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Boot sequence is the order in which a computer searches for nonvolatile data storage devices containing program code to load the operating system (OS). Typically, a Macintosh structure uses ROM and Windows uses BIOS to start the boot sequence. Once the instructions are found, the CPU takes control and loads the OS into system memory.
The devices that are usually listed as boot order options in the BIOS settings are hard disks, floppy drives, optical drives, flash drives, etc. The user is able to change the boot sequence via the CMOS setup.
Boot sequence is also called as boot order or BIOS boot order.
Prior to boot sequence is the power-on self-test (POST), which is the initial diagnostic test performed by a computer when it is switched on. When POST is finished, the boot sequence begins. If there are problems during POST, the user is alerted by beep codes, POST codes or on-screen POST error messages.
Unless programmed otherwise, the BIOS looks for the OS on drive A first, then looks for the drive C. It is possible to modify the boot sequence from BIOS settings. Different BIOS models have different key combination and onscreen instructions to enter the BIOS and change the boot sequence. Normally, after the POST, BIOS will try to boot using the first device assigned 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, an error message is displayed and the system crashes or freezes. Errors can be caused by an unavailable boot device, boot sector viruses or an inactive boot partition.