LESSON 4 : MEMORY STORAGE
What is Storage?
Input
Processing
Output
Storage
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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
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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!! |
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Main Memory
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| 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
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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. |
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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
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Diskette / Floppy Disk
Sizes:
5¼"
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3½"
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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
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.
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Hard Disks
 These
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. |
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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.