Auxiliary or Secondary storage devices can be broadly categorized into two
categories, magnetic and
Magnetic devices get their name from the way they read and write data to and
from a medium. Magnetic storage medium use non-magnetic material for disk,
(aluminum in the case of hard drives and mylar in the case of floppy disk and
magnetic tape) and then that material is coated with tiny iron particles. The
read/write heads, (which are small electro-magnets) polarize the tiny iron
particles to either positive or negative magnetism. The read/write heads can
sense which polarity the iron particles are and interprets positive as "1" and
negative as "0".
Hard Drives, sometimes called hard disks or fixed disk have these attributes;
very high capacity, very high speed, randomly accessed, readable and writable.
Floppy Disk -
Floppy disk have these attributes; very low capacity, very slow speed, randomly
accessed, readable and writable.
Magnetic Tape -
Magnetic tape has these attributes; large capacity, speed is slow but different,
sequentially accessed, readable and writable.
Optical storage devices get their name from the way they read and write data to
and from a medium. Optical devices write data, (not all optical devices can
write data) by burning tiny pits into the medium. When reading the data a laser
is reflected off the medium. If the laser light strikes a pit the light is
defused and does not reflect back to a sensor and this is interpreted as a "0".
If the laser light strikes a "land" ( a place that has not been pitted by the
writing laser) it is reflected back to the sensor and this is interpreted as a
"1". Because of the way optical devices read data they can read only one bit at
a time. Hard drives do not have this limitation so optical devices will always
be slower than hard drives. The data stored by optical devices is immune to
electromagnetic interference and is general much more secure that data stored to
a magnetic device.
Types of Optical Storage Devices
Compact Disk Read Only Memory - have these attributes; medium capacity, medium
speed, randomly accessed, read only.
Compact Disk wRitable - same as CD-ROM but a blank disk can be written to one
time. The disk is then just like a CD-ROM disk
Compact Disk ReWritable - same as CD-ROM but special disks may be repeatedly
Two kinds of impact printers are Daisy Wheel and Dot Matrix.
The attributes of a Daisy Wheel printer are; fair letter quality, slow, loud,
no graphics, can do carbon copies.
The attributes of a Dot Matrix printer are; poor letter quality, slow, loud,
can do graphics, can do carbon copies.
There are two kinds of non-impact printers. They are ink jet and laser.
The attributes of an ink jet printer are; good letter quality, faster,
can do graphics and inexpensive color, inexpensive, can not do carbon copies.
The attributes of a laser printer are; excellent letter quality, fastest,
quietest, most expensive, can do graphics, can not do carbon copies.
Today's monitors are the VGA standard. (SVGA) SVGA must have the ability to
display at least 256 colors at 800 X 600 resolution. All good quality modern
monitors can do this.
Four factors which effect the display of a monitor.
The refresh rate how fast the screen is painted. It is expressed in hertz. A
good refresh rate is 85 HZ or faster.
The resolution is the number of pixels on the screen. It is expressed as the
horizontal number times the vertical number like 640x480 or 800x600.
The dot pitch is the distance between the three dots in a pixel. It is
measured in fractions of a millimeter. .28 is good.
Size is the diagonal measurement of the screen in inches. Most monitors are 15
or 17 inches.
DOS files names were restricted to the 8 and 3 rule. Windows 95 and beyond file names can
be long and include spaces. UNIX systems do not allow spaces in files names so
it is good practice to not have spaces in files names. Windows 95 and
some newer operating systems give files
two names. One is the long name. The other is a special DOS 8 and 3 name.
The three character extension on file names is special. It distinguishes data
files from program files and allows the computer to what type of file it is.
Now some file extensions are four characters.
Also know about formatting a disk; the 4 things which are put on a disk when
its formatted. Know the two ways of measuring the speed of a disk and anything
else you maybe asked on a test. Ha Ha.
John Mauchly and Presper
Eckert was the first to design the computer; they are from the the University of Pennsylvania. The name of the computer
was ENIAC, which stood for Electronic Numerical Intergrator and Computer. It
filled 40 ,9 feet tall cabinets and had about 18,000 vacuum tubes with miles of
wiring this was designed for a weapon of war. It could calculate trajectories
for WWII artillery guns but because of it operating under the army secrecy it
was little known to the public only by computing circles, it was eventually
buried in history under controversy,
jealousy, and lawsuits. ENIAC, Eckert, and Mauchly were forgotten. They also
discovered the first Computer Company, but neither found fame nor fortune
because both men were poor businessmen and lousy marketers. The team that built
the first computer ate, slept, and lived with it. They ran into so many problems
from how to combine vacuum tubes together so they could count numbers without
making mistakes all the way down to how they could make supplies last. Finally
in 1946 this thing was completed<.