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Bootloaders

Before discussing boot managers, how a PC itself boots merits discussion. Today’s computers (that is, x86 PCs with IDE and SCSI hard drives) start by reading the BIOS ROM which is stored on the hard drive. The BIOS either detects or is told by the settings within what devices to look at to boot from. Typical order of devices booting is floppy drive, CD-ROM, and then hard drive. This order can be changed by logging into the PC BIOS. Commonly you hold down the escape or F1 key after powering on the PC to enter the BIOS setup.

After reading the BIOS the PC then looks to the devices. Once it reads a device, for example a hard drive, it reads the Master Boot Record (MBR), then the partition boot sector (specified in the MBR), and then it loads the operating system kernel. Whether you use Windows or Linux the order is the same. DOS, OS/2, and Windows NT all have an optionally installed bootloader called NTLDR that allows you to choose where to boot from. With Linux there are bootloaders as well; the two most common ones are GRUB and LILO.


  

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