When power is first supply power to computer system, a program in read only memory (ROM) executes. After performing some diagnostic checks; a stage-0 window XP boot program is executed.
Stage-0 Boot Program[wp_ad_camp_1]
The stage-0 program checks the presence of one or more boot devices for window. On PC, boot devices are floppy disk, CD-ROM, or a hard disk. Once found the stage-0 boot program, it read the first sector of boot device and send it into the main memory.
Stage-1 Boot Program
The first sector of the boot device is known as the boot sector for the window. It contains a stage-1 boot program. After it has been loaded into memory the stage-0 boot program branches to the stage-1 boot program.
Rather in some cases, boot sector is not initialized. Now, it copy undefined values into the memory. Before transferring control to the next stage in the boot process, the current boot program will check one or more bytes for a “magic” bit pattern. This pattern is use to indicate the sector, which contains valid boot program. While on some systems, operating system reads stage-1 boot program.
While in booting from a hard disk, the disk is divided into the four primary partitions. The partition table contain the starting and ending location of each partition. The partition table define one partition as active partition.
The stage-1 boot program reads the partition table and copy it into memory of first sector of the active partition. The first sector of the active partition have a stage-2 boot program. It is designed to work with the operating system in that partition. It may also capable for loading boot programs or operating systems from other partitions. The stage-2 boot program loads the partition of operating system into memory.
Kernel Mode in window
At the end of boot process, the operating system is loaded into memory and boot program is branched to the operating system. Now, the machine is executing in “kernel mode”.