Hard Drive Basics

Part I

The Evolution of The Hard Disk Drive

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In 1957 the first hard drive was introduced as a component of IBM's RAMAC 350. It required 50 24-inch disks to store 5MB of data and cost roughly $35,000 a year to lease - or $7,000 per MB per year.  For years the use of  hard disk drives was confined to mainframe and minicomputer installations.
Often several rooms or whole floors of buildings were used in housing giant 14 and 8 inch drives costing tens of thousands of dollars each. The PC revolution of the early 1980's changed all that, bringing forth the introduction of the first small hard disk drives.   The first 5.25-inch hard disk drives packed 5 to 10 MB of storage - also at a price of upwards of $10,000.00 - into an external device the size of a small shoe box. At that time, 10 MB was considered too large for a so-called "personal" computer.

The first PCs almost always used removable floppy disks for storage but with the introduction of the IBM PC/XT in 1983, hard disk drives became a standard component of most personal computers. The term "hard disk" is used because the inner disks that holds data are made of a rigid aluminum alloy. These disks, called platters, are coated with a much improved magnetic material that lasts much longer than the coating on a plastic, floppy diskette. The extended life of a hard drive is also due in part to the disk drive's read/write head.  In a hard disk drive, the heads don't come in contact with the actual storage media but in a floppy drive, the read/write head does contact the media, causing wear.

HDD's are designed to contain greater amounts of data than floppy disks and can store and retrieve it many times faster. Hard disks have gone the way of most computer components in that while very expensive when the technology is new, rapid declines in price meant that by the mid-80's, a 20 MB HDD capacity was a standard component of most PCs. (Because floppies are a cheap, removable storage media, floppy drives still are included in most PCs as a means for loading software and transporting and archiving vital data.)

By the mid-1980s,  the 5.25-inch form factor drives had shrunk considerably in terms of height. A standard drive measured about three inches high and weighed only a few pounds, while lower capacity "half-height" drives measured only 1.6 inches high. By 1987, 3.5-inch form factor hard drives began to appear. These compact units weighed as little as a pound and were about the size of a paperback book. They were first integrated into desktop computers and a short time later incorporated into the first truly portable laptop computers. The 3.5-inch form factor drives quickly became the standard for desktop and portable systems requiring less than 500 MB capacity. Height also kept shrinking with the introduction of one-inch high, 'low-profile' drives  Although the HDD has continued to shrink for application in laptops and PDE's with capacity and cost-per-megabyte the leading criteria, the larger form factor drives are still the preferred choice. For this reason, 3.5/5.25 inch drives will continue to dominate for the foreseeable future in desktop PCs and workstations, while 2.5-inch drives will continue to dominate in portable computers.

The drive to smaller form factors is made possible by continuing advances in electronics, disk media, read/write heads, and other disk drive technologies - all of which provide the ability to store ever more data on a given disk surface area. Technology advances have resulted in the doubling of areal density - and thus the megabyte capacity of a disk - about every 18 months.

Today, 3.5-inch, multi-gigabyte drives capable of storing and accessing  billions bytes of data are commonplace in both home PCs and corporate workstations running multimedia, high-end graphics, networking, and communications applications.

Mass Data Storage

RAM ( random access memory ) is extremely fast but rather expensive memory. It is also highly volatile. This means that as soon as the computer is turned off, all of the data stored in RAM is lost. Your computer's mass data storage fulfills the role of  a permanent, non-volatile place to store software and data.. Think of mass data storage as a data warehouse for your computer. It provides a long term repository with easy access to large amounts of information, both software and data. Storage is generally the final destination for the information you're working on: you store the finished version of your work until you are ready to use it again. Access to stored information is often the first thing you need to do your work. Storage then, is very often the first place your processor must access to get the data required for its operations.
When you first start an application program after turning on the computer, the CPU fetches the program and required data from storage. The program and data are copied from the hard disk into RAM, allowing it to execute with greater speed. While hard disk drives are the most common form, they are not the only storage media available to computer users. A variety of options are available, each with unique price/performance characteristics that meet specific requirements.  However, most desktop and workstation users will continue to rely on traditional hard disk drives for their primary storage needs - that is, direct access of programs and data for everyday, on-line use. For this purpose, hard disk drives provide the optimal combination of high capacity, fast access, and low cost.

How It Works: Inside the Hard Disk Drive

The main function of a hard disk drive is to store computer data and instructions in the form of binary numbers (The two distinct magnetization patterns which define digital computer information as 1s and 0s.)  The source of the data and instructions is any software program that involves the computer's processor working in combination with various peripheral components, including the hard disk drive itself.  Although a hard disk drive is the most complex component in your PC, in terms of moving parts, its mechanical operation is really quite simple. In many ways a hard disk drive operates like a jukebox. Inside the drive's case are one or more constantly spinning platters, arranged on top of one another in a stack. While working or playing with your computer, you enter commands through your keyboard or mouse. The hard drive's actuator arm - much like a jukebox's tone arm - acts upon these commands and moves to the proper place on the platter. When it gets to the proper place, the drive's read/write head - like the needle on a tone arm - locates the requested information . The head reads the information, transfers it through the processor into RAM, and in short order, the data you requested either appears on your monitor ready for use.

Reading and Writing Digital Information

The jukebox analogy holds for a hard drive's mechanical operation, but, the way in which a hard drives writes, stores, and reads data is actually similar to the way an audio tape recorder works. Audio tape is coated with a microscopic layer of  metallic particles. Sounds are recorded onto the tape by a recording head. These sounds are played back by a play head. Both the recording and play heads are small electro-magnets through which electrical currents flow in continuously changing patterns that represent the sounds. As the tape passes under the recording head, it is magnetized by these currents. As it passes under the play head, the magnetic patterns are picked up, translated into audio data, and then passed to the amplifier and loudspeaker.

Binary data is written to and read from hard drives in much the same way. Like audio tape, hard drive platters are coated with a magnetic material a few microns thick. But unlike an audio tape recorder, a disk drive uses a single read/write head to both write to and read from the disk.

To write information to a hard drive, an electrical current flowing through a coil in the read/write head produces a magnetic pattern in the coating on the media's surface corresponding to the data being written. To read information from a hard drive, the read/write head converts the magnetic patterns recorded on the media to an electrical current which is amplified and processed to reconstruct the stored data pattern.

Disk drives also differ from audio tape devices in the way in which data is stored. On tape, sound is recorded in continuously changing, analog patterns. On a hard drive, data is recorded as electrical signals. These signals are numerically expressed as 1s and 0s in the two-digit binary system described earlier.

The magnetic coating on the hard drive is composed of microscopic areas called domains. Each domain is in itself similar to a tiny magnet with two opposite poles, positive and negative. Before recording data, the drive uses the read/write heads to polarize these domains in a small region so that the magnetic poles all point in the same direction. Then, the hard drive uses the read/write heads to record data. While all 1s could be recorded as a region with the positive pole to the left and all 0s with the positive pole to the right, the hard drive uses a more efficient recording method called flux reversal. Whenever the drive encounters a 1, the read/write heads reverse the magnetic pole direction. Whenever the drive encounters a 0, the read/write heads do not change the magnetic pole direction.

The data on a hard disk is recorded in tracks, or concentric circles, that are numbered from the outermost edge of the disk to the innermost. Since the hard drive is a random access device, it can retrieve the stored data anywhere on the disk in any order. Sequential access devices, such as tape backup systems are not as quick to retrieve data. They move to a new location on a tape by fast forwarding or rewinding the tape.) A hard drive's read/write head can literally fly directly to a new location once the processor provides the address. This ability is the most important reason disk drives rapidly replaced tape as the primary computer storage technology.

Hard disk drives record data on both sides of a platter when read/write heads are positioned on each side. For example, a hard drive with two platters could have up to four data storage surfaces and four read/write heads; a hard drive with three platters could have up to six data storage surfaces and six read/write heads; and so on. The drive actuator arm synchronizes all the read/write heads so that they stay in perfect alignment as they move together across the platter's surface.

This surface is organized so the hard drive can easily find data. The concentric tracks are divided into units called sectors.  Information is recorded on the outermost track of all platters first. Once the outside track is filled, the heads move inward and begin writing on the next free track. This recording strategy greatly boosts performance since the read/ write heads can record more data at one position before having to move to the next track. For example, if the read/write heads in a four-platter drive are positioned on a specific track, then the drive writes to that same track on both sides of all four platters before moving the read/write head inward to the next track.

The design of hard disk drives makes them quite fast - much faster than either tape or floppy diskette drives. What's more, unlike floppy drives, hard drives virtually eliminate friction between the disk and read/write head, further increasing performance and reducing wear on the heads and media. When you turn on your desktop computer, the platters in your hard disk drive immediately begin spinning at 4,000 rpm or higher. They remain spinning until you turn the computer off or it loses power. This high speed spinning creates a tiny cushion of air above each platter, permitting the tiny read/write heads to float, or fly, just above the surface.

Amazingly, the read/write heads are precisely designed to fly just a few microns above the surface of the platters - a space considerably thinner than a shaft of human hair or even a particle of smoke. Despite the speed and tolerances involved, the heads never touch the surface of the platters while the disk is spinning. When the system is turned off, the platters stop spinning, and the read/write heads touch down in a designated landing zone, separate from the area on the platters where data is stored. If contamination or severe shock cause the heads to touch the surface, the heads or data surface can be damaged, data lost, and, in the most extreme cases, the drive can even be rendered unusable. With today's advanced technology and design crashes are rare because drives are tightly sealed to keep out contaminants and built to withstand shocks in the range of 70 to 100 times the force of gravity (70-100 Gs).

Part 2

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