Small program written on the first sector of the hard disk or on the first sector of a partition which contains an operating system. At boot time that program is loaded then executed. If there are several operating systems the master boot record (mbr) is taking control first and gets a menu displayed on the screen asking the user which OS is to be started. Then once the user has chosen that mbr locates and activates the boot loader which is in the first sector of the partiton where that OS resides. And that boot loader has the function of activating the central core (kernel) of the operating system in order to launch it.

          The mechanism is detailed

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