FAT 12 Research Paper

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FAT 12 file system:

FAT 12 is part of floppy disk file system. Number ‘12’ in FAT 12 system indicates that the FAT consist of 12 bit entries. The storage space of a floppy disk is divided into number of data units often called as sectors. A set of sectors are called clusters in large storage devices. But for the floppy disk, there is only one sector in the cluster. The size sector or a cluster in a floppy is 512 bytes.

Disk organization:

We have seen a common physical layout of FAT file system in figure 7.7. following figure shows the the disk organization of FAT12 system which consist of boot sector, FAT tables, root directory and data area.

Fig 7.8 Disk organization of FAT 12 system.

The boot sector consist of sector 0 which is the
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This root directory located on data area of the disk, has finite size like for FAT 12, 14 sectors *16 directory per sector = 224 possible entries which restricts the total number of files and directories that can be created. The first sector of the data area corresponds to cluster 2 of the file system i.e. the first cluster is always cluster 2. Data area includes file and the directory data and spans the rest of the sectors on the disk.

Table 7.1 summary of disk organization.
XX=32 as 14 sectors are reserved for the root directory in FAT12.

Boot sector:

To use the disk correctly, the set of information needed by the operating system is provided by boot sector which usually exists at sector 0 on the disk. The information from the boot sector is read and any required information is extracted whenever the disk is used. The following table shows the boot sector on a DOS formatted floppy:

Table 7.2 boot sectors on DOS formatted floppy.

i.Total sector count: This field is the 16 bit total count sector on the disk. This count includes all the sectors in total four regions of the volume. This field contains the sector count for FAT 12 and FAT 16
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Directories:

Directories are nothing but the files such as root directory which occupies one or more sectors. Each sectors of 512 bytes of a directory contains 16 directory entries with 32 bytes each. Each directory points to the sub-directory on the disk. The collection of directory entries for a directory specify the sub-directory of that directory.

The following information is included in the sub-directory to which the directory entry points:

FAT 12 file name and extension representation:

In DOS file name have limit of 8 characters for the file name and 3 characters for file extension. Few things to be aware of are:

File/ directory name an d extension should not be null within the directory entry.
File/directory name should always occupy 8 bytes. So in case if the file/directory name is shorter than 8 bytes then the remaining bytes should be filled with spaces (ASCII 32 or hex 0x20). this rule is also applied to 3 character extensions.
File/directory names and extension should always be in uppercase.
Directory name should have extension too.
‘FILE1’ and ‘FILE1.TXT’ are too different files. That means the extension too maters here. File and directory should not have same
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