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Observing the steps of a boot up process
can reveal a great deal about the health of the system and what is
causing problems. The possible causes of errors
can be eliminated or determined during the boot up procedure. If the problem
lies in the boot up process, only some or possibly even none of
those steps will be observed. Knowing the details of each step of the
boot process will help determine what section failed if the system
will not boot up past that step. For example, if the system boots up
all the way to the point where it is checking the floppy drive, and
the floppy drive light does not come on, then the problem exists in
the floppy drive.
The Bootable Disk
One tool that every system administrator should have is a bootable
disk. Often systems have issues that cannot be resolved, or the
operating system has become so corrupt that despite all efforts the
system will not start or boot up. A bootable disk allows
administrators or service technicians to boot from a diskette instead
of the hard drive. This diskette can be used to fix issues that may
arise during the lifetime of the computer. If a system is unable to
boot, simply insert the disk into the floppy drive and restart the
computer, making sure that the BIOS is set to boot from the floppy
drive. The disk possesses the necessary files that are required to
boot the system and bypass the operating system files on the hard
drive that are corrupted. Booting from a bootable disk will bring up a
DOS prompt to allow the administrator to recopy files, inspect the
hard drive partition, or reformat the drive if the bootable floppy has
the fdisk executable on it. It will enable the
administrator to navigate through the files on the hard drive for
problem inspection.
Hidden Files
A hidden file is a file with a special hidden attribute turned on, so
that the file is not normally visible to users. For example, hidden
files are not listed when the DOS
dir command is executed. However,
most file management utilities allow the viewing of hidden files.
DOS hides some files, such as msdos.sys and io.sys, so that
users will not accidentally corrupt them. These two files are the
special files in the operating system boot record. Without them,
the system cannot boot up successfully. The hidden attribute for
any normal file can also be turned on, making it invisible to
casual snoopers. This also prevents users from corrupting those
files. The
attrib command is used to view and change file
attributes.
The Command Interpreter
The command.com program is known as the command interpreter. It is the
most important system file, since the computer cannot boot without it.
command.com contains the most commonly used commands of the operating
system. For example, when a DOS command is entered at the DOS
prompt, the command.com program first examines it to see if it is an
internal or external DOS command. Internal commands are understood by
the command.com program and are executed immediately. External
commands are stored in the \DOS directory and the command.com file
must browse through the \DOS directory to find the command program.
When DOS runs an application, command.com finds the program and
then loads it and gives it control of the system. When the program
is shut down it passes control back to the command interpreter.
DOS Switches
Many of the common DOS switches were discussed in Chapter 4,
"Operating System Fundamentals". In this chapter, a few more
switches that are useful in troubleshooting software problems are
added to the list. As a review, DOS switches are used to configure DOS
commands to perform specific functions. For example, DOS switches are
used in the config.sys file to configure DOS to emulate different
hardware configurations. The
switches
command is useful when using older applications with a new enhanced
keyboard that is incompatible with the operating system. Entering the
line switches=/K in the config.sys file will configure the
keyboard to act like a standard keyboard.
Using these switches can be helpful when troubleshooting
computer systems. They are important to remember because they can
be used to manipulate the DOS commands.
The following is a list of examples of common switches:
- /K – Causes an enhanced keyboard to act as though
it were an older, standard keyboard.
- /F – Removes the two-second delay that occurs when the
"Starting MS-DOS" message is displayed on the screen
(DOS Version 6 only).
- /N – Disables the F5 and F8 keys during system start up
(DOS Version 6 only).
- /W – Tells DOS that the wina20.386 file has been moved to
a directory other than the root directory.
Bootable Disk Utility Files
A DOS boot disk, as mentioned in Chapter 4, can be used to boot the
system to the DOS prompt. This allows a system having boot problems to
start, so troubleshooting can begin. In this regard, there are a few
useful utility files that should be added to the boot disk. These are
the format and fdisk utilities. The significance of these
utilities is discussed next. Figure
shows a list of common files associated with a boot disk.
Format
In the event that the operating system becomes so corrupted that it is
beyond repair, the
format
command can be used to erase the disk and start over. The boot up disk
allows the administrator to boot the computer to a DOS prompt. The
format utility allows the reformatting of the hard drive if needed,
however, all of the information on the hard drive will be erased. The
format
command is also used to prepare a new disk for use with an operating
system.
Fdisk
Fdisk is one of the most commonly used MS-DOS commands, even with
newer operating systems such as Windows 98, 2000, and XP.
Fdisk
allows the technician to delete and create
partitions on the hard drive. This is another useful tool that will
help when troubleshooting a computer system. The boot disk will allow
the technician to boot to a DOS prompt. Once this is completed, type
fdisk
to enter
the screen. It gives access so
changes can be made to the systems hard disk partitions.
Sometimes during troubleshooting calls, the computer will be
unable to boot. This typically means that the Master Boot Record
has been damaged or corrupted in some way. A virus can cause this
or the files could have just been deleted for some reason. The Master
Boot Record (MBR) is a program that is executed when a
computer boots up. Typically, the MBR resides on the first sector
of the hard drive. The program begins the boot process by looking
up the partition table to determine which partition to use for
booting. It then transfers program control to the boot sector of
that partition, which continues the boot process. In DOS and
Windows systems, the MBR is created with the
fdisk/mbr command.
This command is used to rewrite the Master Boot Record so the
system will be able to boot up once again.
Bootable Configuration Files
The two main configuration files that a computer uses to boot up
are config.sys and autoexec.bat. Config.sys is a special file
that contains setup or configuration instructions for the computer
system. The commands in this particular file configure the DOS
program for use with devices and applications in the system and
set up the memory managers in the system. Once the commands in
config.sys have been carried out, DOS begins searching for the
autoexec.bat file. This file contains a list of DOS commands that
will automatically execute when DOS is loaded into the system
(config.sys and autoexec.bat are discussed more thoroughly in
Chapter 4).
Without these files, or if they were corrupted, the system
would still boot. However, these two files are essential for the
complete boot up process to occur with the DOS operating system.
They contain information that is used for things like changing the
operating system for personal uses. They also contain the
requirements of different software application packages. A
troubleshooting call would result if either of these two files
became damaged or corrupted.
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