FAT Chaining

 

Most of the people will not understand the term ”FAT Chaining”. There are two things:

1) FAT

2) Chain

Yes, FAT is File Allocation Table and you will understand chaining at the end of this post. Basic aim of this post is to understand how a file gets retrieved when we click the icon or enter the name of the file on command prompt.

Suppose, we want to access a file present in a Floppy Disk. Now the question arises, Why are we retrieving the file from Floppy Disk and not from Hard Disk?

The reason is that we know the details of the Standard Data Structures of the Floppy Disk (FD). Here, data structures constitute to the following:

1) Boot Sector

2) File Allocation Table (FAT)

3) Root Directory

4) File Area

Now, we will not let ourselves only getting theoretical knowledge, instead we will develop Assembly Language Code for this. So, we divide this topic into two posts. This post will contain information about the above mentioned Data Structures and in the next post we will play with the Interrupts in order to develop the Assembly Level Code.

Now, let’s see how these things going to be used during file retrieval:

The whole area of the FD is divided among the above 4 Data Structures as shown below in Fig.1. Each MS-DOS logical volume is divided into several fixed-size control areas and a files area. The size of each control area depends on several factors like the size of the volume and the version of FORMAT used to initialize the volume, for example, but all of the information needed to interpret the structure of a particular logical volume can be found on the volume itself in the boot sector.  

 

1) Boot Sector:

Boot sector is known as the logical sector 0, contains all of the critical information regarding the disk medium's characteristics (Fig.2).

We are not going into much detail about these fields as they are self explanatory. From Boot Sector, we get all the physical information about the Drive. Let say, we have to find the size of the logical space of the drive. This can be calculated by making use of:

1) Total sectors in logical volume and

2) Bytes per sector

2) File Allocation Table:

Each file's entry in a directory contains the number of the first cluster assigned to that file, which is used as an entry point into the FAT. From the entry point on, each FAT slot contains the cluster number of the next cluster in the file, until a last-cluster mark is encountered.

At the computer manufacturer's option, MS-DOS can maintain two or more identical copies of the FAT on each volume. MS-DOS updates all copies simultaneously whenever files are extended or the directory is modified.

If access to a sector in a FAT fails due to a read error, MS-DOS tries the other copies until a successful disk read is obtained or all copies are exhausted. Thus, if one copy of the FAT becomes unreadable due to wear or a software accident, the other copies may still make it possible to salvage the files on the disk. As part of its procedure for checking the integrity of a disk, the CHKDSK program compares the multiple copies (usually two) of the FAT to make sure they are all readable and consistent.

3) Root Directory:

Following the file allocation tables is an area known as the root directory. The root directory contains 32-byte entries that describe files, other directories, and the optional volume label (Fig. 3).

An entry beginning with the byte value E5H is available for reuse; it represents a file or directory that has been erased. An entry beginning with a null (zero) byte is the logical end-of-directory; that entry and all subsequent entries have never been used.

 

4) Files Area:

The remainder of the volume after the root directory is known as the files area. MS-DOS views the sectors in this area as a pool of clusters, each containing one or more logical sectors, depending on the disk format. Each cluster has a corresponding entry in the FAT that describes its current use: available, reserved, assigned to a file, or unusable (because of defects in the medium). Because the first two fields of the FAT are reserved, the first cluster in the files area is assigned the number 2.

Try to follow the next post which will be more interesting.

Thanks a lot !!!