Initial Startup Process

After the successful execution of the POST, the system BIOS then checks the first hard disk that is powered up.

When the hard disk is the startup disk, the system BIOS reads the master boot record (MBR) and loads it into memory. The system BIOS then transfers the execution of the startup process to the MBR. After the MBR loads a copy of the active partition's boot sector into memory, the boot sector code starts the operating system as defined by the operating system.