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LESSON 4 : MEMORY STORAGE

 

What is Storage?

Input
Processing
Output
Storage

Circle of 4 arrows
Storage refers to the media and methods used to keep information available for later use. Some things will be needed right away while other won't be needed for extended periods of time. So different methods are appropriate for different uses

Remember from early times all the kinds of things that are stored in Main Memory. Thus,

Primary Storage is Main Memory


This keeps track of what is currently being processed.
It's volatile.  (power off erases all data)
  Poof!!     cloud gif

Main Memory

main memory diagram gif
For Main Memory, computers use RAM, or Random Access Memory. This uses memory chips and is the fastest but most expensive type of storage.

Secondary Storage is called Auxiliary Storage

3 file cabinets with center drawer pulled out
This is what is not currently being processed. This is the stuff "filed away", but ready to be pulled out when needed.
It is nonvolatile.  (power off does not erase)
 
Auxiliary Storage is used for:
Input - data & programs
Output - saving results of processing

 
So, Auxiliary Storage is where you put last year's tax info, addresses for old customers, programs you may or may not ever use, data you entered yesterday - everything that is not being used right now.

 

MAGNETIC DISKS

Of the various types of Auxiliary Storage, the types used most often involve some type of magnetic disk. These come in various sizes and materials, as we shall see. This method uses magnetism to store the data on a magnetic surface.

 

Advantages:   high storage capacity
reliable
gives direct access to data

A drive spins the disk very quickly underneath a read/write head, which does what its name says. It reads data from a disk and writes data to a disk. (A name that actually makes sense!)


Types of Magnetic Disks


Diskette / Floppy Disk

Sizes:
5"
large floppy gif
3"
small floppy gif
Both sizes are made of mylar with an oxide coating. The oxide provides the magnetic quality for the disk. The "floppy" part is what is inside the diskette covers - a very floppy piece of plastic (i.e. the mylar)

Other Removable Media

Zip disk

Several other kinds of removable magnetic media are in use, such as the popular Zip disk. All of these have a much higher capacity than floppy disks. Some kinds of new computers come without a floppy disk drive at all.

Each type of media requires its own drive. The drives and disks are much more expensive than floppy drives and disks, but then, you are getting much larger capacities.


Hard Disks

Western Digital hard driveThese consist of 1 or more metal platters which are sealed inside a case. The metal is one which is magnetic. The hard disk is usually installed inside the computer's case, though there are removable and cartridge types, also.
 
Technically the hard drive is what controls the motion of the hard disks which contain the data. But most people use "hard disk" and "hard drive" interchangeably. They don't make that mistake for floppy disks and floppy drives. It is clearer with floppies that the drive and the disk are separate things.

OPTICAL DISKS

An entirely different method of recording data is used for optical disks.

You may guess from the word "optical" that it has to do with light. You'd be exactly right!

To make an optical disk, tiny lasers create peaks and valleys in a plastic layer on a circular disk. In the device that reads the optical disk these peaks and valleys are read as 1's and 0's by shining another laser on the disk.


CD ROM                                                                                                                                              

The most common size of optical disk is the CD-ROM, which stands for Compact Disk - Read Only Memory. It looks just like an audio CD. Almost all software is being distributed on CDs now. The price of the drives that read the disks (but can't write one) has dropped low enough that a new system will come with a CD drive unless you go to some effort to avoid it! Such drives will also play your audio CDs, if you have a sound card and speakers.
 
The CDs that contain commercial software are of the Write Once Read Many (WORM) variety. They can't be changed once they are created. This is where the ROM part comes from.
 
The CD-ROM is useful as a backup medium only when you really need indefinite storage of non-changing material. For data that changes often it is too expensive since a disk can only be used once.
 
What you need for backup storage of changing data are rewritable disks. These use a different material for the laser to work on that can be softened and lasered again. The drives and disks are still quite expensive. But prices are dropping fast.

Advantages


1.  The optical disk is much sturdier than the other media discussed so far. It is physically harder to break or melt or warp.
2.  It is not sensitive to being touched, though it can get too dirty or scratched to be read.
3.  It is entirely unaffected by magnetic fields.
 
Plus you can imprint a pretty label right on the disk!
So for software providers, the optical disk is a great way to store the software and data that they want to distribute or sell.

Disadvantages


1.  The main disadvantage has been cost.
But the cost of a CD-RW drive has dropped drastically and quickly. In 1995 such a drive was around $3000. In the summer of 1997 CD-RW drives were down to just under $1000. In june 2003 a CD-RW that will read at 40X speed, write on CD-R media at 40X speed,  and write on re-writable media at 12X,  can be bought for under $100 US!!

So for commercial use, the read/write drives are quite cost effective. For personal use, they are available, but may not be quite yet cheap enough to use for data storage for most folks.

The cost of disks can add up, too. Recordable disks (one time only) cost about $.30 US each (june 2003). Re-writable disks cost about $.64 to $.85 each.
 
2.  It is not easy to copy an optical disk. (This is an advantage as far as commercial software providers are concerned!) This is balanced by the fact that it is not as necessary to have extra copies since the disk is so much sturdier than other media.


Now you are ready to proceed to the next lesson.

 

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