Difference between revisions of "Disk"

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'''Disk''' storage was, until recently, the primary form of high-speed on-line '''secondary storage''' for computers. ('Secondary' because, unlike [[main memory]], the data on it was not directly accessible to the [[CPU]]; rather, it has to be read into memory before the CPU can look at it.)
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'''Disk''' storage was, until recently, the primary form of high-speed on-line [[secondary storage]] for computers. Disks replaced an earlier form of rotating [[magnetic storage]], [[drum]]s.
  
All disk storage is basically the same; a circular 'platter' is coated with a magnetic coating - very fine particles of magnetic material mixed with a binder, which caused the coating to adhere to the platter. A read/write head, identical in basic concept to the head on a magnetic tape recorder, is used to read and write data on the disk as the disk is rotated.  
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All disk storage is basically the same; a circular '''platter''' is coated with a magnetic coating. A read/write [[head]] is used to read and write data on the disk as the disk is rotated underneath (or above) it, by magnetizing the particles in one of two directions.
  
A separate mechanism is used to move the head in and out; with the head at a fixed position, it writes a '''track'''. Many disks have more than one platter, each with its own read-write heads (usuall two, one for each surface), and the collection of tracks at a given offset is referred to as a '''cylinder'''. Each track is usually divided into a number of '''sectors''' - a complete track usually contains too much data to be usefully read and write as a complete unit.
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In most disks, the platter rotates at high speed, and the heads are shaped to ride on a thin film of air at a very low altitude above the surface of the platter (a small spacing is required to maximize the density of the data on the platter; the smaller the spacing, the more densely the information can be written).
  
Initially, all disks used metal platters (usually aluminium). IBM developed a new form of disk, the [[floppy disk]], which used instead a platter of mylar (a stiff plastic). This was carried in a stiff plastic envelope with a felt layer on the inside. The first floppies were 8 inches in diameter; later they were produced in 5-1/4 inch, and finally 3-1/2 sizes (the latter in rigid plastic containers).
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With the head at a fixed position, it writes a circular '''track'''. Some early disks had a separate head for each track; such disks were known as [[fixed-head disk]]s.
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For most disks, a head is shared between tracks; a separate mechanism is used to move the head in and out, a process known as '''seeking'''. The drive can write data on many tracks, each at a fixed distance from its neighbours. This is mechanically more complex, but uses fewer components overall.
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Many disks have more than one platter, each with its own read-write heads (usually two, one for each surface), and the collection of tracks at a given offset is referred to as a '''cylinder'''.
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A complete track usually contains more data than can easily be read and written as an un-divided unit, and so tracks are usually divided divided up into a number of '''sectors''' or '''blocks'''. This division may be purely logical, and indicated in the magnetic pattern written to the disk, or there may be actual physical means of indicating where each block is. There is also an intermediate (used in most [[floppy disk]]s), where there is a physical mark to indicate the start of the track, but the sectors are logical.
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Most early disk drives used '''packs''', a group of one or more platters which could be installed or removed in a drive, in a process called '''mounting''' a pack. So a system could have multiple packs (for backup purposes, or for infrequently needed data) which could be stored until needed.
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==Low-cost disks==
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Initially, all disks used large metal platters (usually aluminium), complex mechanisms to move the heads in and out at high speed (to reduce seek times), and were generally fairly expensive.
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In the late 1960's, IBM developed a new form of disk, the [[floppy disk]], which used instead a 'platter' of mylar (a stiff plastic); this was much cheaper, but at significantly lower performance. In their drives, the disk rotates much more slowly, and the head makes contact with the surface.  
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The rotating substrate - not referred to as platters, but called the 'magnetic medium' was carried in a stiff plastic envelope, with a thin felt layer on the inside (to keep the medium clean). The entire assembly is usually referred to as a 'floppy disk' (or 'floppy' for short; the term 'diskette' was also used).
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The first floppies were 8 inches in diameter; later they were produced in 5-1/4 inch, and finally 3-1/2 inch sizes (the latter in rigid plastic containers).
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==Recent developments==
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More recently, to increase the amount of information stored on a drive, most disk drives are now sealed. The disks in modern [[personal computer]]s are generally referred to as '''hard drives''' (to distinguish them from the floppy drives which were all the earliest PC's were equipped with).
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==External links==
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* [http://chmhdd.wikifoundry.com/ Hard Disk Drive History]
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[[Category: Device Basics]]

Revision as of 23:42, 20 December 2020

Disk storage was, until recently, the primary form of high-speed on-line secondary storage for computers. Disks replaced an earlier form of rotating magnetic storage, drums.

All disk storage is basically the same; a circular platter is coated with a magnetic coating. A read/write head is used to read and write data on the disk as the disk is rotated underneath (or above) it, by magnetizing the particles in one of two directions.

In most disks, the platter rotates at high speed, and the heads are shaped to ride on a thin film of air at a very low altitude above the surface of the platter (a small spacing is required to maximize the density of the data on the platter; the smaller the spacing, the more densely the information can be written).

With the head at a fixed position, it writes a circular track. Some early disks had a separate head for each track; such disks were known as fixed-head disks.

For most disks, a head is shared between tracks; a separate mechanism is used to move the head in and out, a process known as seeking. The drive can write data on many tracks, each at a fixed distance from its neighbours. This is mechanically more complex, but uses fewer components overall.

Many disks have more than one platter, each with its own read-write heads (usually two, one for each surface), and the collection of tracks at a given offset is referred to as a cylinder.

A complete track usually contains more data than can easily be read and written as an un-divided unit, and so tracks are usually divided divided up into a number of sectors or blocks. This division may be purely logical, and indicated in the magnetic pattern written to the disk, or there may be actual physical means of indicating where each block is. There is also an intermediate (used in most floppy disks), where there is a physical mark to indicate the start of the track, but the sectors are logical.

Most early disk drives used packs, a group of one or more platters which could be installed or removed in a drive, in a process called mounting a pack. So a system could have multiple packs (for backup purposes, or for infrequently needed data) which could be stored until needed.

Low-cost disks

Initially, all disks used large metal platters (usually aluminium), complex mechanisms to move the heads in and out at high speed (to reduce seek times), and were generally fairly expensive.

In the late 1960's, IBM developed a new form of disk, the floppy disk, which used instead a 'platter' of mylar (a stiff plastic); this was much cheaper, but at significantly lower performance. In their drives, the disk rotates much more slowly, and the head makes contact with the surface.

The rotating substrate - not referred to as platters, but called the 'magnetic medium' was carried in a stiff plastic envelope, with a thin felt layer on the inside (to keep the medium clean). The entire assembly is usually referred to as a 'floppy disk' (or 'floppy' for short; the term 'diskette' was also used).

The first floppies were 8 inches in diameter; later they were produced in 5-1/4 inch, and finally 3-1/2 inch sizes (the latter in rigid plastic containers).

Recent developments

More recently, to increase the amount of information stored on a drive, most disk drives are now sealed. The disks in modern personal computers are generally referred to as hard drives (to distinguish them from the floppy drives which were all the earliest PC's were equipped with).

External links