The Hard Drive

The hard disk drive is the main storage device of a modern computer system. Hard drives are random access devices: they can retrieve the stored data anywhere on the disk in any order. (By contrast, sequential access devices, such as audio tape recorders or 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 CPU provides the address. The ability to randomly store and retrieve data is the most important reason why disk drives rapidly displaced tape as the primary computer storage technology.

 Inside the drive's case are one or more constantly spinning aluminum-alloy platters, arranged one on top of another in a stack. Digital information - software programs, files, pictures, even sound - are stored on the platter's surfaces in concentric rings or tracks. 

A read/write head attached to a movable arm - much like a stereo turntable's arm and needle - reads information from and writes information to the drive's platters. 

When you are at work on your computer, you enter commands through your keyboard or mouse. The hard drive's actuator arm - like the turntable's arm - responds to these commands and moves to the proper place on the platter. When it arrives, the drive's read/write head - like the needle on the tone arm - locates the information you've requested. The head reads, or retrieves, the information, transfers it to the CPU, and in short order, the data you requested can be processed.

The magnetic coating on the hard drive is composed of microscopic areas called domains. Each domain is like a tiny magnet with two opposite poles (a positive and a negative). Before recording data, the drive uses the read/write heads to orient the 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. By default the read/write heads magnetise all domains into the same direction. 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.

Hard disk drives record data in tracks, or concentric circles, called cylinders, that are numbered from the outermost edge of the disk to the innermost. The tracks are further divided into sectors - much like a spider's web - producing clusters. It is these clusters that actually hold the data. This disk rotates with a constant turn time, measured in revolutions per minute (rpm). A track is divided into sectors. A hard disk has a read/write head on each side of a disk, which is driven by a servo-motor.

For years, hard disk drives were confined to mainframe and minicomputer installations. Vast "disk farms" of giant 8- and 14-inch drives costing tens of thousands of pounds each whirred away in the air conditioned isolation of corporate data centres. The personal computer revolution in the early 1980s changed all that, ushering in the introduction of the first small hard disk drives. The first 5.25-inch hard disk drives packed 5 to 10 MB of storage - the equivalent of 2,500 to 5,000 pages of double-spaced typed information - into a device the size of a small shoe box. At the time, a storage capacity of 10 MB was considered too large for a so-called "personal" computer. Today’s personal computers usually use 3.5-inch drives and Laptop Computers use 2.5-inch drives. Their storage capacity increases rapidly, so does their access speed and data transfer rate.

Not surprisingly, the march to miniaturisation did not stop at 2.5-inch drives. By 1992, a number of 1.8-inch form factor drives appeared, weighing only a few ounces and delivering capacities up to 40 MB. Even a 1.3-inch drive, about the size of a matchbox, was introduced. Of course, smaller form factors in and of themselves are not necessarily better than larger ones. Where capacity and cost-per-megabyte are the leading criteria, larger form factor drives are still the preferred choice. For this reason, 3.5-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.

How are Hard Drives Constructed?

As mentioned previously, the part that gives a hard disk drive its name is a rigid aluminum-alloy disk, or platter. Although some hard drives use only a single platter, most use two or more platters in a disk stack assembly on a common spindle. The hard drive's spindle motor rotates the platters counter-clockwise at speeds of about 7,500 revolutions per minute (RPM). Drives used in desktop PC and workstations rotate continuously as long as the host system power is on, even when they are not reading or writing. That's why most operating systems have several power saving modes to prolong hard disk life. For example, the "sleep" mode causes the hard drive to stop spinning after a certain period of idle time. During the sleep mode, the drive's electronics remain powered up to provide a fast restart when the computer accesses the hard drive.

The 1s and 0s that comprise the information on a disk are stored as a magnetic pattern in the magnetic coating on the disk. The read/write heads generate these patterns in writing data to the disk platters. Then, when reading from the disk, the read/write head converts the stored magnetic patterns to electrical signals representing the stored data. Hard drives usually have a read/write head on each side of a platter. Each head is held in place by an actuator arm. The combined assembly of heads and disks is often referred to as the head disk assembly or HDA.

The actuator arms, and thus the read/write heads, are moved by a positioning motor, which is controlled by the disk controller. Today's sophisticated drives rely on a positioning technology, called closed-loop positioning, in which positioning information is fed back to the controller electronics, which direct the motor to readjust the position of the read/write heads accordingly. Closed-loop positioning technology ensures highly accurate reading and writing of data.

The signals that the read/write heads pick up from the platters are very weak. The read preamplifier and write driver circuit, mounted on a flex circuit inside the HDA, increase the strength of the signals so that chips on the printed circuit board (PCB) can convert this electrical impulse to a digital signal. The PCB contains the drive electronics which are part of the drive assembly. The drive electronics include:

• A digital signal processor (DSP) to convert the incoming electrical signals to digital signals

• Spindle and actuator motor controller electronics to ensure that the platters spin at the correct speed and that the actuator arms place the read/write heads over the precise spot on the platter

• Interface electronics to communicate with the system's CPU in the proper format

• A microprocessor and associated memory chips to oversee drive operations

The microprocessor and memory electronics on an advanced drive are highly sophisticated; for example, high capacity drives have on-board processing power equivalent to the entire computing power of the first AT/PCs. The base casting is a single piece of aluminum that provides an enclosure for the HDA and a mounting for a PCB. A gasket between the base casting and the top cover acts as a seal to provide a contamination-free operating environment for the read/write heads.

The link between the disk drive and the system bus is through the bus interface connector which takes signals from the drive's interface electronics and passes them to the I/O bus of the host computer. From there, the information is part of the electronic world of the CPU, available to be viewed, printed, edited, and returned again to the hard drive.

To get a better picture of how close the read/write head is to the disk surface, have a look at this illustration:

Organization of the data on the disks

You now know, a hard disk has cylinders, heads and sectors. If you look in your BIOS you will find these values listed. Today, these values are only used for compatibility with DOS, as they have nothing to do with the physical geometry of the drive. The hard disk calculates these values into a logical block address (LBA) and then this LBA value is converted into the real cylinder, head and sector values. Modern BIOS’ are able to use LBA, so limitations like the 504 MB barrier are now gone.

Cylinder, heads and sectors are still used in DOS environments. SCSI drives have always used LBA to access data on the hard disk. Modern operating systems access data via LBA directly without using the BIOS.

Because the hard disk spins at a constant speed, the transfer rate is higher when data is read or written to the outer parts of a disk. The reason is that there is more space for sectors. The number of sectors varies in steps. Usually on a disk there are 10 to 20 zones (called 'notches') with a constant sector number. That’ the reason why you see the steps in the transfer rates.

IDE or SCSI

• IDE = Integrated Drive Electronics

• SCSI = Small Computer System Interface

IDE and EIDE (Enhanced IDE) controllers are integrated in the motherboard and IDE and EIDE hard drives are also cheaper than SCSI drivers. Also, for SCSI you need an extra controller, such as a SCSI card, because not many motherboards have SCSI on board. The advantage of SCSI over IDE/EIDE is that SCSI cards can connect up to 7 devices (Wide SCSI 15 devices) all of which can use the bus at the same time, making the SCSI device perform faster.  IDE/EIDE has a primary and a secondary channel, each capable to take two devices, a master and a slave, totaling in four devices all of which have to take turns in controlling the bus, rather on a time sharing basis. However, IDE/EIDE is the most commonly used interface for today's domestic computers as it is cheaper, easier to implement and less troublesome. A total of four ports is also usually enough for domestic demands. For a speed comparison, have a look at the table in the Input/Output section.

If you purchase a new hard disk it is physically pre-formatted. This means that the cylinder, track and sector information is already written onto the disk. You now have to partition the disk to prepare it for the logical formatting and writing the partition information and boot sector to the disk. You don't have to use the whole hard disk in one single partition though. You can divide it in several partitions. Depending on the operating system there are several file systems option. Famous file systems are FAT (DOS, Windows16), FAT32 (Windows32), NTFS (Windows NT), HPFS (OS/2) and EXT2FS (Linux/Unix).

The main ways to compare hard disk brands is to look at the specifications available from some vendors. Look for drives that are:

• physically small

• spin fast

• have fast seek times

• have large buffers

• a long warranty

Of course, capacity is the main issue for most people, the larger, the better. And always remember that hard drives have warranties, your data does not.

With the prices for hard drives falling all the time you would just be insane not to go for the biggest one possible. As a rule of thumb: always get twice the capacity you think you need. And when making your choice of drive, always consider a major manufacturer.

Connectors

Since IDE/EIDE is the most commonly used interface I will use it here as an example of how to connect your hard drive to the system.

The first thing you have to do is decide for a port. Remember that you have two IDE/EIDE ports available, each capable of supporting two devices. If you only have one hard drive and one other IDE device (such as a CD ROM drive), then it is most advisable to give each device each own port, hence setting the jumpers on both devices to MASTER. If you have more than two devices you should connect the two fastest ones on the same port. Unless unavoidable, it is not advisable, for example to connect a hard drive and a CD ROM drive together as even the slowest of hard drives are a lot faster than the fastest of CD ROMs. That means that the faster device has to wait for any operations of the slower device to finish before it can resume its own operations. In praxis that means you are slowing you PC down.

Once you have decided on which port you want to connect your hard drive you have to tell the drive whether it is the MASTER or the SLAVE. This is done by use of jumper settings. All IDE devices use jumpers to tell them their rank in the port hierarchy. (Don't be mistaken by the fact that you might have an UDMA hard drive. UDMA only stands for Ultra Dynamic Memory Access, but the drive still uses IDE/EIDE architecture.)

Depending on the make of your hard drive, the jumpers will be either at the front or at the rear, sometimes even underneath. The usual options are: MA (Master), SL (Slave) and CS (Cable Select). You can only use Master and Slave in a standard IBM compatible machine. In the illustration on the right, the hard drive is set to 'Master'.

Your hard drive will indicate where to set the jumper in order to close the circuit required for MASTER or SLAVE. You have to set the little 'hat', i.e. jumper, to whatever your drive tells you.

The connectors for power and IDE cable are usually located on the back. The power connector (the little four pin connector at the bottom of the picture) takes the 5V/12V connector provided by the power supply unit. It only fits one way around, as one of its sides has beveled edges.

The IDE cable is the wide, flat ribbon cable that came with your motherboard. It has 40 individual strings, one of which is marked. The marked string (usually red or green) is string number 1. Your hard drive's IDE connector will have a marking to indicate which one is pin one. If there are no markings obey the golden rule of connecting the marked string towards the side of the power connector. 99 percent that's the right way around. Your IDE ribbon cable is likely to have three connectors, one for connection to the motherboard and two for the devices. The connectors are pressed 'in-line' onto he ribbon, therefore it does not matter which connector goes where. Just don't forget to set the jumpers! How to set up your hard drive in the BIOS is covered in the BIOS section.

One last word

Usually you should have no problems fitting your drive into the case. If, however you should run out of space or you want to position your hard drive (which is usually 3.5") into a 5.25" bay, ask your supplier for a set of brackets to bridge the gap.

In these enlightened days of experimentation and multi-operating system PCs it might be a good idea to fit your hard disk drives into carriers. These will enable you to run multiple hard drives with different programs or operating systems on the same system, one at a time. Make sure you set your BIOS to AUTO DETECT though, otherwise your computer will not play ball.