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Introduction to ComputersDay 4

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Storage device

• A functional unit into which data can be– placed– retained (stored)– retrieved (accessed)

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Storage device

• Main Parameters– Location

• Internal storage

• External storage

– Capacity– Speed– Access Method

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Storage devices

• Primary Storage (Main memory) always uses Random Access method.

• Two methods for storing and accessing instructions or data in secondary (external) storage– direct access– Sequential access

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Random Access

• Random Access means, that in any cell in the memory can be accessed in a fixed time irrespective of its physical location.

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Direct access

• Direct access means that the data is stored in a specific location so that any data can be found quickly. e.g. Hard disk, floppy disk, CD-ROM.

• Direct access is the most widely used storage method in external storage devices. The most common direct-access storage medium is the disk.

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Access Time

• RAM– 60 nanoseconds (ns) or less to access memory locations

in RAM

• Secondary Storage – 7 to 9 milliseconds (ms) to access sectors in a hard disk

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Sequential access

• Sequential access means that the data is stored and accessed in a set order, perhaps alphabetically or by date and time. The most common sequential storage medium is magnetic tape on reels or cassettes.

• Sequential-access storage devices are used mostly for backup purposes.e.g. Reel-to-reel magnetic tape, Tape Cartridges

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Magnetic Diskette

• First magnetic diskette was 8” with mini/mainframe computers

• A thin flexible disk is permanently sealed within a rigged protective plastic cover

• Sizes were evolved through 8”,5 1/4” & 3 1/2” (diameter)

• Storage capacity is H/D L/D

3 1/2” 1.44 MB 720MB

5 1/4” 1.2 MB 360KB

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Track

• On a data medium, a path on the recording surface associated with a single read/write head as the data medium moves past it.

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Sector

• A predetermined angular part of a track or band on a magnetic drum or a magnetic disk, that can be addressed.

Most industry-standard PCs use sectors which can store 128 or 256 or 512 or 1024 bytes of information

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The Sector Method

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sectors

tracks

Surface 0track 1sector 2

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Magnetic Diskette (3 1/2 inch)

• Sector= 512 bytes• Track = 18 sectors = 18 * 512 bytes = 9.0Kb• Disk = Double sided = 2 * 80 tracks

= 2 * 80 * 9.0 Kb = 1.44Mb1.44Mb• Size = 3 ½ inch• Capacity = 1.44 Mb

• Access time = 275 ms• Rotational speed = 720 rpm

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Magnetic Diskette (5 1/4 inch)• Rotational speed = 360 revolutions per minute (rpm)• Two Read/write heads capable of addressing 80

cylinders per diskette at the speed of 3 ms from track to track

• Average Access time = 80 milliseconds (ms)• Settling time = 15 ms

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Magnetic Disk (Hard Disk)

• REMOVABLE DISK– Removable disk pack used in earlier Mainframe &

Mini Computers– Disk cartridge - easy to remove like cassettes

• FIXED DISK– Installed in a sealed container and it’s not

removable– most of the fixed disks use the “Winchester”

technology

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Magnetic Disk

Disk consists of several platters (e.g. 3). Each platter has two sides. A number refers to each side (e.g. side 0, 1, 2, 3 for 4 surfaces). A Disc pack may have 20 surfaces or = 11 Platters

-0

123-

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Magnetic Disk

• A disk starts out very unstructured – just a lot of bits of magnetic stuff without any organisation, rhyme or reason. Before the system can start writing records to it, the disk must have a structure- a grid work into which the information can be placed.

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Magnetic Disk

• Formatting a disk is the process of putting the grid work on the disk and building the organisational structure so that file can be found. Once a disk is formatted it is ready for the system to write data to it.

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Magnetic Disk

• Formatting organises disks into numbered rings called cylinders. A cylinder on a single side is referred to as a track. Each track is broken into numbered pie slices called sectors. Each sector stores information.

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Magnetic Disk

• Disk pack = 20 surfaces = 11 Platters• Disk = 2048 cylinders (figure has only 4)• Cylinder = 20 tracks (track in each surface)• Track = 72 sectors (figure outermost has 13)• Sector = 512 bytes• Disk Storage = 512 * 72 * 20 * 2048 bytes = 1.44 GB• Rotational speed = 3600 rpm (revolutions per minute)

= 16.66 ms per revolution

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Magnetic Disk

• The time required to position the read-write heads over the required track is the seek time.

• The time required for the read-write head to come to a complete stop after it is moved is called the settling time.

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Magnetic Disk

• The time required for the disk to rotate to the position where the beginning of the desired bock arrives at the read-write head is latency.Average Rotational delay (latency) = ½ revolution

Track capacity = 72 x 512 = 36 KB

Cylinder capacity = 20 x 36 = 720 KB

Disk capacity = 2048 x 720 = 1.44 GB

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Hard Disk Technology

Removable-pack hard disk system

• Contains 6-20 hard disks of 10 1/2 or 14 inch diameter, aligned one above the other in a sealed unit.

Fixed disk drive• High-speed, high-capacity disk drives that are housed

in their own cabinets.

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Hard Disk Technology

Redundant Arrays of Inexpensive Disks (RAID)• The disk system consists of a number of 5 1/4-inch disk drives

within a single cabinet and sends data to the computer along several parallel paths simultaneously.

• The main purpose is to increase the reliability and availability. I.e. If one disk fails, still no data is lost