Data Area Files Area Root Directory volume disk surface hard disk drive Last Cluster Used LCU Fragmentation of data area

 
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Data Area (or Files Area)

Following the Root Directory the Data Area (or Files Area) starts. Rather we can say that the remainder of the volume after Root Directory is the Data Area.

The data area contains the actual data stored on the disk surface. DOS uses cluster number 2 for the first sector of the data area therefore we should keep it in mind while performing various calculations that the cluster number should start from 2.

When we format a hard disk drive the FORMAT command of DOS does not destroy or overwrite the data on the data area. The FORMAT command only removes the directory entry and FAT entries and it does not touch the actual data area.

This makes the recovery of accidentally formatted hard disk drive possible, which is not possible in all cases of accidentally formatted floppy disk. As in full Format by Windows the Data area of Floppy is filled by F6H characters and all the information is overwritten.

DOS uses a 16-Bit/32-Bit pointer called a “Last Cluster Used” (LCU) pointer to store the last used cluster number. Initially this pointer value is zero, when some information is written on a particular cluster, that cluster number is stored into LCU pointer.

After this, every time if the new information is to be written, DOS searches for a free cluster from the LCU number onwards. This forces new data written on the disk to remain contiguous as the data is written to the new, unused area.

Now understanding of data in case of an accidental deletion would be much easier as the file is in one contiguous area on the disk. Once the end of disk is reached or the system is reset, the LCU pointer value is reset to zero.

But the problem does not last here. It is not practically possible that you only create new files on your disk and you never delete or modify any file from the previously stored ones. Using only LCU has the problems if you do a lot of creation and deletion of files as this will force the new data to move further and further into the inner tracks of the hard disk.

Therefore if any file, which was stored in the disk, is moved or deleted, the clusters in the data area which were occupied by the file are now set as unallocated clusters available in the data area and ready to be written new data on them. By doing this, operating system do not need to move all the data to the inner tracks of the disk.

But a lot of deletion and creation of files fragments the data on the disk and this causes the Fragmentation in the data.

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