The most basic form of disk organization is called disk formatting. Disk formatting prepares the hard disk so that files can be written to the platters and then quckly retrieved when needed.
There are two Disk level formatting.
1. Low level formatting
2. High level formatting
Low level formatting: A low-level format permanently erases the disk and is not reversible because it performs a destructive scan of the disk to find any defects in the recording media. The location of any defect found is recorded as unusable to avoid data problems.
High-level formatting: High-level formatting is done after low-level formatting and after the disk has been partitioned. The high-level format prepares the disk’s partitions by creating a root directory and the File Allocation Table (FAT). The FAT is used to record the location and relationships of files and directories on the disk.
A low-level format should not be done on an IDE or SCSI hard disk. This is performed during manufacturing and should not ever be needed again.
File Allocation Table File System (FAT, FAT12, FAT16)
The most common file system in the PC world is actually a family of file systems. The basic name for this file system is FAT (File Allocation Table); the name comes from one of the main logical structures that the file system uses; the file allocation table. This file system is the one that was used by DOS on the first IBM PCs, and it became the standard for the PCs that followed.
FAT in Concept: The base storage area for hard drives is a drives is a sector, with each sector storing up to 512 bytes of data. If an operating system stores a file smaller that 512 bytes in a sector, the rest of the sector gets wasted. One accepts this waste because most files are far larger than 512 bytes. So what happens when an operating system stores a file larger than 512 bytes? The operating system needs a method to fill one sector, find another that’s unused, and fill it, continuing to fill sector until the file is completely stored. Once the Operating system stores a file, it must resemble which sectors hold that file, so that file can be retrieved later.
FAT16 (16-bit FAT): The FAT16 system uses 16-bit-cluster address numbers, which allow up to 65,535 clusters. Under FAT 16, a cluster can be as big as 32 KB, which translate into a maximum partitions size of (65,536 x 32.768) 2,147,483,648 bytes (2.1 GB). Hence the disk partition size limits to 2 GB. The maximum hard disk capacity supported by FAT16 is 8 GB.
The use of disk space is not very efficient in a FAT16 Partition. For example, if we are storing a 10KB file in a FAT16 volume then a cluster of 32KB will be allocated for this storage, that means 22 KB of that particular cluster is wasted. Such cases will result in inefficient use of disk space. This drawback of FAT16 is overcome in FAT32 by reducing the cluster size.
FAT32 (32-bit FAT): As hard disk continued to increase in size through the 1990s, the limitations of the FAT16 and VFAT file system began to become obvious. The use of large cluster sizes led to a significant amount of wasted hard disk space (slack).
Eventually, hard disk manufacturers started to create drives so large that FAT16 could not be used to format a whole drive in a single partition. PC makers complained that FAT16 was outdated for modern machines. To correct this situation, Microsoft created FAT32. This newest FAT variant is an enhancement of FAT/VFAT file system. It is named FAT32 because it uses 32-bit numbers to represent clusters, instead of the 16-bit numbers used by FAT16. FAT32 was created primarily to solve the two problems mentioned above. It allows single partitions of very large size to be created, where FAT16 was limited to partitions of about 2 GB. FAT32 support partitions up to 32GB. FAT32 uses space more efficiently. FAT32 uses smaller clusters that is, 4-KB clusters for drives up to 8 GB in size, resulting in 10 to 15 percent more efficient use of disk space relative to large FAT or FAT1 drives.
New Technology File System (NTFS): When Microsoft created windows NT it built the operating system pretty much from scratch. It was based on certain existing concepts, of course, but was totally different from older Microsoft operating systems. One of the key elements of NT’s architecture was the file system created especially for the operating systems, called the New Technology File System or NTFS.
Master Boot Record (MBR)
Every hard disk must have a consistent “starting point” where key information is stored about the disk, such as how many partitions it has, what sort of partitions they are, etc. Place where this information is stored is called the master boot record (MBR) which is always located at cylinder 0, head 0, and sector 1, the first sector on the disk. Master boot record contains the following structures: Master Partition Table. This small table contains the descriptions of the partitions that are contained on the hard disk.
Master Boot Code
The master boot record contains the small initial boot program that the BIOS loads and executes to start the boot process.
Partition Types: A hard drive may have up to four partitions. These partitions divide into one of two types: primary and extended.
Primary Partitions
Primary partitions store the OS(s) and if you want to boot from a hard drive, it must have a primary partition. In Windows 9x and 2000, the primary partition is always C:, and you cannot change. A hard drive can have up to four primary partitions.
Active Partition: Active partition comes into play when a hard drive stores multiple primary partitions, each with a valid operating system.For a primary partition to boot, you must set it as the active partition.MBR looks for a primary partition set to “active”.
Boot Managers
Programs specifically designed for the task of booting and they are usually called Boot Managers or boot loaders.
It analyzes the primary partitions on the disk and then presents a menu to you and asks which operating system you want to use.
Boot managers are in many ways indispensable when working with multiple operating systems.
Extended Partition
Extended partitions are not bootable and one hard drive can only have one extended partition.
Extended partitions are completely optional.
When you create an extended partition, it does not automatically get a drive letter instead, you divide the extended partition into “logical drives”.
An extended partition may have as many logical drives as you wish, limited only by the letters of the alphabet for Windows 9x systems.
Partitioning: A maximum of four partitions can be placed on any hard disk. These are sometimes called primary partitions. The limitation of four is one that is imposed on the system by the way that the master boot record is structured.
Only one partition may be designated, at any given time, as active. That partition will be used for booting the system.
One of four partitions may be designated as an extended partition. This partition may then be subdivided into multiple logical partitions. This is the way that two or more logical drives can be placed on a single hard disk.
