- Data rate - The data rate is the number of bytes per second that the drive can deliver to the CPU. Rates between 5 and 40 megabytes per second are common.
- Seek time - The seek time is the amount of time between when the CPU requests a file and when the first byte of the file is sent to the CPU. Times between 10 and 20 milliseconds are common.
- The other important parameter is the capacity of the drive, which is the number of bytes it can hold.
Storing Data
The data in a computer is stored using binary code. Within the computer's memory, 1s and 0s are stored as electrical impulses. On magnetic media, the 1s and 0s can be stored as either magnetic or nonmagnetic areas on the drive surface. Although there are magnetized and non-magnetized positions on the hard disk drive, the 1s and 0s of the binary code are stored in terms of flux reversals. These flux reversals are actually the transitions between magnetized and non-magnetized positions on the hard drive surface.
The goal of a hard disk drive is to quickly and directly access data stored on a flat surface. To do this, two different motions are required. As the disk spins, the R/W heads move across the platter perpendicular to the motion of the disk. The R/W heads are mounted on the ends of the actuator arms (much like the arm of an old record player). A critical element in hard disk drive design is the speed and accuracy of these actuator arms. Early hard disk drives used a stepper motor to move the actuator arms in fixed increments or steps. This early technology had several limitations:
- The interface between the stepper motor and actuator arm required that slippage be kept to a minimum. The greater the slippage, the greater the error.
- Time and physical deterioration of the components caused the positioning of the arms to become less precise. This deterioration eventually caused data transfer errors.
- Heat affected the operation of the stepper motor negatively. The contraction and expansion of the components caused positioning accuracy errors. (Components expand as they get warmer and contract as they cool. Even though these changes are very small, they make it difficult to access data, written while the hard drive is cold, after the disk has warmed up.)
- The R/W heads need to be "parked" when not in use. Parking moves the heads to an area of the disk that does not contain data. Leaving the heads on an area with data can cause that data to be corrupted. Old hard disk drives had to be parked with a command. Most drives today automatically park the heads during spin-down.
NOTE
Older hard disk drives (pre-EIDE or SCSI-2) require that the heads be parked before moving the computer. With these units it is recommended that you use the appropriate command to park the heads. The actual command can vary depending on the drive manufacturer, but you can try typing park at an MS-DOS prompt. Newer computers, including laptops, do not require that the drives be parked. Hard disk drives with stepping motor actuator arms have been replaced by drives that employ a linear motor to move the actuator arms. These linear voice coil motors use the same type of voice coil found in an audio loudspeaker, hence the name. This principle uses a permanent magnet and a coil on the actuator arm. By passing electrical current through the coil, it generates a magnetic field that moves the actuator arm into the proper position.
Voice coil hard disk drives offer several advantages:
- The lack of mechanical interface between the motor and the actuator arm provides consistent positioning accuracy.
- When the drive is shut down (the power is removed from the coil), the actuator arm, which is spring-loaded, moves back to its initial position, thus eliminating the need to park the head. In a sense, these drives are self-parking.
There is a drawback to this design: Because a voice coil motor can't accurately predict the movement of the heads across the disk, one side of one platter is used for navigational purposes, and so is unavailable for data storage. The voice coil moves the R/W head into an approximate position. Then the R/W heads on the reserved platter use the "map" to determine the head's true position and make any necessary adjustments. This is why hard drive specifications list an odd number of heads.
Head-to-disk interference (HDI) is a fancy term for head crash. These terms describe the contact that sometimes occurs between the fragile surface of the disk and the R/W head. This contact can cause considerable damage to both the R/W head and the disk. Never move—or even pick up—a hard disk drive until it is completely stopped; the momentum of the drive can cause a crash if it is moved or dropped during operation. Picking up a disconnected hard disk drive that is still spinning is not a good idea either. The rotation force of the platters can wrench it out of your hands, and the drive is not likely to survive the trip to the floor.
Hard disk drives are composed of one or more disks or platters on which data is stored. The geometry of a hard drive is the organization of data on these platters. Geometry determines how and where data is stored on the surface of each platter, and thus the maximum storage capacity of the drive. There are five numerical values that describe geometry:
- Heads
- Cylinders
- Sectors per track
- Write precompensation
- Landing zone
Write precompensation and landing zone are obsolete, but often seen on older drives. Let's take a look at each of these components.
TIP
All hard disk drives have geometry factors that must be known by the BIOS to read and write to the drive. Knowledge of the geometry is required to install or reinstall a hard drive. New PCs and drives often have technology that lets the BIOS get the information directly from the drive. You still need to know the figures, however, in case this technology fails.
The number of heads is relative to the total number of sides of all the platters used to store data. If a hard disk drive has four platters, it can have up to eight heads. The maximum number of heads is limited by BIOS to 16. Hard disk drives that control the actuator arms using voice coil motors reserve a head or two for accuracy of the arm position. Therefore, it is not uncommon for a hard disk drive to have an odd number of heads. Some hard disk drive manufacturers use a technology called sector translation. This allows some hard drives to have more than two heads per platter. It is possible for a drive to have up to 12 heads but only one platter. Regardless of the methods used to manufacture a hard drive, the maximum number of heads a hard drive can contain is 16.
Data is stored in circular paths on the surface of each platter. Each path is called a track. There are hundreds of tracks on the surface of each platter. A set of tracks (all of the same diameter) through each platter is called a cylinder. The number of cylinders is a measurement of drive geometry; the number of tracks is not a measurement of drive geometry. BIOS limitations set the maximum number of cylinders at 1024.
A hard disk drive is cut into tens of thousands of small arcs, like a pie. Each arc is called a sector and holds 512 bytes of data. The number of sectors is not important and is not part of the geometry; the important value is the number of sectors per track. BIOS limitations set the number of sectors per track at 63.
All sectors store the same number of bytes—512; however, the sectors toward the outside of the platter are physically longer than those closer to the center. Early drives experienced difficulty with the varying physical sizes of the sectors. Therefore, a method of compensation was needed. The write pre-compensation value defines the cylinder where write pre-compensation begins.
NOTE
The write pre-compensation value is now obsolete, but is often seen on older drives.
A landing zone defines an unused cylinder as a "parking place" for the R/W heads. This is found in older hard disk drives that use stepper motors. It is important to park the heads on these drives to avoid accidental damage when moving hard disk drives.
Cylinders, heads, and sectors per track are known collectively as the CHS values. The capacity of any hard disk drive can be determined from these three values.
The maximum CHS values are:
- 1024 cylinders
- 16 heads
- 63 sectors per track
- 512 bytes per sector
Therefore, the largest hard disk drive size recognized directly by the BIOS is 504 MB. Larger drive sizes can be attained by using either hardware or software translation that manages access to the expanded capacity without direct control by the system BIOS: 1024 × 16 × 63 × 512 bytes/sector = 528,482,304 bytes (528 million bytes or 504 MB) There are many hard disk drives that are larger than 504 MB. These drives manage to exceed this limitation in one of two ways: Either they bypass the system BIOS (by using one of their own) or they change the way the system BIOS routines are read.
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NOTE: For more information and clarification with regard on this topics, feel free to read “A+ Certification Training Kit / Microsoft Corporation.--3rd Ed.” PUBLISHED BYMicrosoft PressA Division of Microsoft CorporationOne Microsoft WayRedmond, Washington 98052-6399 Copyright © 2001 by Microsoft Corporation
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