invoked. You will need this information when you low
level format some fixed disk drives. If the write
precompensation value is the same as the highest
numbered cylinder on the disk, it means that the disk
does not require write precompensation.
Creating a DOS Partition
Upon completion of the low-level format, a fixed
disk to be used in a personal computer needs to be
partitioned. To partition a disk, run the DOS FDISK
program. Partitioning a fixed disk divides the disk into
one or more logical drives. The drive must be
partitioned even if the entire drive will be one large
partition. DOS 3.3 allows a maximum partition of 32M.
DOS versions 4.0 and greater allow DOS partitions of
up to 4 gigabytes. If you have a drive larger than 32M,
and are using DOS 3.3, you can divide the disk into two
logical drives to fully use the disk. Refer to the
primary partition as drive C and the extended partition
as drive D.
Running FDISK on the disk prepares the DOS boot
sector so the high-level format program will operate
correctly.
Partitioning will also allow you to have two
different operating systems on the same disk. The
primary partition will have DOS, where the extended
partition can be set-up to run with OS/2, UNIX, or some
other operating system.
High-Level Format
The last step in preparing a fixed disk for use in a
personal computer is to run the DOS high-level format
program. This program creates the FAT and an empty
root directory so DOS can manage files. If the drive is
to be used to boot the computer, this format will also
write the two hidden system files and the
COMMAND.COM file. Use the command FORMAT
C: /S to create a bootable disk. If the disk is to be used
for data storage only, do not be use the/S switch.
RECOVERING DATA FROM FIXED
DISK DRIVES
Loss of data on a fixed disk drive can result from
several causes. These range from accidental erasure to
infection by a computer virus to actual hardware failure.
When disaster does strike, the main objective is to
recover as much data as possible from the disk.
Recovering Data From an Erased File
There are many ways that a file can be accidentally
erased. The important thing in recovering an erased file
is detecting the error quickly. DOS does not actually
erase the data areas of a file when you delete it, DOS
merely changes the code in the FAT to indicate that the
cluster is available for use. Therefore, to completely
recover an erased file, you must try the recovery before
DOS reuses the clusters that the file was in.
You can manually recover an erased file by using
the DEBUG program in DOS. This method is long and
tedious. Several commercial programs are available
that will try to restore an erased file. These programs
will look at the deleted directory entry to find where the
starting cluster of the file was, then check the size of the
file to determine how many clusters the file should have
occupied. The recovery program will then check the
FAT and see if the clusters are available. For example,
if a file occupied clusters 75 to 79, a check of the
directory entry would show that the beginning of the
file was cluster 75. The program would then try to
recover all the data in clusters 75 to 79. The problem
arises if the file was fragmented. That is, the file was
in clusters 75, 83, 100, and 101. In many cases when
the file is fragmented, it cannot be recovered.
To avoid file fragmentation, there are also several
file unfragmenter programs for use in personal
computers. These programs will check the disk for
fragmented files, and rewrite the fragmented files so
they are contiguous.
Computer Viruses
A computer virus is any program designed to be
willfully destructive. A virus can be spread by several
methods. The methods include loading the virus from
a bulletin board system and loading a virus onto your
fixed disk from a floppy disk.
When the word of a virus infection is spread, the
first reaction of many users is to panic. Knowing how
a virus is spread can help you find the source of the
virus.
In IBM personal computer systems and
compatible systems, a virus can only be spread in .COM
and .EXE files. Some viruses maybe harmless pranks,
such as displaying a message on the screen every time
the virus is activated. Others are much more harmful
and may format your fixed disk or they may erase the
FAT or master boot record (MBR).
The three common types of viruses are the worm,
the Trojan horse, and the logic bomb.
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