External Memory

 The devices that provides baqckip storage are called external memory or auxuallary 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.
The most common form of external memory is a hard disc which is permanently installed in the computer and will typically have a capacity of hundreds of megabytes. A hard disc is a flat, circular oxide-coated disc which rotates continuously. Information is recorded on the disc by magnetizing spots of the oxide coating on concentric circular tracks. An access arm in the disc drive positions a read/write head over the appropriate track to read and write data from and to the track. This means that before accessing or modifying data the read/write head must be positioned over the correct track. This time is called the seek time and is measured in milliseconds. There is also a small delay waiting for the appropriate section of the track to rotate under the head. This latency is much smaller than the seek time.
It is important to note that the CPU can only directly access data that is in main memory. To process data that resides in external memory the CPU must first transfer it to main memory. Accessing external memory to find the appropriate data is slow (milliseconds) in relation to CPU speeds but the rate of transfer of data to main memory is reasonably fast once it has been located.

1. Serial access types : such as magnetic tapes
2. Random access types :such as magnetic tapes

Magnetic tapes:

explained in Auxiliary memory section

Magnetic Disks 

explained in Auxiliary memory section

Optical Disk

   It random access storage device that is written and read by light. The most common optical discs in use are CDs and DVDs. As removable media, optical discs have advantages over the older removable magnetic disk cartridges. They weigh less, have higher capacities and are not subject to head crashes or corruption from stray magnetic fields. They also have a 30-year life and are less vulnerable to extremes of hot and cold. However, flash drives and external hard disks that plug into the USB port have given optical discs a run for their money as a transportable storage medium. Following are the major types.

a.Read-Only (Factory Pressed)
Read-only discs are pressed from a master at the time of manufacture and cannot be erased. They include the music CD, CD-ROM, DVD-ROM, DVD-Video and BD-ROM (Blu-ray).

b.Write-Once (Burnable)
Write-once discs are recorded in the user's environment but cannot be erased. They include the CD-R, DVD-R, DVD+R, BD-R (Blu-ray), WORM discs, as well as magneto-optic (MO) discs in WORM mode.

c.Rewritable (Phase Change and Magneto Optic)
Rewritable discs can be written and re-written numerous times. Employing phase change technology, consumer-oriented products include CD-RW, DVD-RAM, DVD-RW, DVD+RW and BD-RE (Blu-ray)

 

Dvd Technology 

•  Multi Layer
• Very high capacity 4.7 g per layer r
• Full length movie on a single disk 
• Using MPEG compression
• Finally standardized
• players can only play correct regions films
• Loads of trouble with standards

DVD Writable

• First Generation DVD drives may not read the first generation DVD w disk
• First generation DVD drives may not read CD-RW disks