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The DOS Boot Process
The system boot sequence is the series of steps that the system performs when it is turned on (or rebooted with the reset switch, for example). This always starts with the special boot program software that is in the system BIOS ROM. The BIOS has several steps that it must perform to test the system and set it up, before any operating system can be loaded. These steps are described in detail here.
Once the BIOS has completed its startup activities, the last thing it does is to begin the process of loading the operating system. It does this by searching for a boot device containing boot code to which it can hand off the boot process. It will search for boot devices in the order specified by the BIOS setting that controls the boot sequence. If it cannot find a boot device it will terminate with an error.
Assuming that the BIOS finds a boot sector on a device, the process of loading the operating system begins. If the operating system is DOS, or any variant of Windows that starts out by booting the equivalent of DOS--which is all of them other than Windows NT or Windows 2000--then a specific operating system load sequence commences, which is normally called the DOS Boot Process. In the case of Windows, additional steps are added to the end of the process after the underlying DOS operating system has loaded.
The process below outlines how booting from the hard disk functions. Booting from the floppy disk differs only in the first few steps, because the floppy disk's structures are slightly different. Floppies cannot be partitioned, and hence have no master boot record or partitions. This means that the steps where the master boot record are searched are skipped.
Here are the steps in the DOS boot process:
At this point the operating system code itself has control of the PC. In the case of 32-bit Windows versions like Windows 95/98/ME, the steps above are only the beginning of the process. The initial DOS operating system files control the loading and execution of many more routines as the boot progresses, which perform tasks such as reading the system registry, initializing hardware devices and starting the graphical operating system shell. In fact, it is surprising in some ways just how many different pieces of code have a hand in starting up the PC.
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