS load completes.  One way to acquire a forensically sound image of a computer hard drive is to boot from the CD-ROM and acquire the image with the hard drive in read-only mode.

In a typical boot process, the BIOS would load and then look at the first sector of the primary hard drive to locate the boot sector for the installed operating system(s).