Normally Windows operating system boots according to the following sequence:
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1. BIOS loads the MBR, verifies 55 AA signature and transfers control to Master Boot Record.
2. MBR scans the partition table for active partition, then it loads the first sector from active partition, verifies 55 AA signature, and transfers control to Boot Sector.
3. Boot Sector loads 15 additional sectors from the start of the disk (Volume Boot Record) and transfers control to first additional sector of VBR.
4. VBR loads BOOTMGR file from root directory of boot partition into the memory and transfers control to it.