A file allocation table (FAT) is a table that an operating system maintains on a hard disk that provides a map of the clusters (the basic unit of logical storage on a hard disk) that a file has been stored in. When you write a new file to a hard disk, the file is stored in one or more clusters that are not necessarily next to each other; they may be rather widely scattered over the disk. A typical cluster size is 2,048 bytes, 4,096 bytes, or 8,192 bytes. The operating system creates a FAT entry for the new file that records where each cluster is located and their sequential order. When you read a file, the operating system reassembles the file from clusters and places it as an entire file where you want to read it. For example, if this is a long Web page, it may very well be stored on more than one cluster on your hard disk.

Until Windows 95 OSR2 (OEM Release 2), DOS and Windows file allocation table entries were 16 bits in length, limiting hard disk size to 128 megabytes, assuming a 2,048 size cluster. Up to 512 megabyte support is possible assuming a cluster size of 8,192 but at the cost of using clusters inefficiently. DOS 5.0 and later versions provide for support of hard disks up to two gigabytes with the 16-bit FAT entry limit by supporting separate FATs for up to four partitions.

With 32-bit FAT entry (FAT32) support in Windows 95 OSR2, the largest size hard disk that can be supported is two terabytes! However, personal computer users are more likely to take advantage of FAT32 with 5 or 10 gigabyte drives.

NTFS (NT file system) is the system that the Windows NT operating system uses for storing and retrieving files on a hard disk. NTFS is the Windows NT equivalent of the Windows 95 FAT (file allocation table) and the OS/2 HPFS (high performance file system). However, NTFS offers a number of improvements over FAT and HPFS in terms of performance, extendibility, and security.

Notable features of NTFS include:

Use of a b-tree directory scheme to keep track of file clusters

Information about a file's clusters and other data is stored with each cluster, not just a governing table (as FAT is)

Support for very large files (up to 2 to the 64th power or approximately 16 billion bytes in size)

An access control list (ACL) that lets a server administrator control who can access specific files

Integrated file compression

Support for names based on Unicode

Support for long file names as well as "8 by 3" names

Data security on both removable and fixed disks

How NTFS Works

When a hard disk is formatted (initialized), it is divided into partitions or major divisions of the total physical hard disk space. Within each partition, the operating system keeps track of all the files that are stored by that operating system. Each file is actually stored on the hard disk in one or more clusters or disk spaces of a predefined uniform size. Using NTFS, the sizes of clusters range from 512 bytes to 64 kilobytes. Windows NT provides a recommended default cluster size for any given drive size. For example, for a 4 GB (gigabyte) drive, the default cluster size is 4 KB (kilobytes). Note that clusters are indivisible. Even the smallest file takes up one cluster and a 4.1 KB file takes up two clusters (or 8 KB) on a 4 KB cluster system.

The selection of the cluster size is a trade-off between efficient use of disk space and the number of disk accesses required to access a file. In general, using NTFS, the larger the hard disk the larger the default cluster size, since it's assumed that a system user will prefer to increase performance (fewer disk accesses) at the expense of some amount of space inefficiency.

When a file is created using NTFS, a record about the file is created in a special file, the Master File Table (MFT). The record is used to locate a file's possibly scattered clusters. NTFS tries to find contiguous storage space that will hold the entire file (all of its clusters).

Each file contains, along with its data content, a description of its attributes (its metadata).

To go from 95 to NT, you will need to fdisk and delete your current partition unless it's already FAT16, make a new FAT16 partition and go about installing from there.

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