File X has 15 data blocks. The block addresses of the 15 blocks are 8, 9, 10, 11

Question

File X has 15 data blocks. The block addresses of the 15 blocks are 8, 9, 10, 11, 12, 13, 5, 4, 21, 22, 23, 24, 25, 26 and 27.

1-How the data block information is managed using FAT if file X is in DOS? Draw part of the FAT that describes the data blocks.

2-How the data block information is managed using i-node if file X is in Linux? Draw part of i-node that describes the data blocks.

3-How the data block information is managed using MFT if file X is in Windows Vista? Draw part of MFT that describes the data block.

Explanation / Answer

Filesystems are containers of files, that are stored, probably in a directory tree, together with attributes, like size, owner, creation date and the like. A filesystem has a type. It defines how things are arranged on the disk. For example, one has the types minix, ext2, reiserfs, iso9660, vfat, hfs.

The traditional DOS filesystem types are FAT12 and FAT16. Here FAT stands for File Allocation Table: the disk is divided into clusters, the unit used by the file allocation, and the FAT describes which clusters are used by which files.

Let us describe the FAT filesystem in some detail. The FAT12/16 type is important, not only because of the traditional use, but also because it is useful for data exchange between different operating systems, and because it is the filesystem type used by all kinds of devices, like digital cameras.

First the boot sector (at relative address 0), and possibly other stuff. Together these are the Reserved Sectors. Usually the boot sector is the only reserved sector.

Then the FATs (following the reserved sectors; the number of reserved sectors is given in the boot sector, bytes 14-15; the length of a sector is found in the boot sector, bytes 11-12).

Then the Root Directory (following the FATs; the number of FATs is given in the boot sector, byte 16; each FAT has a number of sectors given in the boot sector, bytes 22-23).

Finally the Data Area (following the root directory; the number of root directory entries is given in the boot sector, bytes 17-18, and each directory entry takes 32 bytes; space is rounded up to entire sectors).

The first sector (512 bytes) of a FAT filesystem is the boot sector. In Unix-like terminology this would be called the superblock. It contains some general information.

First an explicit example (of the boot sector of a DRDOS boot floppy).

The first cluster of the data area is cluster #2. That leaves the first two entries of the FAT unused. In the first byte of the first entry a copy of the media descriptor is stored. The remaining bits of this entry are 1. In the second entry the end-of-file marker is stored. The high order two bits of the second entry are sometimes, in the case of FAT16 and FAT32, used for dirty volume management: high order bit 1: last shutdown was clean; next highest bit 1: during the previous mount no disk I/O errors were detected.

(Historically this description has things backwards: DOS 1.0 did not have a BIOS Parameter Block, and the distinction between single-sided and double-sided 5.25" 360K floppies was indicated by the first byte in the FAT. DOS 2.0 introduced the BPB with media descriptor byte.)

Since 12 bits is not an integral number of bytes, we have to specify how these are arranged. Two FAT12 entries are stored into three bytes; if these bytes are uv,wx,yz then the entries are xuv and yzw.

Possible values for a FAT12 entry are: 000: free, 002-fef: cluster in use; the value given is the number of the next cluster in the file, ff0-ff6: reserved, ff7: bad cluster, ff8-fff: cluster in use, the last one in this file. Since the first cluster in the data area is numbered 2, the value 001 does not occur.

DOS 1.0 and 2.0 used FAT12. The maximum possible size of a FAT12 filesystem (volume) was 8 MB (4086 clusters of at most 4 sectors each)

FAT16 uses the above BIOS Parameter Block, with some extensions:

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