System Boot Order

System Boot order is the order of devices in which the computer’s Basic Input Output System (BIOS) searches the disk drives to find the operating system files to boot from.  Hard drives, floppy drives, optical drives, flash drives, and network resources are all typical devices that are listed as boot order options in the BIOS.

Normally, after the POST operation, BIOS will try to boot from the first device listed in the BIOS boot order. If that device is not suitable for booting, then the BIOS will try to boot from the second device listed, and this process continues till the BIOS finds the boot code from the devices listed. If the boot device is not found then an error message is displayed and the system crashes or might even freeze. Errors can be caused by an unavailable boot device, boot sector viruses or an inactive boot partition.

For example, when the system is booted, first the BIOS look drive A (i.e., the floppy disk) for the OS and then the drive C (i.e., the hard disk) and so on. If the OS is stored in drive A then the BIOS will no need look further; but, if the BIOS does not find the OS in drive A then it will next look to drive C, and so on depending on the configuration of the boot sequence. In order to speed up your computers boot procedure, one can use the BIOS setup program to make it search the drive C first via the CMOS setup.

General Boot Sequence

  • Turn on the Power button
  • CPU pins are reset and registers are set to specific value
  • CPU jump to address of BIOS (0xFFFF0)
  • BIOS run POST (Power-On Self-Test) and other necessary checks
  • BIOS jumps to MBR (Master Boot Record)
  • Primary Bootloader runs from MBR and jumps to Secondary Bootloader
  • Secondary Bootloaders loads Operating System

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