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Immediately after the computer is started, the CPU runs the instruction located at the memory location FFFF0h or boot sector of the BIOS.
This holds jump instruction that transfers execution to the place where the BIOS start-up program is found. A Power-On Self Test (POST) is run to see that the devices connected to the computer are working properly.
Now, the BIOS will go through a list of devices already configure till it finds the one that is bootable. If it does not come across any such device then an error is thrown and the boot process halts. Incase a bootable device is found then it loads and executes its boot sector.
When we talk of a hard drive, this is referred to as the master boot record (MBR). The MBR code verifies the partition table for an active partition and when found, it loads that partition's boot sector and executes it.
