The last step in the PC start-up is reading the operating system. The start-up program is instructed to find the Master boot sector. The boot sector is the very first sector on either hard disk (C) or floppy drive A.
By default, the PC will look for a boot sector in floppy drive A. That is why the PC "drops dead" if there is a different diskette in A drive. If there is no diskette in A drive, the start-up program will search for the boot sector on hard drive C. When the boot sector is found, a small program segment (boot-strap) is read from there. The boot-strap then takes over control of the PC. The start-up program has done its job. Now DOS, Windows, or another operating system takes control.
Here is an illustration of the start-up process:
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