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Hardware Fundamentals

A Hard Drive is the workhorse of the computer system.Obtained name as inside the drive that is used to hold the data are hard and inflexible plattersInflexible platter is made of aluminumInternal or external drives*

Typical configurations seen in disks todayPlatter diameters: 3.7, 3.3, 2.6RPMs: 5400, 7200, 10000, 150000.5-1% variation in the RPM during operationNumber of platters: 1-5Mobile disks can be as small as 0.75Power proportional to: (# Platters)*(RPM)2.8(Diameter)4.6Tradeoff in the drive-designRead/write headReading Faradays LawWriting Magnetic InductionData-channelEncoding/decoding of data to/from magnetic phase changes

HDD ORGANISATION:*

HDD OrganizationArm AsSEmblyArmHeadCylinderSpindlePlatterTrack

Head floats above the disk surface on a cushion of moving air, which is created by the disk spinningHead Gap = 0.0003mm*

Electro magnetic coilIs used to read or write data to the disk*

To read data from the disk, a coil is passed over the surface.The magnetic domain on the surface induces / creates a current in the coil.The direction of this current depends on the magnetic alignment of the surface domain underneath the coil.*

To write data to a disk, a magnetic field is created to change the alignment of the particles.This is done by passing a current through the head coil in one direction or the other.This causes a magnetic field (in one alignment or the other) which changes the alignment of the particles on the disk surface.*

Heads park in a landing zoneUnused area in the center of platter (when off)Unused AREA as parking place for headsReferred to as Lzone, LZ, ParkNeeded for older drives using stepper motors

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Head crash is when the head touchs the disk surface whilst spinning, due to vibration/shockFerric oxide coating is scratched, creating bad sectors, and in severe cases, damages the heads.*

Parts of the disk that can no longer hold data.Shows up as a CRC error (CRC checksum does not match that written on disk)*

aka bad sector remappingHard disk drives have extra sectors, which are used to replace any bad sectors that could possibly occur*

In a hard disk, the disks are called plattersThe platters are usually double-sided (data is recorded on both sides of the platter)If the platter is double-sided it will have two read/write heads, one for each surfacePlatters spin between 3500 and 10,000 rounds per minute (RPM)The platters are made up of aluminum & coated with a magnetic medium

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Hermetic sealing means that the drive is completely sealed, so that nothing, not even air, can get into the driveKeeps out dust, dirt, fingerprints, hair, moisture and other contaminantsAll modern HDDs are hermetically sealedHermetically sealed reduces head crashesAllows the read/write heads to float closer to the disk surface*

The first hermetically sealed HDD had two 30MB platters - nicknamed the Winchester drive (after a rifle, the Winchester 30/30)*

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Geometry is used to determine the location of the data on the hard driveCHS (cylinders, heads, sectors)

Used to be critical to know geometryHad to enter into CMOS manually

Today, geometry stored on hard driveBIOS can query hard drive for geometry data

USES OF HARD DISK GEOMETRY :*

Disk surface is marked out into circular tracks, one inside the otherConcentric circles of data1000+ tracksThe tracks are numbered from the outside, from 0 to 1000+Each track is divided into sectors*

Track=concentric circleSector =small arc of trackCan store 512 bytesTracks and sectors

Hardware Fundamentals

A sector is a part (an arc) of the track (512B) 17-43 sectors per track

As you move further out from the centre of a platter, the distance around the track (the circumference of the track) gets largerThis means that, the further from the centre of the disk the more space on the trackTo make better use of this space, the number of sectors per track increases as you move away from the centre.*

If there is more than one disk in the drive, then the tracks that are directly above or below each other are known collectively as a cylinderTracks of same diameter, simultaneously accessible across both surfaces of all the platters*

Hard drives store data in tiny magnetic fields called fluxes

The flux switches back and forth through a process called flux reversal

Hard drives read these flux reversals at a very high speed when accessing or writing dataFluxes in one direction are read as 0 and the other direction as 1ENCODING DATA:*

Run length limited (RLL) Data is stored using runs that are unique patterns of ones and zeroesCan have runs of about seven fluxes

Partial Response Maximum Likelihood (PRML) Uses a powerful, intelligent circuitry to analyze each flux reversalCan have runs of about 16 to 20 fluxesSignificantly increased capacity (up to 1 TB)

Encoding methods used by hard drives are

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Modified Frequency Modulation (MFM)Early PC HDDs (Intel 8088, 8086) used MFM technique to write data to the HDDMFM have a fixed number of 17 sectors per trackWastes space on outer tracksRun Length Limited (RLL)Modern HDDs use RLLencoding for writing data to the HDDRLL stores data more efficiently than MFM by using bit level data compression (Zone Bit Recording (ZBR)) to compress more sectors into the outer tracks*

The stepper motor technology and the voice coil technology are used for moving the actuator armMoves the arms in fixed increments or stepsOnly seen in floppies today

The voice coil technology uses a permanent magnet surrounding the coil on the actuator arm to move the armAutomatically parks drive over non-data area when power removed

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**To read data from the disk or tape, a coil is passed over the surface. The magnetic field on the surface creates a current in the coil. The direction of this current depends on the magnetic alignment of the surface underneath the coil: particles oriented to "north" produce the opposite current to particles oriented to "south".

*To write data to a disk or tape (that is, to record data on a disk or tape), a magnetic field is created to change the alignment of the particles. The magnetic field is generated by passing a current through the coil in one direction or the other. This causes a magnetic field (in one alignment or the other) which changes the alignment of the particles on the disk/tape surface.1s and 0s are written by changing the orientation of the ferric oxide particles to either "north" or south". *A head crash occurs when the head touches the surface of the disk, causing damage. This damage results in bad sectors - parts of the disk that can no longer hold data. In severe cases, the heads themselves can be damaged, affecting the ability of the drive to read or write data at any location on the disk.

**Hard disks usually have extra sectors that can be used to replace damaged sectors.Hard disk drives automatically replace bad sectors on the disk. This saves the operating system from having to keep a table of bad sectors,or even having to know about them.

*The disks spin at 5,200 - 10,000 revolutions per minute (RPM). Drives spin at constant angular velocity (CAV). This means that the disks spin at the same number of revolutions per minute all the time. There is a comparison of CAV with CLV in the lesson on optical storage.[ Top ]

*If a hard drive has more than one platter (almost all modern hard drives do) then the tracks immediately above or below each other are called a cylinder. For example, cylinder number 256 consists of track number 256 from each platter.All tracks in the same cylinder can be accessed at the same time.

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