Booting Sequence:

 WHAT IS BOOT SEQUENCE?


The boot sequence—sometimes called BIOS boot sequence or BIOS boot order—is the order of devices listed in BIOS that the computer will look for an operating system on.

Although a hard drive is usually the main device a user may want to boot from, other devices like optical drives, floppy drives, flash drives, and network resources are all typical devices that are listed as boot sequence options in the BIOS.



There isn't a single order that everyone needs to order all the time. Under normal conditions, the hard-drive should be listed first, but these lists aren't actually helpful.

The order of booting depends upon specific situations. If you're downloading Windows OS, you should change the boot order to the disc or removable device is listed first in order to start the operating system


More on BOOT Sequence:

After the power on self test, the BIOS (basic input output system) will attempt to boot from the first device in the boot order. If that device isn't bootable, then the device will move on to the next device listed in the order.

If you have two hard drives installed and just one controls the operating system, that particular device must be listed first in the order. If not, it is possible BIOS will hang there , thinking the hard drive should have an OS present but in actual reality it doesn't. Just change the boot order to have the actual OS on top, and it will boot properly.

For example: pressing the F9 to change the BIOS systems to the default settings.


Running POST: After the BIOS starts, it starts a process called POST (Power-on Self Test) which tests all the hardware devices and makes sure that it has no issues. Moreover, if the POST does find some issues, the booting process stops and the computer, as a result, stops to boot.

Loading MBR to RAM: After running POST, the BIOS proceeds to load the MBR (Master Boot Record) from the bootable devices into the RAM. The MBR consists of 512 or more bytes located at the beginning sector of the bootable devices (which could either be an HDD, SSD or hard drive)

Running the BootLoader: After loading the MBR into RAM, the BIOS runs the first instruction loaded into the MBR. The first instruction is typically the bootstrap code, aka the bootloader, which is a program written in machine language that loads the operating system into RAM.




Each operating system has it's own bootloaders. . For example: GNU GRUB, LILO (Linux Loader) and rEFlnd are a few LINUX Bootloaders.

Running the OS: Once the OS is loaded into the memory, the OS starts running . Further, the OS starts it's own initialization process (which includes loading device drivers , setting up libraries etc.) . Finally, the OS initialization is finished, it starts a shell that displays a login prompt to the user.


Sources:

The following are the resources ive used to complete my research and solidify my conclusions on the Booting Sequence and all concepts of a BOOTLOADER.




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