The first sector on the hard disk is critical to the startup process. This sector contains the Master Boot Record and the Partition Table.
If the startup disk is a floppy disk, the first sector on the disk is the Partition Boot Sector.
For more information about the Master Boot Record, the system partition, and the Partition Boot Sector, see "Disk Sectors Critical to the Startup Process," presented earlier in this chapter.
After the POST on x86-based computers, the system BIOS attempts to locate the startup disk. If there is a floppy disk in drive A, the system BIOS uses drive A as the startup disk. If there is no disk in drive A, it then checks the first hard disk that is powered up.
Note
Some system BIOS versions enable the user to reconfigure the order in which it checks the floppy disks and hard disks for the startup disk.
When the hard disk is the startup disk, the system BIOS reads the Master Boot Record, and loads it into memory. The system BIOS then transfers execution to the Master Boot Record. The code in the Master Boot Record scans the Partition Table for the system partition. When the Master Boot Record finds the system partition, it loads sector 0 of the partition into memory, and executes the code in the sector. Sector 0 on the system partition can be a program or diagnostic program, or a Partition Boot Sector that contains startup code for an operating system. The Partition Boot Sector code starts the operating system in a manner defined by the operating system.
If there is no system partition on the first hard disk, the Master Boot Record displays errors such as the following:
If there is a floppy disk in drive A, the system BIOS loads the first sector on the disk into memory. If the floppy is bootable, the first sector on the disk is the Partition Boot Sector. If the floppy disk is not bootable, you see errors such as:
After a RISC-based computer completes the POST routine, the resident ROM firmware selects a startup disk by reading a boot precedence table from nonvolatile RAM (NVRAM). What the firmware does depends upon the platform (Alpha, PPC, or MIPS computer) and the information in the NVRAM. For example, to startup from the floppy disk, the NVRAM must define it as an alternate boot selection.
The NVRAM also defines the:
The system partition can be on any hard disk on RISC-based computers. You can use the Boot selection menu to set up or change the system partition. See "Manage Boot Selection Menu," in Chapter 19, "What Happens When You Start Your Computer," of the Windows NT Workstation Resource Guide for more information about changing the NVRAM on RISC-based computers.
The system partition on a RISC-based computer must be formatted with the FAT file system.