4. The computer loads and initializes the NTLDR file, which is the operating system
loader.
Note Windows XP Professional Setup modifies the boot sector during installation so that
NTLDR loads during system startup.
There are a number of problems that can occur during the preboot sequence, including
the following:
Improper hardware configuration or malfunctioning hardware If the BIOS
cannot detect a hard drive during its POST routine, startup fails early during the
preboot sequence and usually presents a message stating that a hard drive cannot
be located.
Corrupt MBR If your MBR becomes corrupt (a fairly common action taken by
viruses), you can generally repair it by using the Recovery Console, which is cov-
ered in Lesson 3, “Troubleshooting Problems Using Startup and Recovery Tools.”
Antivirus software can prevent and often repair a corrupt MBR.
Floppy or USB disk inserted If you see an error message stating that there is a non-
system disk or a disk error, or stating that no operating system could be found, a
common reason is that a floppy disk or a universal serial bus (USB) flash memory
disk is inserted in the drive during startup. On most computers, BIOS is configured
by default to try starting using the floppy drive or an available USB drive before it
attempts to start by using the hard drive.
What Happens During the Boot Sequence
After the computer loads NTLDR into memory, the boot sequence gathers information
about hardware and drivers in preparation for the Windows XP Professional load
phases. The boot sequence uses the following files: NTLDR, BOOT.INI, BOOT-
SECT.DOS (optional), NTDETECT.COM, and NTOSKRNL.EXE.
The boot sequence has four phases: initial boot loader phase, operating system selection,
hardware detection, and configuration selection (described in the following sections).
Initial Boot Loader Phase
During the initial boot loader phase, NTLDR switches the microprocessor from real
mode to 32-bit flat memory mode, which NTLDR requires to carry out any additional
functions. Next, NTLDR starts the appropriate minifile system drivers. The minifile sys-
tem drivers are built into NTLDR so that NTLDR can find and load Windows XP Pro-
fessional from partitions formatted with file allocation table (FAT), FAT32, or NT file
system (NTFS).
Operating System Selection
During the boot sequence, NTLDR reads the BOOT.INI file. If more than one operating
system selection is available in the BOOT.INI file, a Please Select The Operating System
To Start screen appears, listing the operating systems specified in the BOOT.INI file. If
you do not select an entry before the timer reaches zero, NTLDR loads the operating
system specified by the default parameter in the BOOT.INI file. Windows XP Profes-
sional Setup sets the default parameter to the most recent Windows XP Professional
installation. If there is only one entry in the BOOT.INI file, the Please Select The Oper-
ating System To Start screen does not appear, and the default operating system is auto-
matically loaded.
Note If the BOOT.INI file is not present, NTLDR attempts to load Windows XP Professional
from the first partition of the first disk—typically C:\.
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