External Memory

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Ahah! got a new topic to post !

Let's started with external memory..

 External Memory

External memory which is sometimes called backing store or secondary memory, allows the permanent storage of large quantities of data. Some method of magnetic recording on magnetic disks or tapes is most commonly used. More recently optical methods which rely upon marks etched by a laser beam on the surface of a disc (CD-ROM) have become popular, although they remain more expensive than magnetic media. The capacity of external memory is high, usually measured in hundreds of megabytes or even in gigabytes (thousand million bytes) at present. External memory has the important property that the information stored is not lost when the computer is switched off.

We can divided types of external memory into three:

Magnetic Disk

◦RAID

◦Removable

Optical

◦CD-ROM

◦CD-Recordable (CD-R)

◦CD-R/W

◦DVD

Magnetic Tape

Magnetic Disk

       The primary computer storage device. Like tape, it is magnetically recorded and can be re-recorded over and over. Disks are rotating platters with a mechanical arm that moves a read/write head between the outer and inner edges of the platter's surface. It can take as long as one second to find a location on a floppy disk to as little as a couple of milliseconds on a fast hard disk.

Why Magnetic Disk Storage?

• The base technology has a simplicity and elegance by the use of
magnetics (dc motor to spin disks, voice coil for positioning of head,
magnetic disk for storage and magnetic heads for reading and writing)

• The technology offers an unequaled set of performance tradeoffs in
terms of capacity, access time, data rate and cost/GB

• Magnetic disk storage has dominated on-line real-time storage since its
creation and now information storage via the Internet, thus integral part
of today's societies.

• The technology is still advancing at a dramatic rate.

• Conclusion: magnetic disk storage will be the major means of
information storage for many more decades

Read/Write Mechanism

•Recording & retrieval via conductive coil called a head

•May be single read/write head or separate ones

•During read/write, head is stationary, platter rotates

•Write

—Current through coil produces magnetic field

—Pulses sent to head

—Magnetic pattern recorded on surface below

•Read (traditional)

—Magnetic field moving relative to coil produces current

—Coil is the same for read and write

•Read (contemporary)

—Separate read head, close to write head

—Partially shielded magneto resistive (MR) sensor

—Electrical resistance depends on direction of magnetic field

—High frequency operatio

__Higher storage density and speed

Data Organization and Formatting 

• Data is organized on a platter using a set of concentric rings named tracks 

o Each track is the same width as the head 

o Adjacent tracks are separated by gaps – minimizes errors due to 

misalignment or magnetic field interference 

• Tracks divided into sectors 

o Data is transferred to and from disk in sectors – blocks of sectors 

o Adjacent sectors separated by intratrack gaps Disk Velocity 

• A bit stored near the center of a rotating disk passes a fixed point slower than a 

bit stored near the outside edge of the disk 

• Spacing between bits is increased on the tracks towards the outside edge 

• Constant angular velocity (CAV) – rotating the disk at a fixed rate     

o Sectors are pie shaped and the tracks are concentric 

o Individual tracks and sectors can be directly addressed 

o Head is moved to the desired track and then waits for the desired sector 

o Wastes disk space on the outer tracks – lower data density 

• Multiple zone recording 

o The number of bits per track is constant 

o Zones towards the outer edge contain more bits and more sectors 

o Read and write timing changes form one zone to the next 

o Increased capacity traded for more complex circuitry

Physical Characteristics:

Disk Performance Parameters 

General timing diagram of disk I/O transfer: 
• Seek time 
o Time it takes to position the head at the desired track – moveable head 
system 
• Rotational delay 
o Time it takes for the beginning of the desire sector to reach the head 
o Floppy disks rotate at a rate between 300 and 600 rpm 
o Hard disks rotate at a rate between 3600 and 15000 rpm 
• Access time 
o Sum of the seek time and the rotational delay 
• Transfer time 
o Time required for the data transfer 
o Dependent on the rotation speed of the disk

Well that's all from me for now. I hope you are having a wonderful day!

Big Squeezes! xo