Booting Up the System

Booting Up the System

What the BIOS actually does during its boot sequence varies slightly from manufacturer to manufacturer, but here are the steps performed during the boot sequence:

1. When the PC is powered on, the internal power supply initializes. The power supply doesn't immediately provide power to the rest of the computer. First, it determines whether it can supply the proper voltages that the PC's components require. The power supply sends out a POWER GOOD signal when it determines that it can supply reliable power to the rest of the PC. When the chipset receives this signal, it issues a SYSTEM RESET signal to the processor.

2. When the processor receives the SYSTEM RESET signal, it accesses the jump address for the start of the BIOS boot program at its hard-wired preset address and loads it into RAM.

Remember The jump address contains the actual address of the BIOS boot program on the ROM BIOS chip. The jump address is typically located at address FFFF0 (hexadecimal) or 1,048,560 (decimal), which is at end of the first megabyte of system memory. For more information on hexadecimal and other number systems used in the PC.

3. With the primary part of the BIOS now loaded to RAM, the POST process begins.

If any fatal errors happen during the POST process (problems that prevent the PC from operating normally), the appropriate error beep codes sound or perhaps an error message displays, and the boot process stops. At this point in the boot process only the system speaker (because it's technically part of the motherboard) can notify the user of errors.

4. If all is well, the boot sequence continues and the system BIOS loads the device BIOS of the video adapter (if there is one) and loads it to memory.

As your PC boots, the video adapter's information displays on the monitor.

5. Any other device-specific BIOS routines, such as those for the hard disks or SCSI devices, are loaded.

Information, usually including the manufacturer and the BIOS version, displays. The BIOS begins a series of tests on the system, including a run-up count of the amount of memory detected on the system. Because the display is now available, any errors found in this process are displayed on the monitor as an error message instead of a beep code played through the system speaker.

6. The system determines if the devices listed in the CMOS configuration data are present and functioning, including tests for device speeds and access modes.

7. Remember: The serial and parallel ports are assigned their identities (COM1, COM2, LPT1, and so on), and a message is displayed for each device found, configured, and tested.

If the BIOS program supports Plug and Play (PnP), any PnP devices detected are configured.

Although it usually goes by much too fast to read, the BIOS displays a message for each device it finds and configures.

8. The configuration is confirmed.

The BIOS displays a summary screen that details the computer as the BIOS sees it. This summary screen signals that the system is verified and ready for use.

9. The BIOS looks in the CMOS data to determine which disk drive to use for the operating system.

If the boot device is the hard disk, the BIOS looks for the master boot record. If the boot device is a floppy disk or a CD-ROM, it looks at the first sector of the disk for the operating system's boot program. If the boot program is not found on the first device listed, the next device indicated is searched, and then the third, and so on until the boot program is found.

If no boot device is found, the boot sequences stops and an error message ("No boot device available") is displayed.