Hard Disk Drive (HDD) - Guide - Page: 2/4

Technology and Specifications

Technical Settings

Although you may never need to know the specific settings for your hard disk, your BIOS does. The settings are essentially map guides - detailing how much room (in bytes), how many tracks, sectors, heads and cylinders are on your hard disk.

Tracks: Hard Disk platters arrange data into concentric circles, rather than one large spiral, as some other mediums use. Each circle is called a Track.

Sectors: The smallest addressable unit on a Track. Sectors are normally 512 bytes in size, and there can be hundreds of sectors per track, depending on location.

Heads: The devices used to write and read data on each platter.

Cylinders: The number of tracks on a platter. This one is a bit hard to explain: Platters on a hard disk are stacked up, and so are the heads. Because of this, all of the heads move simultaneously, so they can read separate tracks, but technically at the same physical location (only on at a different platter). If you combine the concentric circles on each platter being accessed by the drive heads, you get a Cylinder.

The Cylinder number keeps track of how many there are from inside to out, although it means the same thing as the amount of tracks on one platter.

Interrupt 13h Problem

Early on in the evolution of PCs, the standard for hard disks was to for the BIOS to use Interrupt 13h for setting hard disk information. However, the IDE interface also needs to set the information, but lacks the same number of bits for each part! Because of this, each number is reduced to the lowest of the two, as shown in the diagram below.

Interrupt 13h Workaround

Breaks through the 528 MB barrier through the use of a Logical Block Address (LBA). By modifying the BIOS to translate the information that is received into a 28-bit LBA, and instructing the BIOS to load the LBA driver from the hard disk, the Hard Disk is given enough room (bitspace) to load all of its information regardless of BIOS limits.

Maximum Cylinder Problem

PC's ran into problems again with Maximum Cylinder Limit. Although most systems by then had employed translation to get past the first Int13h problem, the amount of bitspace allocated was not enough to get past 4096 cylinders, which was quickly being surpassed. This limited hard disk space to 1.97 or 2.1 Gigabytes. The problem was only solved through a new BIOS translation mode, or a new BIOS altogether.

The 8GB BIOS problem

The final and most pervasive limit to hard disks. The problem is no longer truncated numbers, but the actual total numbers the BIOS can recognize at all. The only way to get past this problem was through changes to the BIOS to enable Int13h extensions. It's problematic for some disk utilities, but with newer operating systems such as Windows 95 and Windows 98, the OS is already set up to recognize it.

Super Paramagnetic Limit

The density at which opposite magnetic charges begin to degrade each others signatures, resulting in data loss. The limit is at roughly 20 Gigabits per square inch, which is 4 times greater than today's popular 5 Gigabit per square inch limit.

Read/Write Heads

Essentially electronically controlled magnets. The heads are responsible for converting electrical signals into magnetic data streams on the hard disk and vice versa. Despite their importance, the R/W heads are also the most volatile part of the hard disk assembly.

Because each head is literally a microscopic distance from touching the platters, there is a danger of collision between the the two. This is called a Head Crash, and can have catastrophic effects on your disk, including data loss or worse - physical platter damage. Although today's disks use heads that are even closer to the platters, superior shock suppression and disk enclosure technologies keep problems to a minimum.

There have been several different technologies for Read/Write heads, and each of them has brought dramatic increases in storage size. The most recently used is IBM's Giant Magnetoresistive Head (GMR), which has provided the latest 14-19 gigabyte densities. Newer GMR models have the ability to handle 10 gigabits (not gigabytes) of data per square inch.

The latest and most powerful technology, however, comes from Seagate's subsidiary, Quinta. Using a new 'Optically Assisted Winchester Technology' (OAW), they have managed to squeeze an incredible 40 gigabits of data into a square inch, well beyond the super paramagnetic limit. This breakthrough is obtained through the combination of fiber-optics, MEMS mirrors and specific RE-TM platter media. Quinta estimates the ability to hold at least 100,000 sectors per track while lowering part costs and increasing interface speed.

File Allocation Table Systems - FAT

Every computer needs a system to keep track of files on the hard disk - otherwise there are just random sectors on the disk with no way to interpret them. The system used is called the File Allocation Table.

FAT16: The file system used for MS-DOS. 16-bit numbers are used to represent cluster numbers, which allows for partitions of up to 2 Gigabytes. It was efficient for its time, but cluster sizes are far too large for today's large hard disks. It is also restricted in cluster size.

FAT32 / VFAT: The preferred file system for Microsoft Windows 95 and Windows 98. It is an extension to FAT16, providing 32-bit numbers for clusters. Like FAT16, there are cluster size restrictions.

NTFS: The file system for Microsoft Windows NT. NTFS is considerably better than FAT32 and has no cluster size restrictions, although at a certain point the slack space consumes so much of the hard disk that alternate systems are needed. NTFS also provides permission controls and RAID support.

These are the mainstream PC file systems - other notable systems include HPFS - IBM's OS/2 File System, the Unix File System, and the 64-bit BeOS file system.

Slack Space - The Cluster Size Problem

With any file system, each file is allocated 'clusters' to be placed into. Since each Cluster is a locked size, not every one can be filled - If a system uses 16 KB clusters, but the file is only 2 KB, the remaining space - 14 KB, is automatically wasted and unusable for other files. This can result in hundreds of megabytes in lost space.

The only solution is to partition your hard disks or use the FAT32 file system. By creating multiple partitions you lower the potential amount of lost space - you still lose some, but since the cluster size on each drive is lower you can greatly reduce the loss.

The limits of FAT16 are obvious. Partitions of 512 MB or less are the most attractive. However trying to manage 512MB partitions on multi-gigabyte drives is a real pain, not many people want to deal with 12 logical drives on a 6.5GB hard disk. On top of that with several programs installed the Windows9X folder itself can easily grow to over 500MB.

FAT32 is the solution for large disk drives. There is a small performance loss in using FAT32 because of the increased amount of clusters however, the benifits far outway the performance loss. Managing the one or two partions is much easier than trying to figure out which drive your information is on or asking yourself "Did I backup that seventh partion or not?".