Although the primary purpose of an MBR is to start the bootstrap process, the code in the MBR is not entirely needed. This is because the partition table of an MBR also indicates which partition is bootable. Upon a reboot or power up, the BIOS of a PC reads the partition table and determines how to start up a system. It is only when the partition table does not define a bootable partition that the code of the MBR be executed.