What is a Hard Disk?
Hard disks were invented in the 1950s. They
started as large disks up to 20 inches in diameter holding just a few
megabytes. They were originally called "fixed disks" or
"Winchesters" (a code name used for a popular IBM product). A hard
disk drive consists of a motor, spindle, platters, read/write heads, actuator,
frame, air filter, and electronics.
The first computer with a hard
disk was IBM’s RAMAC, which was used during the 1960 Olympics to calculate
sports results. A bit later, in 1962, removable disk
packs were
developed – a forerunner of the floppy disk. In 1964, the CRC algorithm was
introduced. It provided greater security by checking and comparing data before
and after it was written to the disk. In 1971, the first 8-inch diskettes came
onto the market.
But it wasn’t until the middle of the
1980’s that people began to use hard disks in more standard PC’s, and since
then development has surged ahead. The capacity of a standard hard disk has
actually become a thousand times greater in the period 1990-2000.
The standard user’s need for disk space
(e.g. for digital photos, video and music) has grown in step with this, so that
120-250 GB of disk space or more is normal in many PC’s – a figure which will
double over the next few years.
Hard disks are constantly being developed
which have greater capacity and speed (the two go together, as we shall see),
and there is therefore a constant need for new types of hard disk controllers.
The companies leading the development are Maxtor, Western Digital, IBM/Hitachi
and Seagate.
The frame mounts the mechanical parts of
the drive and is sealed with a cover. The sealed part of the drive is known as
the Hard Disk Assembly or HDA. The drive electronics usually consists of one or
more printed circuit boards mounted on the bottom of the HDA.
How Hard Disk Works?
Data on a hard disk is
stored in microscopic areas called magnetic domains on the magnetic material.
Each domain stores either a 1 or 0 value. Similar to a floppy disk, a hard disk
records its data in concentric circles or tracks, which are numbered from the
outermost edge to the innermost edge of the platter. These tracks are further
subdivided into smaller units called sectors which typically store 512 bytes of
data each. Zoning may be needed to further optimise the data storage as the
outer circumference would normally pack more sector units than the inner
circumference.
·
When a command is made
to store some data on a disk, the following chain of events occurs:
·
The data flows into a
cache where it is encoded using special mathematically derived formulae,
ensuring that any subsequent errors caused by noise can be detected and
corrected.
·
Free sectors on the
disk are selected and the actuator moves the heads over those sectors just prior
to writing. (The time it takes the actuator to move to the selected data track
is called the "seek" time.)
·
Once over the data
track, the heads must not write the data until the selected free sectors on
that track pass beneath the head. This time is related to the rotation speed of
the disk: the faster the speed, the shorter this "latency" period.
·
When it's time to
write, a pattern of electrical pulses representing the data pass through a coil
in the writing element of the recording head, producing a related pattern of
magnetic fields at a gap in the head nearest the disk. These magnetic fields
alter the magnetic orientations of bit regions on the disk itself, so the bits
now represent the data.
When a command is made
to read some data on a disk, a similar process occurs in reverse. After
consulting the table of stored data locations in the drive's electronics, the
actuator moves the head over the track where the chosen data is located. When
the correct sectors pass beneath the head, the magnetic fields from the bits
induce resistivity changes in the sensitive MR or GMR materials located in the
reading elements within the head. These elements are connected to electronic
circuits, and the current flowing through those circuits change with the
resistivity changes. The current variations are then detected and decoded to
reveal the data that had been stored on the disk.
Types of Hard Disks
There are few different types of hard
disks, but other than its physical size, the different type of interfaces of
the hard disk is main difference.
1. IDE: Integrated
Drive Electronics. IDE drives are also known as PATA drives (Parallel advance
technology attachment)
2. SATA: Serial
advance technology attachment
3. SCSI
Small Computer System Interface. SCSI is
pronounced as scuzzy.
1. IDE / PATA (Integrated Drive Electronics Drive / Parallel Advance Technology Attachment Drive)
- IDE/PATA Drives have usually Interface Connector 40 pins & Power Connector 4 Pins.
- IDE/PATA Drives offer 133 MB/sec transfer rate.
- It sends 8 bit data at a time.
- PATA Cables are used to connect PATA HDD. Two drives can be connected in a single pata cable. One as master and other as slave. The configuration of master and slave is done by different combination of jumpers in the HDD.
2.SATA (Serial AdvancTechnologyAttachment Drive)
- SATA Drives have usually 7 pins, 4 pins in pair of two for sending and receiving data and rest 3 pins are grounded and Poer Connector 15 pins & 4 Pins .
- SATA Drives offers generally 300MB/sec transfer rate.
- It sends data bit by bit.
- SATA Cables are used to connect SATA HDD. Only one drive can be connected in a single sata cable
3. SCSI (Small Computer System Interface Drive)
- SCSI Drives have usually 50 to 68 pins and Power Connector 4 Pins
- SCSI Drive offers generally 640MB/sec transfer rate.
- This drives are hot swappable.
- SCSI cables are used to connect SCSI HDD. Maximum of 16 drives can be connected in a single scsi cable. Each hdd have a 8 bytes hexadecimal code known as WWN (world wide name) for its identification in the cable.
0 comments:
Post a Comment