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COMPUTER BITS BY JENNIFER BONNITCHA How to cope with damaged directories & file allocation errors Continuing on with troubleshooting tips for your PC, this month we look at problems such as non-DOS discs, damaged directories, allocation errors and invalid clusters. Each disc used by DOS must first be subdivided into tracks and sectors. You can think of tracks as a series of concentric circles covering the disc surface (Fig. l). The tracks are numbered from the outer edge to the inner edge as track 0, track 1, track 2 and so on. Additionally, discs with more than one side have their sides numbered, starting with 0, then side 1. Finally, tracks are divided like pie slices into sectors, starting with sector 1, then sector 2 and so on. Typically, sectors have a storage capacity of 512 bytes. You can work out disc capacity with the following formulas: (1) No. bytes per sector x No. sectors per track= no. bytes per track; (2) No. bytes per track x total no. tracks = disc storage capacity Sector 7 Sector 3 Fig.1: how a computer disc is divided into tracks & sectors by the FORMAT procedure. SILICON CHIP Disc formatting Tracks and sectors are constructed during the disc FORMAT procedure, which also places a copy of the DOS boot record in sector 1, track 0, side 0. This record has a table containing information about the disc and machine language code that loads two system files into memory. Format also constructs the File Allocation Table (FAT) and the file directory. The file directory is just like a table of contents for a disc. Every file on the disc has a corresponding entry in the disc's file directory; each entry is composed of 32 bytes which in turn are partitioned into 8 fields. Finally, each of these 8 fields contains information used by DOS in file management, such as a file's name, extension, time and date stamp, starting cluster, etc. File allocation table Sector 8 Sector 1 52 Hard discs consist of one or more platters, each with two surfaces. These platters are also divided into tracks and the tracks into sectors. All tracks with the same radius form a cylinder through the platters. Note, however, that the layout of hard drives varies according to the manufacturer and disc capacity. While the disc directory tells you which files are present on a disc, the FAT tells DOS where a file's contents are actually located. DOS divides each file's contents into clusters which are numbered according to their physical location on the disc. The first cluster starts with the sector immediately following the last sector of the file directory; the second cluster follows the first and so on. An invalid entry for the FAT is a cluster number that points to a non-existent cluster on the disc. The entry is either 1 (which is invalid since DOS starts with entry 2 on the disc) or a number higher than the number of clusters for the disc. Without the FAT, DOS would be unable to access any files. It is so important to the running of your computer that a second copy is stored on each disc. Should you experience an Invalid Cluster message while running CHKDSK, you can be reasonably sure it is the result of an unexpected system shutdown or a poorly behaved program. You can expect to see one of two messages from CHKDSK if this is the case: <filename> First cluster number is invalid, entry truncated; or <filename> has invalid cluster, file truncated In the first case, the starting cluster number in the file's directory entry doesn 't make any sense since DOS doesn't know where the file's chain is in the FAT. The second message indicates corruption in the FAT file chain at the point of the invalid cluster. In either case, DOS determines that the file's entry in the file directory indicates that the file is larger than the amount of disc space allocated for it in the FAT. The errors typically occur during an update operation, so you could try using a backup of the fil e (which will be out-of-date but intact), or you could try recovering the file . You have some chance of manual repair, provided you do not use CHKDSK/F. The IF switch in this instance truncates the file at the invalid cluster number so that its size matches the allocation specified by the FAT. The part of the file beyond the invalid cluster is then lost. Recovery, while not impossible requires some luck and a good knowledge of both DOS and the FAT. If you are desperate, try a disc-sector editor such as the Norton Utilities and follow the associated instructions. The general idea is to change the wayward entry to point to the next cluster of the disc. More orphans CHKDSK may report orphaned and invalid clusters within the FAT. Usually the orphaned clusters belong to the file whose FAT chain is broken. You should run CHKDSK/F and convert the lost chains/clusters to files then examine each of the files created by the procedure. If the information is worth saving, use the copy command: COPY <filename>+FILE000l .CHK+FILE0000.CHK [d:}<filename> IB The /B switch causes COPY to treat the file(s) as a binary file. If a file is to be copied, this switch tells COPY to copy the entire file based on the size stored in the file directory. If the file is to be a copy, the switch tells.COPY not to place an end-of-file marker at the end of the file. Examine the result of your file concatenation command then delete the CHK files created by CHKDSK, together with the originally suspect file. Should you get a message indicating an invalid starting cluster, DOS has lost track of the entire FAT chain(s) for the file. Chances are good that the chains still exist on the disc. Once again run CHKDSK/F to truncate the file and generate a series of CHK files containing the orphaned clusters. Once again examine each of the CHK files and try to piece together the file. Allocation errors Inconsistencies between the file size in the directory entry and the number of clusters allocated to the file in the FAT produce the following message: <filename> Allocation error for file, size adjusted As a general rule, the file's information has been successfully written to the disc; the problem is simply that the file size in the directory entry is too large or small. Although serious problems seldom occur as a result of this procedure, you may find some extra information at the end of the file in question. Possibly some information may be deleted which will cause the program using the file to object. Finally, ensure that the allocation error is for a file. Should CHKDSK report this error for a directory, you may be in serious trouble. A good starting point once again is the Norton Utilities' Disk Doctor. The program is menudriven and easier to understand than CHKDSK; it will diagnose and repair many logical and some physical disc problems. While there are certainly other programs which will help with these problems (MACE Utilities, PC Tools, etc), Norton Utilities is my personal favourite. Any one of the utilities is a good investment for all computer users. Non-DOS discs From time to time, you may receive the following message, even when you have previously used a disc: Probable non-DOS disk Continue (YIN)? CHKDSK checks each disc to make sure it is a DOS disc , since it has the capability to alter the information contained therein. By examining the first two entries of the FAT (where the media ID is stored) and the second sector of the disc, CHKDSK can determine the disc's status. If the media ID byte is invalid, CHKDSK reports a potential nonDOS disc. In this situation, it's possible that the disc is either unformatted or has been formatted under a different operating system. If you get this message periodically, you may have a program or hardware problem which mangles the FAT. Once again the disc editor can repair the damage - compare the two copies of the FAT, then correct the media ID byte and write the sector back to the disc. Directory errors All subdirectories have two entries placed when the directory is created - the "." (dot) and" .. " (double dot).· The single dot entry designates the parent of the current directory while the double dot is the current directory. Should CHKDSK report that the "." or " .. " entry has a bad attribute (or link or size), the first or second subdirectory entry is defective. The only DOS attribute usually set to ON for a subdirectory is the directory attribute. If CHKDSK reports a bad link, the starting cluster number for the directory is incorrect. Badly behaved programs can cause the special subdirectory entries to have problems with their file attributes, size or FAT linkage. Run CHKDSK/F to repair most of the above problems, provided the parent directory is undamaged. If the parent should happen to be damaged, CHKDSK will display more warning messages. You may get the following: Processing cannot continue Unrecoverable error in directory Convert directory to file (YIN)? The first message indicates that the disc structure may be so badly damaged that CHKDSK is confused. Respond "N" then try re-booting DOS and run the command again. If you continue to get the message, try to back-up the disc then run CHKDSK/F and convert the subdirectory to a file. However, in converting the subdirectory to a file, all files in this and subsequent subdirectories will become orphaned clusters. You could try to rebuild the files you did not back-up or delete the subdirectory and files , then use a previous back-up. SC MARCH 1992 53