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 freed 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. 10-29