Master Boot Record (MBR)
Master boot record is the information present in the first sector of any hard disk. It contains the information regarding how and where the Operating system is located in the hard disk so that it can be booted in the RAM.
MBR is sometimes called master partition table because it includes a partition table which locates every partition in the hard disk.
Master boot record (MBR) also includes a program which reads the boot sector record of the partition that contains operating system.
What happens when you turn on your computer?
Due to the fact that the main memory is volatile, when we turn on our computer, CPU
cannot access the main memory directly. However, there is a special program called as BIOS stored in ROM is accessed for the first time by the CPU.
BIOS contains the code, by executing which, the CPU access the very first partition of hard disk that is MBR. It contains a partition table for all the partitions of the hard disk.
Since, MBR contains the information about where the operating system is being stored and it also contains a program which can read the boot sector record of the partition, hence the CPU fetches all this information and load the operating system into the main memory.