A hard disk's highest level structure consists of its Master Boot Record and its primary partitions.
The MBR resides in the very first physical sector of the disk drive
and is not part of any partition.
(This means primitive Boot PROM - BIOS - routines can access the MBR.)
A hd must have at least one primary partition and no more than four.
The MBR contains a tiny executable program and a table describing the primary partitions.
The MBR also records which primary partition is ACTIVE.
The Boot PROM/FLASH combo is capable of loading the MBR into RAM and executing it.
The MBR executable is capable of loading the program in the secondary boot sector of the active partition.
What's a partition?