STRUCTURE OF DISK

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    The dos diskette is divided into four major areas, they are

    1. Boot area               (System area)
    2. FAT area                (System area)
    3. Root directory area     (System area)
    4. Data area
 

    1.BOOT AREA:
            The boot area is a the first sector on the disk which holds the instructions associated with booting of the computer. Normally a diskette has its boot records in its very first sector (sector - 1, head - 0, track -0). You can see the instructions in the boot sector using diagonistic software such as PC TOOLS, debug ,etc,. You can use the DEBUG utility provided with DOS to load the boot sector. To load the boot sector follow these simple steps.

       step 1. c:\dos>DEBUG    (enter)
       step 2. -L 100 0 0 1    (enter)
       step 3. -u 100          (enter)

            Normally the boot sector is 512 bytes in size. The boot sector is otherwise called as DOS BOOT RECORD (DBR). The boot sector is subdivided into two major sections. They are

            1. Bios Parameter Block (BPB)
            2. Boot instructions

        1.BPB (Bios Parameter Block)
                    The bios parameter block holds some of the important information about the disk like bytes per sector, sectors per track, sectors in boot record ,etc,. These informations are useful in manuplating the disk. Note that the information is just a data part of the boot record the control is transfered to the boot instruction before the BPB via a JMP instruction. See BPB structure for more details.

        2.Boot instructions
                    This part is the executable part of the boot record. It is responsible for the setting up of operating system and it plays an important role in the system boot up sequence. I will not be dealing boot instructions in depth which is beyond the scope of this discussion.

    In harddisks in addition to DOS BOOT RECORD (DBR) it has MASTER BOOT RECORD (MBR) which holds the partition information. Diskettes do not have MBRs. The physical location of MBR and DBR is given below.

           Floppy                Harddisk

        DBR       track 0                  track 0
                  sector 1                 sector 1
                  head 0                   head 1

        MBR        ----                    track 0
                                           sector 1
                                           head 0

How to find the number of sectors in boot area ?

    The number of sectors used by boot area is found by checking the BPB. The BPB has a field in which the number of sectors in boot is stored. Normally its count is one. See BPB structure for more details.

    2.FAT AREA:
            The FAT (File Allocation Table) is important area inside a disk. It is the address book of the disk. The information which are stored on the disk is identified with the help of FAT. If you are familiar with data structures then the structure of FAT will be more clear to you.
            FAT is a group of dedicated sectors holding the information about how the data inside the disk is organised. Normally there are two copies of the same FAT because if one of the FAT is corrupted then the other may be used.

    STRUCTURE OF FAT :

           There are three types of FAT, they are
                1. FAT12
                2. FAT16
                3. FAT32
                These three FATs can be seen in plenty, so the discussion will be appropriate if we deal with these three. Basically, if you are clear with FAT12 then I suppose that FAT16 and FAT32 will not be difficult
at all. I will not be dealing with FAT32 here because I think that FAT12 and FAT16 will serve the purpose.

FAT12:
    FAT 12 is the File Allocation Table scheme used in the diskettes. The 12 at the end specifies the number of bits used for representing a cluster. So for FAT12 a cluster is represented by 12 bits in the FAT entry and for FAT16 a cluster is represented by 16 bits in the FAT entry. Same is the case with FAT32. Each entry in the FAT represents a cluster number. Note that the cluster numbering order starts from 2 and not 0. So the first cluster in a disk is cluster number 2. The first entry (byte) in the FAT is the media discriptor which varies according to the type of diskette used. For a fixed drive it is F8h and for 1.44M floppy it is F0h. The next two bytes are FFh. The structure of FAT is more clearly shown below.
 

 1. Boot record.

 2. FAT (2 copies of FAT the two copies will be identical.)

    First copy of FAT

    F0  FF  FF  <--------- Cluster information starts ---------->
    ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..
    ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..
    ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..

    Second copy of FAT

    F0  FF  FF  <--------- Cluster information starts ---------->
    ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..
    ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..
    ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..  ..

    Note that F0h represents the media descriptor and it varies according to diskette type. The following two bytes are FFh. From the fifth byte onwards the cluster 2 information starts.

 3. Root directory.

 4. Data area.

How cluster information is stored in the FAT ?

    The cluster information is similar to a linked list. Each FAT entry represents the next cluster in the cluster chain. For example

    F0  FF  FF  03  40  00  ..  ..     --> FAT12
                ^
                |
                cluster 2 information

    F8  FF  FF  00  0300  04  ..  ..  --> FAT16
                ^
                |
                cluster 2 information

    From the above cluster 2 information we can see that the after reading cluster 2 we should read cluster 3 and then cluster 4. To understand the FAT entry consider the FAT16 example. Each underlined item represents a entry in FAT. In the cluster 2 entry we have 0003. So it implyies after cluster 2 comes the cluster 3, but on cluster 3 entry we have 0004 so after cluster 3 comes the cluster 4 and so on. Since we deal with FAT16 note that each FAT entry has 2 bytes (16 bits).
    FAT16 is easy to understand but FAT12 is a bit difficult. All diskettes have FAT12 scheme. Consider the previous FAT12 example

    F0  FF  FF  03  40  00  ..  ..     --> FAT12
                ^
                |
                cluster 2 information

     Since we deal with FAT12 12 bits represent a cluster. To get the Cluster 2 entry swap the cluster 2 entry and the cluster 3 entry so after swapping the bytes we get, 40 03. Since the cluster number is even only conider the last 12 bits (underlined) which is 003. Hence the cluster 2 points to cluster 3. Similarly to get cluster 3 entry swap cluster 3 entry and the next entry so we get 00 40. Since the cluster number is odd consider only the first 12 bits which is 004. Hence cluster 3 points to cluster 4.

   In general, for FAT12 swap the adjecent FAT entry bytes, check if the cluster number is even or odd, if even then AND the swaped entry bytes with FFFh else if odd then shift right (logical shift) the swaped entry bytes by 4 bits. By doing this algorithm you can get the next cluster number from a FAT entry.

    FAT16 is much simple. Every two bytes represent a cluster information so there is no need to play with the bits. Just read the two bytes and do your own manuplation. In C the int data type (2 bytes) will be ideal for this case.

How to find the correct FAT entry byte given a cluster number ?

    In FAT12 three bytes hold information about two clusters so multiply the given cluster number by 3/2. The result may cross over the total FAT entries available. Hence if the result is even then AND the result with FFFh else shift right (logical shift) the result by 4 bits. Now, the final result is got this result repersents the offset from the first byte of the FAT entry to the required FAT entry byte. An example will explain it more clearly. Consider the given cluster number is 2.

    step 1 : Multiply by (3/2)       ->  (3/2) * 2 = 3
    step 2 : Even cluster so & FFFh  ->  3 & FFFh = 3
    step 3 : Offset from first byte of FAT = 3

    So the cluster 2 FAT entry byte can be found after skipping 3 bytes form the first byte of the FAT. This is shown below.

    F0  FF  FF  03  40  00  ..  ..     --> FAT12
    Skip 3 bytes ^
                 |
                 cluster 2 information

 In FAT16  2 bytes hold information about one cluster so just multiply the cluster number by 2 and use the result as the offset from start of FAT. So FAT16 is easier to manuplate than FAT12.

How to find the number of sectors in FAT ?

    The number of sectors in FAT varies according to the diskette type. The BPB holds information about how many copies of FAT is present (Normally 2) and it also holds information about how many sectors are present in each FAT copy. So you can easily calcuate the total sectors present in the FAT copies which is given below.

    Number of sectors in FAT = Number of FAT copies * Number of sectors
                               (See BPB structure)    in one FAT copy
                                                      (See BPB structure)

    3.ROOT DIRECTORY AREA:
                        The root directory area is another important system area in the diskette. It is the last system area of the disk. It contains the root directory's file(s) name(s) and directory name(s) and its internal details (similar to a table). Each entry in the root area is 32 bytes long. Within these 32 bytes contains the file/subdirectory name, the size of the file (0 if its a subdirectory), the starting cluster number of the file/subdirectory. Note that the entries in the root directory area contain information about only the root directory's file names and the root directory's subdirectory's name. For a file entry the starting cluster will point to the first cluster of the file contents. The successive clusters can be obtained by manuplating the FAT entries which is discussed in the above section. For a subdirectory entry the starting cluster denotes to another table of contents very similar to the root directory structure. It is important to note that this subdirectory table lies on the data area. After this root directory area comes the data area.

How to find the number of sectors in root directory area ?

    Normally each entry in the root directory area is 32 bytes long. The maximum number of root directory entries can be obtained from the boot sector's BPB(Bios Parameter Block discussed in Boot area). Now to calculate the total sectors in the root directory apply the following formula.

    Number of sectors in                Maximum number of
    root directory         =    32  *   root directory entries  /512
    area                                (See BPB structure)

    The above formula is fairly simple. 32 denotes the number of bytes for each entry which is constant, 512 denotes the sector size which is mostly 512 bytes. Thus the number of sectors in the root area is calculated.

    4.DATA AREA:
            After the root directory area finishes comes the data area. This area dose not come under system area because it dose not hold system information. The data area holds the actual file contents and subdirectory entry tables in form of clusters. The first cluster number in the data area is cluster 2 which is immediatly after the root directory area. Any damage in the data area can be fixed by marking that area as bad in FAT. There no much technical information about data area execpt that it holds the actual contents of the diskette.

How to mark a bad cluster in the FAT ?

        we've discussed that FAT holds information about each and every cluster, so a bad cluster can be marked by changing the contents of the FAT entry corresponding to the cluster number. We know how to find the correct FAT entry byte given the cluster number. Now change the FAT entry to FF7h for FAT12 or FFF7h for FAT16. The change will look like this shown below

    F0  FF  FF  f7  7f  ff  ..  ..     --> FAT12
                ^
                |
                cluster 2 information

    F8  FF  FF  ff  f7ff  f7  ..  ..  --> FAT16
                ^
                |
                cluster 2 information

    From the above examples for FAT12 and FAT16 it is clear that cluster 2 and 3 are bad clusters since the FAT entry bytes are FF7h for FAT12 and FFF7h for FAT16.
 

General information :

    Any damage in the data area can be fixed by marking it bad in the FAT but is any system area is damaged then the problem is serious. The disk may become unusable.
 

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