When a computer is turned on , the computer 's BIOS finds the primary bootable device ( usually the computer 's hard disk ) and loads the initial bootstrap program from the master boot record ( MBR ) , the first 512 bytes of the hard disk , and then transfers control to this code . When a computer is turned on , the computer 's BIOS finds the primary bootable device ( usually the computer 's hard disk ) and loads the initial bootstrap program from the master boot record ( MBR ) , the first 512 bytes of the hard disk , then transfers control to this code .