HARD DISKHARD DISK

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The best way to understand how a hard disk works is to take a look inside. (Note that The best way to understand how a hard disk works is to take a look inside. (Note that OPENING A HARD DISK RUINS ITOPENING A HARD DISK RUINS IT, so this is not something to try at home unless you , so this is not something to try at home unless you have a defunct drive.) have a defunct drive.)

Here is a typical hard-disk drive: Here is a typical hard-disk drive: It is a sealed aluminum box with controller electronics attached to one side. The It is a sealed aluminum box with controller electronics attached to one side. The

electronics control the read/write mechanism and the electronics control the read/write mechanism and the motormotor that spins the platters. The that spins the platters. The electronics also assemble the magnetic domains on the drive into bytes (reading) and electronics also assemble the magnetic domains on the drive into bytes (reading) and turn bytes into magnetic domains (writing). The electronics are all contained on a small turn bytes into magnetic domains (writing). The electronics are all contained on a small board that detaches from the rest of the drive:board that detaches from the rest of the drive:

Underneath the board are the connections for the motor that spins the plattersUnderneath the board are the connections for the motor that spins the platters Removing the cover from the drive reveals an extremely simple but very precise Removing the cover from the drive reveals an extremely simple but very precise

interior: interior:

The The plattersplatters - These typically spin at 3,600 or 7,200 rpm (rotations per minute) when - These typically spin at 3,600 or 7,200 rpm (rotations per minute) when the drive is operating. These platters are manufactured to amazing tolerances and are the drive is operating. These platters are manufactured to amazing tolerances and are mirror-smooth (as you can see in this interesting self-portrait of the author... no easy mirror-smooth (as you can see in this interesting self-portrait of the author... no easy way to avoid that!). way to avoid that!).

The The armarm - This holds the read/write heads and is controlled by the mechanism in the - This holds the read/write heads and is controlled by the mechanism in the upper-left corner. The arm is able to move the heads from the hub to the edge of the upper-left corner. The arm is able to move the heads from the hub to the edge of the drive. The arm and its movement mechanism are extremely light and fast. The arm on drive. The arm and its movement mechanism are extremely light and fast. The arm on a typical hard-disk drive can move from hub to edge and back up to 50 times per a typical hard-disk drive can move from hub to edge and back up to 50 times per second -- it is an amazing thing to watch! second -- it is an amazing thing to watch!

Nekada Davno….Nekada Davno….

Cuvanje podatakaCuvanje podataka

In order to increase the amount of In order to increase the amount of information the drive can store, most information the drive can store, most hard disks have hard disks have multiple plattersmultiple platters. This . This drive has three platters and six drive has three platters and six read/write heads: read/write heads:

The mechanism that moves the arms on The mechanism that moves the arms on a hard disk has to be incredibly fast and a hard disk has to be incredibly fast and precise. It can be constructed using a precise. It can be constructed using a high-speed linear motor high-speed linear motor

Data is stored on the surface of a platter Data is stored on the surface of a platter in in sectorssectors and and trackstracks. Tracks are . Tracks are concentric circles, and sectors are pie-concentric circles, and sectors are pie-shaped wedges on a track, like this:shaped wedges on a track, like this:

A typical track is shown in yellow; a A typical track is shown in yellow; a typical sector is shown in blue. A sector typical sector is shown in blue. A sector contains a fixed number of bytes -- for contains a fixed number of bytes -- for example, 256 or 512. Either at the drive example, 256 or 512. Either at the drive or the or the operating systemoperating system level, sectors level, sectors are often grouped together into are often grouped together into clustersclusters

Kapacitet i PerformanseKapacitet i Performanse

A typical desktop machine will have a hard disk with a capacity of A typical desktop machine will have a hard disk with a capacity of between 10 and 40 between 10 and 40 gigabytesgigabytes. Data is stored onto the disk in the . Data is stored onto the disk in the form of form of filesfiles. A file is simply a named collection of . A file is simply a named collection of bytesbytes. The . The bytes might be the bytes might be the ASCII codesASCII codes for the characters of a text file, or for the characters of a text file, or they could be the instructions of a software application for the they could be the instructions of a software application for the computer to execute, or they could be the records of a data base, computer to execute, or they could be the records of a data base, or they could be the pixel colors for a GIF image. No matter what it or they could be the pixel colors for a GIF image. No matter what it contains, however, a file is simply a string of bytes. When a contains, however, a file is simply a string of bytes. When a program running on the computer requests a file, the hard disk program running on the computer requests a file, the hard disk retrieves its bytes and sends them to the CPU one at a time. retrieves its bytes and sends them to the CPU one at a time.

There are two ways to measure the performance of a hard disk: There are two ways to measure the performance of a hard disk: Data rateData rate - The data rate is the number of bytes per second that - The data rate is the number of bytes per second that

the drive can deliver to the CPU. Rates between 5 and 40 the drive can deliver to the CPU. Rates between 5 and 40 megabytes per second are common. megabytes per second are common.

Seek timeSeek time - The seek time is the amount of time between when - 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 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 CPU. Times between 10 and 20 milliseconds are common.

The other important parameter is the The other important parameter is the capacitycapacity of the drive, which of the drive, which is the number of bytes it can hold.is the number of bytes it can hold.

Cassette Tape vs. Hard DiskCassette Tape vs. Hard Disk Let's look at the big differences between cassette tapes and hard disks: Let's look at the big differences between cassette tapes and hard disks: The magnetic recording material on a cassette tape is coated onto a thin The magnetic recording material on a cassette tape is coated onto a thin

plastic strip. In a hard disk, the magnetic recording material is layered plastic strip. In a hard disk, the magnetic recording material is layered onto a high-precision aluminum or glass disk. The hard-disk platter is onto a high-precision aluminum or glass disk. The hard-disk platter is then polished to mirror-type smoothness. then polished to mirror-type smoothness.

With a tape, you have to fast-forward or reverse to get to any particular With a tape, you have to fast-forward or reverse to get to any particular point on the tape. This can take several minutes with a long tape. On a point on the tape. This can take several minutes with a long tape. On a hard disk, you can move to any point on the surface of the disk almost hard disk, you can move to any point on the surface of the disk almost instantly. instantly.

In a cassette-tape deck, the read/write head touches the tape directly. In In a cassette-tape deck, the read/write head touches the tape directly. In a hard disk, the read/write head "flies" over the disk, never actually a hard disk, the read/write head "flies" over the disk, never actually touching it. touching it.

The tape in a cassette-tape deck moves over the head at about 2 inches The tape in a cassette-tape deck moves over the head at about 2 inches (about 5.08 cm) per second. A hard-disk platter can spin underneath its (about 5.08 cm) per second. A hard-disk platter can spin underneath its head at speeds up to 3,000 inches per second (about 170 mph or 272 head at speeds up to 3,000 inches per second (about 170 mph or 272 kph)! kph)!

The information on a hard disk is stored in extremely small magnetic The information on a hard disk is stored in extremely small magnetic domains compared to a cassette tape's. The size of these domains is domains compared to a cassette tape's. The size of these domains is made possible by the precision of the platter and the speed of the made possible by the precision of the platter and the speed of the medium. medium.

Because of these differences, a modern hard disk is able to store an Because of these differences, a modern hard disk is able to store an amazing amount of information in a small space. A hard disk can also amazing amount of information in a small space. A hard disk can also access any of its information in a fraction of a second.access any of its information in a fraction of a second.

Jos o BrziniJos o Brzini

Desktop disks rotate at either Desktop disks rotate at either 5400 or 7200 rotations per 5400 or 7200 rotations per minute. minute.

Server disks rotate at 10000 Server disks rotate at 10000 or 15000 RPM. or 15000 RPM.

Laptop disks frequently rotate Laptop disks frequently rotate at a slower speed of 4200 at a slower speed of 4200 RPM, although some match RPM, although some match the desktop 5400 speed and the desktop 5400 speed and one or two even go to 7200. If one or two even go to 7200. If a disk were to rotate at 6000 a disk were to rotate at 6000 RPM then each rotation would RPM then each rotation would take .01 seconds take .01 seconds

IDEIDE

IDE stands for Integrated Disk Electronics. By the early '90s, IDE stands for Integrated Disk Electronics. By the early '90s, however, computer chips had become cheap and powerful enough however, computer chips had become cheap and powerful enough to put all the control logic for motors, recording arm positioning, and to put all the control logic for motors, recording arm positioning, and digital to analog conversion on the disk itself. Thus the "electronics" digital to analog conversion on the disk itself. Thus the "electronics" became "integrated" on the disk (IDE).became "integrated" on the disk (IDE).

Over a decade, every piece of technology changes. The good news Over a decade, every piece of technology changes. The good news is that chips get faster and smarter. Each year the engineers made it is that chips get faster and smarter. Each year the engineers made it better and faster by building extensions on top of the original better and faster by building extensions on top of the original architecture. Better chips allow smarter logic without increasing the architecture. Better chips allow smarter logic without increasing the cost.cost.

Once every computer came with an IDE controller chip and cables, it Once every computer came with an IDE controller chip and cables, it made sense to adapt other devices to use the same interface. The made sense to adapt other devices to use the same interface. The most important such device is the CD or DVD reader or writer.most important such device is the CD or DVD reader or writer.

DMADMA

"Direct Memory Access" or DMA. The operating system reserves an "Direct Memory Access" or DMA. The operating system reserves an area of memory to hold data for the I/O operation. It passes the area of memory to hold data for the I/O operation. It passes the address of this memory to the disk controller. The disk controller address of this memory to the disk controller. The disk controller transfers data directly between the designated area of memory and transfers data directly between the designated area of memory and the buffer memory on the hard drive. Meanwhile, the CPU is running the buffer memory on the hard drive. Meanwhile, the CPU is running some other task. The CPU only has to be interrupted when the some other task. The CPU only has to be interrupted when the transfer is complete.transfer is complete.

DMA becomes essential for multimedia applications where large DMA becomes essential for multimedia applications where large amounts of data must be processed and performance is important. amounts of data must be processed and performance is important. The first DMA operated at what amounts to 16 MHz. Subsequent The first DMA operated at what amounts to 16 MHz. Subsequent generations increased the speed to 33, 66, 100, and 133 MHz. generations increased the speed to 33, 66, 100, and 133 MHz. Anything above 33 MHz is also called Ultra DMA or UDMA. All Anything above 33 MHz is also called Ultra DMA or UDMA. All modern mainboards support UDMA. Unfortunately, for disk UDMA to modern mainboards support UDMA. Unfortunately, for disk UDMA to operate at the highest possible speed, the disk has to support the operate at the highest possible speed, the disk has to support the speed, and the cable has to be properly connected speed, and the cable has to be properly connected

40 or 80 Wire Cable40 or 80 Wire Cable The first generation of DMA transferred data at a 33 MHz rate and did not The first generation of DMA transferred data at a 33 MHz rate and did not

require a change to the disk cable. However, to transfer data at any higher require a change to the disk cable. However, to transfer data at any higher speed, the old grey cable with 40 wires was not good enough. A wire is also speed, the old grey cable with 40 wires was not good enough. A wire is also an antenna. When two wires run next to each other for a distance, as in the an antenna. When two wires run next to each other for a distance, as in the ATA cable, The electric activity on one wire generates an electromagnetic ATA cable, The electric activity on one wire generates an electromagnetic signal over the length of the wire, This is then picked up by the wire next to it signal over the length of the wire, This is then picked up by the wire next to it and produces a weak duplicate of the original activity. At high speed this and produces a weak duplicate of the original activity. At high speed this "crosstalk" becomes a problem. The solution is to transmit the data over "crosstalk" becomes a problem. The solution is to transmit the data over twisted pairs of wires, but this would require a complete reengineering of the twisted pairs of wires, but this would require a complete reengineering of the ATA interface.ATA interface.

Fortunately, the crosstalk was so weak that a less complicated solution was Fortunately, the crosstalk was so weak that a less complicated solution was possible. Although the ATA plug continued to have 40 pins, a new generation possible. Although the ATA plug continued to have 40 pins, a new generation of "blue" ATA cables now has 80 wires. Every other wire is a dummy, it of "blue" ATA cables now has 80 wires. Every other wire is a dummy, it carries no signal and is connected to ground. By this trick, between every two carries no signal and is connected to ground. By this trick, between every two "adjacent" signal carrying wires there is one dummy wire to receive and "adjacent" signal carrying wires there is one dummy wire to receive and dissipate the electromagnetic crosstalk. This is sufficient to bump the Ultra dissipate the electromagnetic crosstalk. This is sufficient to bump the Ultra DMA transfer up to 100 or 133 MHz.DMA transfer up to 100 or 133 MHz.

To go higher than 133 MHz, you need an entirely different technology. Serial To go higher than 133 MHz, you need an entirely different technology. Serial ATA transfers data over a much smaller cable using serial transmission of bits ATA transfers data over a much smaller cable using serial transmission of bits at a much higher clock rate. Currently, SATA transfers data at 150 Megabytes at a much higher clock rate. Currently, SATA transfers data at 150 Megabytes per second and a second generation 300 Megabyte per second version per second and a second generation 300 Megabyte per second version should be available shortly. Even 150 megabytes is more than twice the should be available shortly. Even 150 megabytes is more than twice the transfer speed of the fastest hard disk.transfer speed of the fastest hard disk.

““Master" and "Slave"Master" and "Slave" IDE was designed to support either one or two devices. IDE was designed to support either one or two devices. In the original IDE design, the first device plugged into the cable had to In the original IDE design, the first device plugged into the cable had to

perform all the supervisory functions required of the device end of the perform all the supervisory functions required of the device end of the protocol. When a second device was plugged in, it would allow the first protocol. When a second device was plugged in, it would allow the first device to continue to supervise the device end of the bus. Following the device to continue to supervise the device end of the bus. Following the common jargon of electrical engineering, the device in control of the bus common jargon of electrical engineering, the device in control of the bus is called the Master and the device that follows that control is called the is called the Master and the device that follows that control is called the Slave.Slave.

A more descriptive analogy for the relationship between the two devices A more descriptive analogy for the relationship between the two devices would be an analogy to dancing partners, where one "leads" and the would be an analogy to dancing partners, where one "leads" and the other "follows".other "follows".

Each disk has a set of pins on the back and a "jumper" that can be Each disk has a set of pins on the back and a "jumper" that can be removed or plugged onto a pair of pins to close a circuit. Following removed or plugged onto a pair of pins to close a circuit. Following instructions typically written on a label attached to the disk, the jumper instructions typically written on a label attached to the disk, the jumper can be set to tell the device to be Master, Slave, or CS for "Cable can be set to tell the device to be Master, Slave, or CS for "Cable Select". If set to CS, then the device senses if it is connected to the Select". If set to CS, then the device senses if it is connected to the black connector at the end of the cable (and then becomes Master) or to black connector at the end of the cable (and then becomes Master) or to the grey connector in the middle of the cable (and then becomes Slave).the grey connector in the middle of the cable (and then becomes Slave).

The simplest solution would be to always The simplest solution would be to always use Cable Select. However, in practice use Cable Select. However, in practice this doesn't typically work. A tower, or this doesn't typically work. A tower, or any other configuration with room for any other configuration with room for expansion, typically positions the first expansion, typically positions the first disk in the lowest available 3.5 inch bay. disk in the lowest available 3.5 inch bay. If you buy a second disk and put it in the If you buy a second disk and put it in the bay immediately above the first disk, the bay immediately above the first disk, the only way the cable will attach is to plug only way the cable will attach is to plug the new disk into the black end the new disk into the black end connector and the old disk into the grey connector and the old disk into the grey middle connector, So you must, in this middle connector, So you must, in this situation, jumper the old disk to be situation, jumper the old disk to be Master and the new disk to be Slave to Master and the new disk to be Slave to get the system to boot properly.get the system to boot properly.

A mainboard has two standard ATA A mainboard has two standard ATA plugs for two cables. Each cable has plugs for two cables. Each cable has plugs for two devices. So a standard PC plugs for two devices. So a standard PC supports a total of four disk/CD devices. supports a total of four disk/CD devices.

Floppy disk drajvFloppy disk drajv

The floppy disk drive (The floppy disk drive (FDDFDD). The first floppy drives ). The first floppy drives used an 8-inch disk (later called a "used an 8-inch disk (later called a "diskettediskette" as it got " as it got smaller), which evolved into the 5.25-inch disk that was smaller), which evolved into the 5.25-inch disk that was used on the first IBM Personal Computer in 1981. used on the first IBM Personal Computer in 1981.

The 5.25-inch disk held 360 kilobytes compared to the The 5.25-inch disk held 360 kilobytes compared to the 1.44 megabyte capacity of today's 3.5-inch diskette. 1.44 megabyte capacity of today's 3.5-inch diskette.

The 5.25-inch disks were dubbed "The 5.25-inch disks were dubbed "floppyfloppy" because the " because the diskette packaging was a very diskette packaging was a very flexible plastic flexible plastic envelopeenvelope, unlike the rigid case used to hold today's , unlike the rigid case used to hold today's 3.5-inch diskettes. 3.5-inch diskettes.

The major parts of a FDD: The major parts of a FDD:

Read/Write HeadsRead/Write Heads: Located on both sides of a diskette, they move together on the : Located on both sides of a diskette, they move together on the same assembly. The heads are not directly opposite each other in an effort to prevent same assembly. The heads are not directly opposite each other in an effort to prevent interaction between write operations on each of the two media surfaces. The same interaction between write operations on each of the two media surfaces. The same head is used for reading and writing, while a second, wider head is used for erasing a head is used for reading and writing, while a second, wider head is used for erasing a track just prior to it being written. This allows the data to be written on a wider "clean track just prior to it being written. This allows the data to be written on a wider "clean slate," without interfering with the analog data on an adjacent track. slate," without interfering with the analog data on an adjacent track.

Drive MotorDrive Motor: A very small spindle motor engages the metal hub at the center of the : A very small spindle motor engages the metal hub at the center of the diskette, spinning it at either 300 or 360 rotations per minute (RPM). diskette, spinning it at either 300 or 360 rotations per minute (RPM).

Stepper MotorStepper Motor: This motor makes a precise number of stepped revolutions to move : This motor makes a precise number of stepped revolutions to move the read/write head assembly to the proper track position. The read/write head the read/write head assembly to the proper track position. The read/write head assembly is fastened to the stepper motor shaft. assembly is fastened to the stepper motor shaft.

Mechanical FrameMechanical Frame: A system of levers that opens the little protective window on the : A system of levers that opens the little protective window on the diskette to allow the read/write heads to touch the dual-sided diskette media. An diskette to allow the read/write heads to touch the dual-sided diskette media. An external button allows the diskette to be ejected, at which point the spring-loaded external button allows the diskette to be ejected, at which point the spring-loaded protective window on the diskette closes. protective window on the diskette closes.

Circuit BoardCircuit Board: Contains all of the electronics to handle the data read from or written to : Contains all of the electronics to handle the data read from or written to the diskette. It also controls the stepper-motor control circuits used to move the the diskette. It also controls the stepper-motor control circuits used to move the read/write heads to each track, as well as the movement of the read/write heads toward read/write heads to each track, as well as the movement of the read/write heads toward the diskette surface. the diskette surface.

The read/write heads do not touch the diskette media when the heads are traveling The read/write heads do not touch the diskette media when the heads are traveling between tracks. Electronic optics check for the presence of an opening in the lower between tracks. Electronic optics check for the presence of an opening in the lower corner of a 3.5-inch diskette (or a notch in the side of a 5.25-inch diskette) to see if the corner of a 3.5-inch diskette (or a notch in the side of a 5.25-inch diskette) to see if the user wants to prevent data from being written on it. user wants to prevent data from being written on it.