Booting the Computer

Booting the Computer

Very shortly, the Pentium boot process is as follows. Every Pentium includes a parentboard (previously called a motherboard before political correctness hit the computer industry). On the parentboard is a program called the system BIOS (Basic Input Output System). The BIOS includes low-level I/O software, including procedures to read the keyboard, write to the screen, and do disk I/O, among other things. Nowadays, it is held in a flash RAM, which is nonvolatile but which can be updated by the operating system when bugs are found in the BIOS.

When the computer is booted, the BIOS is started. It first checks to see how much RAM is installed and whether the keyboard and other basic devices are installed and responding correctly. It starts out by scanning the ISA and PCI buses to detect all the devices attached to them. Some of these devices are normally legacy (i.e., designed before plug and play was invented) and have fixed interrupt levels and I/O addresses (possibly set by switches or jumpers on the I/O card, but not changeable by the operating system). These devices are recorded. The plug and play devices are also recorded. If the devices present are different from when the system was last booted, the new devices are configured.

The BIOS then decides the boot device by trying a list of devices stored in the CMOS memory. The user can modify this list by entering a BIOS configuration program just after booting. Typically, an attempt is made to boot from the floppy disk, if one is present. If that fails the CD-ROM drive is queried to see if a bootable CD-ROM is present. If neither a floppy nor a CD-ROM is present, the system is booted from the hard disk. The first sector from the boot device is read into memory and executed. This sector includes a program that usually inspects the partition table at the end of the boot sector to decide which partition is active. Then a secondary boot loader is read in from that partition. This loader reads in the operating system from the active partition and starts it.

The operating system then queries the BIOS to get the configuration information. For each device, it checks to see if it has the device driver. If not, it asks the user to insert a CD-ROM containing the driver (supplied by the device's manufacturer). Once it has all the device drivers, the operating system loads them into the kernel. Then it initializes its tables, creates whatever background processes are required, and starts up a login program or GUI.


parentboard, bios, operating system