Hard Drives (continued...)

Partitions, FAT and Translation
Normally, when you first install a hard drive, it won't be recognized by your computer. You must use your CMOS setup program to enter the drive's configuration according to the manufacturer's specs.

Once the drive is recognized, it has to be partitioned. This creates a Master Boot Record (MBR) in the very first sector of your drive. The MBR tells your computer's BIOS if the drive is split up into two or more logical drives, how big each logical drive is, and if it is active (bootable; contains operating system files). It also tells your BIOS where to look to find the OS files to initially start up your computer. The BIOS knows to look for the MBR in the first sector of the drive.

A partitioning program is usually supplied with most operating systems. FDISK is the utility supplied with most Microsoft products. However, there are third party products out there, such as PartitionIt and Partition Magic, that do the same job and provide several other valuable features.

After partitioning, the hard drive must be formatted by the operating system. Each logical drive or partition must also be formatted. This creates an OS boot record that consists of two tables, the FAT and the DIRECTORY, for each logical drive. The operating system doesn't recognize the actual physical organization of the hard drive. Through the hard drive controller's translation, it views it as a long list of sectors, which it divides up into clusters. The size of drive, whether physical or logical, designates cluster size.

The FAT (file allocation table) is the file system developed for MS DOS, Windows 3.1, and early versions of Win95. It's sometimes called FAT16 because each entry uses 16 bits. Later versions of 95 and Windows 98 use FAT32.

The FAT (File Allocation Table) keeps track of which files are using which clusters, and which clusters are available for allocation. The Directory Table keeps track of the starting cluster of each file, filenames, length of each file, date created, attributes and other directories. When an OS accesses a file, it first checks the Directory for the starting cluster number, then follows the FAT. The entry in the FAT for that cluster will contain the number of the next cluster associated with the file. When the entire file has been retrieved, the entry in the FAT associated with the last cluster accessed will contain an EOF (End of File) marker.

When people talk about the File Allocation Table, they are usually talking about both the actual table and the Directory.

Translation
For your BIOS to recognize your hard drive properly, it needs to know the number of cylinders on each platter, the number of heads, and the number of sectors/track.

cylinders x heads x sectors per track x 512(bits per sector) = capacity

If you look at the list of drive types in your BIOS you'll see that the largest is about 132 MB. I know it's important to stay backward compatible, but surely they could get rid of the first 46 HD choices. The 'user defined' (usually 47 or 48) type allows you to enter configuration for drives larger than this.

Just a few years ago, the BIOS could only recognize drives with a maximum of 1024 cylinders, 16 heads and 63 sec/track. So the largest capacity drive that could be recognized was 528 MB. This method of translation is called CHS.

Enhanced BIOS was developed to be compatible with other methods of translation. It could now recognize three different methods:

CHS - Cylinders, Heads, Sectors per Track.

LARGE - This method converted the actual physical CHS information on the HD to CHS information that the BIOS could understand by fooling it into thinking the drive didn't have more than 1024 cylinders. It would support drives with a capacity up to 1 Gigabyte.

LBA - LBA converted the physical organization of the drive into blocks that the BIOS saw as a long string of addresses. This method supports drives larger than 1 Gig.

To know which methods your BIOS supports, you have to check in your BIOS Setup for the particular options (under Mode or Trans.). If your BIOS does not support the larger drives, your only options are to upgrade your BIOS, upgrade your motherboard, or use a software overlay of some sort. The latter is usually supplied with a new drive or available as shareware on the Internet. There are third party disk management utilities that also provide other features.

Note: Swapping HDs to another machine with a different BIOS, or changing BIOS translation modes, can sometimes destroy the data on your drive.

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