Rigid Operating System

• CPU executing in real mode (addrs correspond to actual physical addrs)
• CPU looks at reset vector (FFFFFF0h for 32/64 bit) for firmware entry point
• Usually this location is a jump instruction that moves execution to location of firmware start-up program
• check and initialize required devices: memory, PCI bus, PCI devices
• firmware goes through list of non-volatile storage devices looking for bootable device
• bootable device is one that can be read from and where last 2 bytes of the first sector contain the little-endian word AA55h (MBR boot signature)
• once a bootable device is found, it loads boot sector to linear address 7C00h and transfers execution to boot code.
• for a hard disk, this address is the master boot record - conventionally the code here checks the partition table for a partition set as bootable i.e. the one with the active flag set.
• if an active partition is found, mbr loads and executes boot sector code from that partition - this code is called volume boot record
• boot sector code is first-stage boot loader - must fit into first 446 bytes of the MBR to leave room for the 64 byte partition table with 4 partition entries + the 2 byte boot signature
• this configuration differs for different first-stage boot loaders
• vbr loads and executes operating system boot loader - this is the second-stage boot loader

• detect available RAM
• pre-initialize CPU and hardware
• look for bootable disk
• boot operating system kernel