Chapter 15 PC Troubleshooting 669
Initial Startup Phase
In the initial startup phase, the POST completes and then looks for the boot
device where the master boot record (MBR) is stored. The BIOS configuration
determines the order for the computer system to locate the next boot device.
The boot device could be the floppy drive, the hard disk drive, the CD or DVD
drive, or the USB Flash drive. The exact order can be changed in the BIOS Setup
program and stored in the CMOS memory. In general, the computer uses the
hard disk drive as the boot device. Exceptions are when a floppy is used to
startup the computer or when an installation CD or DVD is used to install an
operating system or for system recovery.
After identifying the location of the MBR, the BIOS loads the MBR into
RAM. For Windows 95, 98, and Me, the BIOS then loads the
io.sys
file into RAM.
You should recall that the
io.sys
file contains generic drivers necessary for
communicating with hardware devices such as the monitor, floppy drive, hard
drive, and keyboard. This
io.sys
file then loads the file allocation table (FAT)
into RAM. The FAT is a table of all the files on the hard drive along with their
attributes and locations. For Windows 2000 and XP, the BIOS loads the
ntldr
file
into RAM, and for Windows Vista, it loads the Windows Boot Manager (bootmgr).
Keep in mind, you cannot use a non-bootable CD, DVD, or floppy to start the
computer. When a non-bootable media is encountered during the boot sequence,
an error message will appear on the screen. Some possible errors include the
following:
✔ Non system disk.
✔ Missing Ntloader (ntldr).
✔ Hard disk errors.
In an EFI system, a GUID Partition Table (GPT) is used instead of an MBR
to locate partitions on a physical disk(s). This table overcomes the partition
limitations imposed by the MBR. Before GPT, Microsoft operating system
partitioning was based on the limitations of the MBR. Partitions could consist of
four primary partitions or three primary partitions and one extended partition
subdivided into logical partitions. This is an archaic partitioning system, which
evolved from the DOS file system. The maximum number of partitions that can
be supported by the Microsoft operating systems is 128. Since the GPT does
not have the same limitations of MBR-based partitions, you can have almost an
unlimited number of partitions using GPT. EFI does not require a GPT partition
and can be used with a partition system based on MBR. Also, EFI can use a disk
system that contains both GPT and MBR partitions.
Since the rest of the boot sequence is different for each of the three types of
operating systems, the rest of this section is organized by operating system. Each
operating system boot sequence discussion continues from the initial startup
phase to the last phase for that particular operating system.
Windows 95, 98, and Me
In the previous stage, the
io.sys
file was loaded into memory, and the
io.sys
file
loaded the FAT into memory. With the FAT in memory, the
io.sys
file can locate
the
msdos.sys
file. The
msdos.sys
file is a text file that contains references to items
such as where the Windows files are located and options for displaying the boot
menu.