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Logical Drives and Partition TablesThe basic unit of a FAT file system is a logical drive. A logical drive is a contiguous portion (or possibly all) of a disk consisting of the following components in the given order:
Diskettes, and all devices formatted as diskettes, will contain exactly these components. With the introduction of MS-DOS 2.0, hard disks with much larger capacity than diskettes needed to be supported. Since MS-DOS was not able to handle such large volumes with a single logical drive, the concept of a partition table was added. The partition table only appears on hard disks. This table can contain up to 4 entries, each entry describing a single logical drive in a portion of the disk. A special partition type, the Extended Partition, can implement a linked list of any number of logical drives. Each logical drive described by a partition has exactly the format described above: boot record, FAT(s), root directory, and data area. For backward compatibility, the first sector of a hard disk still has the basic structure of a boot record; it just adds the partition table to the end of the sector. Such a boot record with a partition table is called a Master Boot Record. If a hard disk is booted, the master boot record is executed which in turn will scan the partition table for a bootable logical drive. If one is found, the boot record of that logical drive is loaded and executed.
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