Hard disk drive is used to store data and programs. It is one kind of memory. But the information stored in it doesn’t vanish when the computer is off.
The movement of disks and heads and the transfer of data between computer and hard disk are controlled by a circuitry called hard disk controller. The computer orders everything to the controller and the rest of the controlling is just the controller’s headache.
In the older computers the hard disk controller was built in a separate circuit board called disk controller card or I/O card. Nowadays, the hard disk controller is built in the drive itself. The motherboard is connected with the controller by a cable. To get or store any data in a hard disk, the processor orders the controller. Then the controller supplies the data and controls the hard disk through the cable and the hard disk does the work as ordered.
Introduction to Drive Interface Drive interface is the world wide predefined standard by which a controller can talk to its drive. This standard defines the cable and connectors between the controller and the hard disk and the language by which they talk together.
We can get controllers from different industries and also hard disk drives from different industries. But there is no problem because of the interface. For example, if we connect the controller of one industry having IDE interface with a hard disk drive of another industry having IDE interface, they can work together because the interface (i.e., the language) between them is the same. Different drive interfaces are available. We have discussed here some common interfaces like ST506, ESDI, SCSI and IDE.
Here ‘ST’ stands for ‘Shugart Technologies’ because this interface is first defined by this industry. It has the following characteristics:
It is an advanced design of ST506. Some problems of ST506 interface are removed in this modified design. It has the following characteristics:
Cabling same as ST506 (20-wires data and 34-wires control).
SCSI stands for Small Computer System Interface. SCSI is pronounced as “scuzzy”. It has the following characteristics:
IDE stands for Integrated Drive Electronics. It is also a modified version of ST506. To get a more reliable but cheaper interface technology, IDE is designed. It has the following characteristics:
EIDE stands for Enhanced IDE. It is designed by Western Digital. It has the following characteristics:
what is hard drive in computer
When it comes to store data permanently and accessing them quickly, Hard disk drive (HDD) systems (hard disks or hard drives for short) are used as one of the most popular options. Although there are instances of external and removable hard drives, hard disks typically reside inside the computer, where they are semi-permanently mounted with no external access. HDD can hold more information than other forms of storage. Hard drives are known as conventional drives to contrast them from newer and fancier solid-state drives. HDDs use a magnetic storage medium and thus it has become convention in today’s world.
Components of a Hard Drive System
Although we see a Hard Drive as a single object, actually it consists of the following three essential components of a Hard Drive:
Controller
The component that controls the drive to operate and to encode data onto the platters is known as controller chip. It controls the process of sending signals from data to the various motors in the drive and receiving signals from the sensors inside the drive. Most of todays hard disk technologies incorporate the controller and drive into one assembly. The most common and well-known of these are PATA and SATA.
Hard Disk
This is the physical storage medium where data is stored. The drive systems of Hard disk store information on small discs which are under 1 inch to 5 inches in diameter, and called platters which are stacked together and placed in an enclosure.
Basic Construction of a Hard Disk With every computer the hard disk is sealed to prevent any sort of physical damage. To describe the basic construction of a hard disk the following list of
terms will be used:
- Platters
- Read/write heads
- Tracks
- Sectors
- Cylinders
- Clusters (allocation units)
Platters, also called surface, lie inside the sealed case of the hard drive, where the actual data is stored by the read/write heads. Their quantity can be one or more. Each platter has its own read and write head with each surface containing a large number of concentric tracks. The heads can move together along both in and out directions of the radius of the surfaces. If we place the head in a given position, the corresponding tracks on each surface can be accessed. This group of tracks is called a cylinder.
The tracks are divided around their length into sectors where inner tracks are shorter than outer ones which generally have fewer sectors. In other words, sectors are the magnetic domains representing the smallest units of storage on the disc platters. Magnetic-drive sectors commonly store only 512 bytes (1 2KB) of data each.
Capacity of a Hard Disk
The basic hard disk geometry consists of three components:
i) The number of sectors that each track contains.
ii) The number of read/write heads in the disk assembly.
iii) The number of cylinders in the assembly.
This set of values is known as CHS (Cylinders/Heads/Sectors). The number of cylinders is the number of tracks that can be found on any single surface of any single platter. Because the number of cylinders indicates only the number of tracks on any one writable surface in the assembly, the number of writable surfaces must be factored into the equation to produce the total number of tracks in the entire assembly. This is where the number of heads comes in. There is a single head dedicated to each writable surface, two per platter. By multiplying the number of cylinders by the number of heads, you produce the total number of tracks throughout the disk assembly. By multiplying this product by the number of sectors per track, you discover the total number of sectors throughout the disk assembly.
Dividing the result by 2 provides the number of kilobytes the hard drive can store. This works because each sector holds 512 bytes, which is equivalent to 12KB. Each time you divide the result by 1024, you obtain a smaller number but the unit of measure increases from kilobytes to megabytes, from megabytes to gigabytes, terabyte and so on.
HDD Speeds
The electronics within the HBA and controller get faster and they are capable of requesting data at higher and higher rates. The platters, spinning at a constant rate, can access information as fast as a given fixed rate. Manufacturers increase the speed at which the platters spin from one generation of drives to the next to make information available to the electronics more quickly. Thus, multiple speeds coexists in the marketplace for an unpredictable period until demand dies down for one or more speeds.
Standard Interfaces for Hard Drives
Standard interfaces define the physical and logical means by which a hard
drive is connected to the motherboard.
Basically they are 3 types:
i) Advanced Technology Attachment (ATA)/PATA / IDE
ii) Serial ATA (SATA)
iii) Small Computer System Interface (SCSI)
Advanced Technology Attachment (ATA)
The ATA interface was introduced in 1986 and was the first successful attempt at standardizing the interface between a motherboard and its hard drives. During the 1990’s up to the mid-2000’s, virtually every PC manufactured had an IDE interface to pass information back and forth between the motherboard and the higher volume storage devices such as hard drives, CDROMs, and
tape drives.
This interface is also referred to as the Integrated Drive Electronics (IDE) interface. With advent of Serial ATA (will be discussed later) the traditional ATA is termed as Parallel ATA or simply PATA.
Most of the motherboards that are ATA-equipped will contain one or two built in 40-pin IDE host adaptors. The image below shows two IDE host adaptors side-by-side on a motherboard, one labeled IDE1 and other labeled IDE2. IDE1 is the primary interface. Each interface can handle up to two devices, so a motherboard with two IDE host adaptors can handle up to four drives.
IDE standard (or ATA) has a number of enhancements as follows:
IDE (ATA-1): IDE is the original definition where each adapter had two devices on them, one assigned as a master (device 0) and the other as a slave (device 1). The devices themselves usually had to have jumper or some mechanism, to determine which was which. The interface used
DMA to allow devices to transfer data directly to memory bypassing processor intercession. The interface allowed for data transfer rates up to 4.16 Megabytes per second.
EIDE (ATA-2): EIDE increased performance over IDE by increasing the total hard drive size supported to 137.4 Gigabytes and increasing the maximum data transfer rate to 16.67 Megabytes per second. BIOS limitations, however, limited drive size to 8.4 Gigabytes.
ATA-3: ATA-3 provided for improved reliability and password protection to access drives.
ATA-4: ATA-4 used better DMA support and integration of AT Attachment Program Interface (ATAPI).
A common interface for CDROMs is provided by this. Data transfer rates were allowed as high as up to 33.33 Megabytes per second. ATA-4 also defined the use of 80 conductor cables where alternating wires in the cable are connected to ground in order to reduce the effects of electrical interference.
ATA-5: ATA-5 added auto detection for the cable type and increased data transfer rates (up to 66.67 Megabytes per second).
IDE Connections: The connection for the IDE interface is a 40-pin connector on the back of the hard drive. The pins are spaced 0.1" apart and are in two rows of twenty. The pins are numbered, and typically (but you should verify this before you connect the drive) pin 1 is located nearest the connector for the power. The ribbon cable used to connect the hard drive to the motherboard has a single red wire identifying ”pin 1”. The power connector also has a red wire. These red wires should be next to each other when connected to a standard hard drive.
Jumpers: Each IDE interface allows up to two drives on a single cable which will be needed to be able to configure the drives to make one device as device 0 and the other as device 1. A small plastic clip called a jumper can configure a device as a primary or secondary drive as an usual approach.
A jumper, the blue block is used to straddle two pins on a circuit board to create a ”short circuit” or electrical connection that the hard drive controller can detect. The jumper with plastic coating on the outside has a metal strip inside of the plastic which connects the two pins.
Jumper Protocols: Two different protocols can be used for jumpering PATA devices.
i) Master-slave: With this protocol, one device is jumpered as master and the other is jumpered as slave. If you have only one hard drive then it should be set as master. When connecting more than one hard drive to a computer on the same IDE controller, you generally have to assign one as the primary (master) and one as the secondary (slave).
ii) Cable Select: The second protocol known as cable select is the protocol where both devices are jumpered as cable select. The position on the cable dictates which is the master and which is the slave. The end device is master while the device on the middle of the cable is slave. You can use either of these protocols but you cannot mix them on the same data cable.
Serial ATA (SATA)
Serial ATA, introduced in 2003, was a replacement for ATA which is now sometimes referred to as PATA for parallel ATA. The benefits of such technology are smaller cabling and faster transfer times. The smaller cabling meant better air flow unlike ribbon cables which are wide and block air flow inside of the computer case. Benefits of serial interface include fewer conductors which resulted in allowing for a smaller cable and more airflow inside the case. Additionally, smaller cables means smaller connectors providing more space on the motherboard and simpler wiring.
As for the faster rates, remember from our description of crosstalk that the higher the frequency, the worse the transmission of electromagnetic signals. With fewer conductors running next to each other, crosstalk becomes much less of an issue.
A brief description of the changes in the SATA interface is presented below. The original SATA definition allowed for a data transfer rate of 1.5 gigabits per second (Gbit/s). This generation of SATA was not noticeably faster than the last generation of ATA devices.
The second generation of SATA allows multiple device transactions to occur simultaneously in addition to increasing the bit rate to 3.0 Gbit/s. Because of the faster data rate, 3.0 Gbit/s SATA requires a cable that is capable of supporting the higher rate. (1.5 Gbit/s cables will work, just not for high demand applications.)
6.0 Gbit/s SATA exists, but for the most part, this data rate far exceeds that which today’s hard drives are capable of driving.
Small Computer System Interface (SCSI)
Another interface definition that computers use to communicate to hard drives is the Small Computer System Interface or SCSI. Like ATA, there are a number of variations on the interface definition, but unlike ATA, not all of the interfaces are backwards compatible with earlier drives or controllers.
The initial standardization of the interface was made as SCSI-1 which had an 8-bit wide data bus and a clock speed of 5 MHz. Two primary evolutions of the SCSI interface have been present since then, SCSI-2 and SCSI-3. There are also many sub-variations that defined different speeds and bus widths. The current iteration is the SCSI-3 Ultra 320 which has a 16 bit wide data bus and a clock speed of 80 MHz. This allows for a transfer rate of 320 Megabytes per second. In addition, up to 16 devices may be connected to this interface.
SCSI has better support for multiple drives making it a superior interface for applications such as RAID systems. RAID (Redundant Array of Independent Discs) is a system which uses multiple hard drives and make them appear as one. Different configurations of RAID can be found which are meant to improve performance and reliability. Aditionally, SCSI can be used as an interface to peripherals like scanners, CD-ROM/RW drives, printers, and other storage devices. The SCSI interface tends to be more expensive than ATA, and as such tends to be used for high-performance applications such as servers.
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