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WELCOME TO PC-LEARN
THE SHAREWARE COMPUTER TUTORIAL FOR BEGINNERS
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PC-LEARN is SHAREWARE: please make disk copies for your friends
and office associates. Please pay the registration fee to continue
legal use of your copy of PC-LEARN and receive two valuable BONUS
DISKS! Information on registration is contained in the tutorials
marked "registration" and "print registration" on the main menu.
Some businesses use PC-LEARN to teach new employees about computer
use. Site and LAN licenses are available. Custom versions with
your company or club address, logo, telephone or special "custom"
information or tutorials are available. Contact the author
listed in the registration section.
Watch the information bar at the bottom of the screen for clues
about what keys you can use! The F1 key on your keyboard brings
up small help screens. The F2 key or the escape key brings up menus.
Use right/left arrow keys to select a menu and up/down arrow keys
to select a menu item. Then press enter/return key to examine the
menu item and thus read a tutorial. Simple!
To move around within the various tutorials in PC-LEARN use
the PAGE UP (Pg Up) and PAGE DOWN (Pg Dn) keys to move one
screen up or down. The up/down arrow keys move you one line.
These keys are common to all tutorials. REMEMBER: You can jump
between tutorials by pressing the ESCAPE key or F2 to return
to the menu. Our first tutorial follows in a few paragraphs.
Always glance at the reminder bar at the bottom of the screen
for clues about what keys to use!
PC-LEARN is normally used on a hard drive equipped computer.
A special floppy disk version of PC-LEARN is available for
those who do not have a hard drive. Contact the author.
WHAT IS PC-LEARN?
PC-Learn is for beginners! PC-LEARN is a series of short, basic
tutorials every new PC owner should read. PC-LEARN is the
diskette "booklet" which accompanies the instructions you
received with your computer. I've had the good fortune to teach
many beginners during the first few weeks when the computer is
born into a waiting office or home. The most important thing
missing from the packing box is a teacher who can answer those
endless questions and provide those necessary "insider tips."
The most effective way to use PC-LEARN isn't very high tech.
Just read it on screen and make notes. Print a paper copy of
tutorials you like. PC-LEARN is an essential distillation of
hundreds of books, magazines, advertisements and many hours
of instructional time with beginners.
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PRINTING PC-LEARN ON YOUR PRINTER
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If you have a printer, you can make a paper copy of any tutorial
by using the print file/print tutorial option from the main
menu. Simply start reading the tutorial you want, then use
the main menu to select the print file/tutorial option.
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WHAT CAN YOU DO WITH A COMPUTER?
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The variety of software programs which are available for a PC
(personal computer) and the tasks these software programs
accomplish is stunning. Ultimately the reason for owning one of
these marvelous machines is the variety of opportunities
software presents.
IBM estimates over 130,000 programs exist for the PC family of
computers! These programs range from the predictably practical
to elegantly imaginative to utterly eccentric. All are worth
investigating. Local software stores, public domain and
shareware sources from local computer clubs contain more
possibilities. Addresses for free software and hardware catalogs
are listed in PC-LEARN under the PC-LEARN menu options SOFTWARE
and also RECOMMENDED READING/BIBLIOGRAPHY.
A list of interesting computerized activities follows . . .
Trace your family roots (genealogy programs)
Learn to speed read (reading tutorials)
Improve typing speed or learn keyboarding skills tutorials)
Manage your career (aptitude tests, resume writing)
Plan a workable diet and appropriate meals (diet programs)
Track stock market performance
Design a floorplan for a house, boat, garden
Write the great American novel (word processor)
Budget and manage finances (money managers and spreadsheets)
Learn a foreign language (language tutorials)
Design a personal aerobics/exercise program
Play games (computerized chess, bridge, checkers, monopoly)
Learn to fly a plane (flight simulation programs)
Prepare brochures and newsletters (desktop publishing)
Visit with others and obtain free software (modems/bbs use)
Automate a professional office (physician, lawyer, architect)
Prepare/forecast your taxes (tax return preparation programs)
Track horse race odds
Chart and graph business data
Prepare for the college SAT tests
And the list goes on! This is only a SMALL sample of some
practical and entertaining programs which ACTUALLY EXIST for use
with a personal computer. Most beginners guides start with a
discussion of DOS. Let's do that in a later article, but first a
little honesty to set the stage . . .
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A TYPICAL ODYSSEY FOLLOWS . . .
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Grownups who understand computers least will tell you to decide
on one or two practical goals (word processing or accounting)
and apply the computer to a task which will make you money or
save you time. Grownups also unwisely suggest you should become
organized in attempting these goals . . .
Children and computer users (is there a difference?) who
understand computers best will suggest that you to PLAY to see
what happens. Let the machine guide you. As you delve into the
computer, ask important questions: What kind of machine will
keep me addictively awake until 4 am? How can I afford a modem?
Can I write my own software?
Week one - 6pm to 8pm. The marriage
The new computer is home. You can load DOS. Your spouse is
skeptical about this "fantastic computer consulting business you
are going to run out of the garage."
One way to start is to try learning a basic software function
like word processing. Purchase a software package for beginners
like Microsoft Works or First Choice. Or locate a free shareware
copy of GALAXY or PC-WRITE from a computer user club. If you are
just starting, nothing is as impressive, practical and fun as
jumping from a typewriter to a word processor. Spouse is now
mildly impressed that you have mastered the word processor but
unaware of the savage reality which lies ahead . . .
Week two - 6pm to 10pm. The honeymoon
Now grab some games and play for a while. Study the visuals, the
logic and the joy of computing. Continue delving deeper with the
word processor. Join a computer User Club. The addiction is
taking form . . .
Week 3,4,5. 6pm to midnight. Reality dawns
Study DOS and practice with the DOS tips contained in PC-LEARN.
DOS lets you control the machine for faster, smoother results.
Borrow a book on DOS from the library - see the suggested books
and reading references contained elsewhere on this disk. DOS is
a pain? Use a DOS shell software package like Xtree, DirMagic,
Qfiler or PcTools which lets you manipulate the PC without
having to learn too much about DOS. Get a copy of the PC-SIG
shareware catalog and try some low cost and free shareware. PC-
SIG address listed elsewhere on this disk under software
resources.
Week six. 6pm to 3 am. Addiction phase/personality shift
Next move on to a good database program and index some christmas
card lists or business data you use frequently. Tackle
spreadsheets or number grids with ASEASYAS or a commercial
program such as lotus 123 or Microsoft Works. Databases can act
like spreadsheets and spreadsheets can act like databases.
Software starts to blend together.
Good programs have layers like an onion. You can go deeper for
greater layers of complexity and reward, but (as with the onion)
you can be left crying as you peel the layers. However, once you
get this far you find your computer curiosity is overwhelming.
Spouses of male computer-holics refer to "the man and his
midnight mistress." However, being an "equal opportunity
software package," the author of PC-LEARN hastens to add that
women also find the PC addictive and equally infatuating!
Week seven through 14. 6pm to 6am. Loved ones move away.
Computer user does not notice or care . . .
Now start to learn in greater detail. Go to the library and get
some back issues of Personal Computing (interesting and basic),
PC magazine (practical), and Byte (sophisticated). Try to see
the pattern: you can use a computer for an infinite variety of
tasks all of which may be at the same time fun/productive/silly/
lucrative/exhausting. Spouse realizes belatedly that this is a
full blown addiction and finds solace by reading and writing
personals ads which leads to discovery of a dynamite divorce
attorney . . .
End of first six months. Fatal discovery: programming
Next plunge is to try programming. Basic or turbo pascal allow
you to write programs that do exactly what you want.
Alternatively there are keyboard macro programs (smartkey,
prokey, newkey) that do "sort of programming." At this point
you begin to see that the machine has logic and always does
exactly what you tell it to do, even if you don't care for the
result!
Terminal phase. Severe addiction. Purchase of modem
Getting a modem on your computer to "have a little fun" is sort
of like getting a little bit pregnant. Basically things swell
out of normal proportion, if you follow the drift. Modems let
you hook into large databases, neighborhood "bulletin boards,"
and all kinds of expensive long distance telephone things.
Incredible 4 day "no food or sleep" modem orgies begin. Modems
are fascinating, global, thoroughly useful and BIG TROUBLE!
Human Perspective phase. Back to the real world
After sampling everything, somewhere about 1 to 3 years after
you first started, the computer will integrate nicely into the
real world again. You start to think about camping trips,
cooking classes, church on Sunday, bowling, the routine things.
Bedtime settles in at 10 pm. Going near a computer is simply
like walking by a typewriter or can opener. The computer is
there when you need it, does the job and is essential to your
normal daily routine. Loved ones return home again after
discovering your alimony payments are unreliable and usually
spent on computer toys anyway.
Time to go a little deeper:
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INTRODUCTION TO COMPUTER TECHNOLOGY - INPUT, STORAGE, OUTPUT
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Before we examine computer technology let's cover two items
which seem to confuse EVERY computer beginner. It's a wonder
computer manufacturers don't include these two ESSENTIAL points
in instruction books. First item. Working with floppy diskettes.
Note the little picture below which places a floppy diskette in
the proper orientation as if you had an overhead "bird's eye"
view of someone properly inserting a diskette into a drive.
Insert diskette into drive in
the direction of the arrows.
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█████x█████ <---- Read write slot on face of disk.
█████▄█████ (Marked with an X)
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Write protect cutout--> ██████████
███████████ <---- Label faces you and is here.
You might smile, but many beginners still get a little confused
on this. A standard floppy diskette is about 5 1/4 inches
square. To insert a floppy diskette, first remove it from the
paper or plastic slipcover which protects it. The proper way to
insert a floppy diskette in most drives is with the diskette's
printed label side UP and the diskette edge closest to the
"read/write" opening (a slot-shaped hole) going into the disk
drive first. The edge with the little square "write protect"
cutout will then be to the left. Finally, on most drives you
close or turn a little handle to complete the operation.
If the diskette write protect cutout notch is uncovered then you
CAN read and write data to the diskette. If the write protect
cutout is covered with a piece of tape, then you can READ
information from the diskette but you CANNOT write information
to the diskette. It is a safeguard feature you may wish to use.
If you wish to write data to the diskette, make sure the notch
is uncovered. Keep fragile diskettes away from smoke, hair, dirt
and ESPECIALLY sources of magnetism such as motors, loudspeakers
or even childrens magnetic toys which will ERASE your data!
The second item is very simple. Many times an instruction manual
refers to "booting up" or "booting DOS" before you can start a
program. This simply means inserting your DOS diskette in a
drive and starting the machine with the DOS diskette in place.
When you see the familiar A> or C> prompt symbol, you have
booted up! If you have a hard drive which starts the machine
automatically, the hard drive "boots DOS" for you and you do NOT
need to use the DOS diskette. This seems simple, but many
beginners are confused by the term "booting up."
Time to move on to basic computer technology! You can continue
using the PAGE DOWN key or jump to another tutorial such as DOS
or other topic by pressing the escape key to return to the main
PC-LEARN menu at this (or any) time.
Computers vary widely in size and use. However all computers are
similar in what the hardware does. So-called microcomputers
(like your desktop pc) are designed for personal use, relatively
low price, and modest data processing tasks. Minicomputers are
moderate sized (a small refrigerator size) and perform more
complex tasks with larger amounts of data. Minicomputers might
be used in a small engineering office or a local bank branch to
send transaction data to a head office computer. Mainframe
computers are large, expensive and process billions of
characters of data rapidly and fill entire rooms. Finally
supercomputers are built to minimize distance between circuit
boards and operate at very high speed for complex uses such as
designing airplanes, animating complex movie sequences
graphically or solving complex engineering formulas having
billions of steps mathematically. Supercomputers are built for
raw speed.
Some terms apply to all computers. INPUT is how data gets into a
computer. The keyboard and mouse are familiar INPUT devices.
OUTPUT references how data is provided from the computer. A
Monitor or printer are good examples of OUTPUT devices. PRIMARY
STORAGE or MEMORY is the computer's immediate data storage area
- usually this is in small integrated circuit chips which hold
data ONLY while power is supplied. This PRIMARY STORAGE area is
thus temporary. More permanent SECONDARY STORAGE is used when
computer power is off or when data overflows primary storage.
This is usually floppy or hard disk drives but can include paper
tapes, punch cards, or even non-volatile magnetic bubble
memories.
How do computers store data and programs? For the PC (personal
computer) storage of data can take place either in an integrated
circuit chip or IC when the machine is on or a magnetic disk
when the machine is turned off.
The magnetic disk used to store information works in a manner
similar to a tape recorder - magnetic impressions are placed on
the tape and can be later replayed. Magnetic sound tape as a
long strip of plastic with a thin coating of a metallic, easily
magnetized powder glued to the surface of the plastic strip.
When a electrically driven coil is placed near the surface of
the plastic strip, thousands of little magnets are created on
the surface of the tape as it rapidly streams beneath the coil.
Later these little magnets can induce current to flow in the
coil as the tape is pulled past the coil a second time. Thus the
information or music is replayed. During recording, the
electrical coil receives electric pulses which produce small
magnetic "blips" along the tape. During playback, the coil is
passive and the little magnetic pulses passing below its surface
create electric pulses in the coil which are amplified.
A magnetic computer disk works in the same fashion but spins in
a circle like a music record rather than moving in a straight
line like recording tape. Magnetic computer disks are available
in two basic types: floppy and hard disks. A hard disk can hold
considerably more information than a floppy disk - frequently
millions of computer words (or "bytes") while a floppy disk
holds less than a million in many cases. However what the floppy
disk loses in capacity in gains in the advantage of portability
since it can easily be removed from the pc and stored which is
not true of the hard disk.
On a typical music cassette tape you will find two channels
(left and right speakers) and a total of four tracks (side A of
the tape and side B.) Think of this as four lines of
"information" running the length of the music tape. On a
computer disk data is stored in a similar manner except there
are far more tracks of information and of course the tracks are
arranged in circles on a flat surface like a music record or
compact CD disk.
Tracks of computer information are written to and read from the
computer disk by a read/write coil (head) that moves rapidly
across the surface of the disk in a fashion similar to a record
player needle on a music record. Most current disks (360K IBM
format) have 40 tracks which are numbered from 0 to 39. The low
numbers are towards the edge of the disk - the high numbers
towards the center.
Tracks, the circular data paths on the disk, are divided into
still smaller units called sectors with the number of sectors
varying with the exact DOS operating system you use on your PC.
MS-DOS version 2.0 and higher versions use nine sectors per
track. DOS 2.0 and above can read the older eight sector disks
created by DOS version 1.1 but the reverse is not true. Each
track is divided into the same number of sectors like pieces of
apple pie. The sectors contain the magnetic bits or pulses of
information which the computer records in a special index
(called the file allocation table or FAT) so that it can quickly
move from sector to sector sniffing out information on the disk.
When you format a disk you ask the computer to inspect the
magnetic surface of the disk for any errors, prepare it for use
by future data and create an index "file allocation table (FAT)"
which is like a card index for a large library of books.
Formatting a disk is a little like taking a blank piece of paper
and using a pencil and ruler to turn it into graph paper with
both horizontal and vertical lines. What was blank before now
has little cells or file drawers which can hold information.
The file allocation table is so crucial to keeping track of
where the data is on the disk that DOS (the disk operating
system) usually keeps two copies in case of errors. Without a
file allocation table the disk is like a large public library
with no card catalog index and (worse still) every light in the
building has been turned off! Certain utilities contained in DOS
(i.e., the debug utility) and other software programs can adjust
or repair the file allocation table but generally this is a
delicate operation a beginner should not attempt.
Floppy disks are available in two types: single and double
sided. This means that the manufacturer guarantees only one (or
both) sides of the disk as capable of holding magnetic pulses.
Usually both sides of all disks are chemically coated, but the
manufacturer may have found defects and advises use of only one
side. IBM compatible machines usually use double sided, double
density disks (abbreviated as DSDD on the package.) Single
density disks record magnetic pulses or computer bits at 2,768
bits per inch and double density at 5,876 bits per inch. A
single sided disk may work in a machine for a while, but you DO
stand a risk that the data may be lost in time on the second
"non-certified" side of a single sided disk. Do NOT turn over a
disk and attempt to use the other side! Two problems arise: the
disk spins in the opposite direction which may cause data errors
and the small write protect notch is in the wrong location which
may damage the floppy drive mechanism.
What is the difference between a bit and a byte? The IBM PC and
its clones generally use 512 bits (electrical pulses) to make up
a byte (computer word.) Bits are like alphabet characters and
bytes are like the words made up from alphabet characters. So
how many bytes are stored on a disk? 40 tracks per side x 2
sides per disk x 9 sectors per track x 512 bytes per sector =
368,640 bytes stored per disk assuming DOS version 2.0 or later.
Basically this means about one third of a million pieces of data
information - quite a bit!
On the side of all floppy disks is a small square notch. If the
notch is uncovered, data can be freely written to the disk. If
covered with tape, the PC will NOT write to the disk but CAN
read from the disk. This is called the write protect tab. Be
careful when handling disks! Since the read/write magnetic head
on a floppy rides delicately in contact with the disk, tiny
obstructions can cause it to jump, skip or scratch the disk and
lose your data. Fingerprints, smoke, hair and moisture can cause
problems. Always handle a floppy disk by the edges of its
protective plastic "jacket" and replace it in a paper or plastic
Tyvek slipcover sleeve when not in use. In addition, magnets, x-
rays, televisions and other sources of stray magnetism can cause
a floppy disk to lose data.
Hard disks have many of the same characteristics as floppy
disks, but are managed and maintained in a different manner as
we will see in a later expanded tutorial on hard disks within
PC-LEARN. In brief, however, hard disks use aluminum platters
rather than flexible plastic mylar. Usually several platters are
stacked together within a single hard drive unit. The number of
stacked platters determine the data capacity of the hard drive
unit. Because the hard disk platter spins much faster and holds
data packed more tightly that a floppy disk, the hard drive unit
is usually sealed in a metal shroud or container to eliminate
dust or other contaminants. A sealed hard drive is sometimes
referred to as a Winchester disk or Fixed drive. Where a floppy
disk might hold approximately 360,000 bytes (abbreviated as
360K), a hard drive holds 10 Megabytes (million bytes) or more.
As we will discuss later, backing up (making spare copies of
hard drive data onto floppy or tape) is a necessary task since
hard drives can and do fail - taking precious data with them.
The bottom line is that once you get started with a computer,
quite quickly your data becomes far more valuable than the
computer in which it resides!
Since we have briefly covered data storage we need to talk about
data input. Two primary input devices are central to getting
data into a pc. The keyboard and the mouse. We will discuss the
keyboard in greater detail in a later tutorial. The mouse is an
alternate input device which is rolled or moved across the
desktop to position a cursor or pointer on the computer screen.
The mouse also contains several buttons to help select items on
data on the monitor screen. A mouse is not necessary for
computer input - it is an optional device.
Another introductory topic is that of output devices such as a
monitor, printer or plotter.
A plotter is a device which uses a motor to move pens or drawing
implements in tightly controlled horizontal and vertical motions
on a piece of paper or film. The computer can control a plotter
to combine on one piece of paper differing pen colors and text
and pictures stored within the computer. Computer plotter can be
purchased with flat table or flat bed configurations or in
models which move the pen(s) back and forth with gears that also
drive the paper movement at the same time.
The printer is probably the most common and useful output device
attached to your computer. There are many types of modern
computer printer with differing speeds and capabilities. The
most common printer is the dot matrix printer which provides
characters made up from tiny dots of ink on paper. The Daisy
wheel printer uses a rapidly spinning wheel to imprint each
letter separately like any ordinary typewriter. Line printers
print entire lines of text in one sweep then move to the next
line and are thus very fast. Ink jet printers produce characters
made from individual dots of ink sprayed onto the paper. Thermal
printers contain tiny wires which burn and thus darken special
thermal paper into tiny letters and dots which we can read.
Finally laser printers use a rapidly scanning laser to sensitize
a polished drum with an entire page of information quickly and
look and work roughly like an office copier. The first three
types of printer are classified as impact printers since
something strikes the paper which the later three are non impact
printers.
The oldest printer design is the thermal printer which
maintained some popularity and was easy to manufacture, however
the use of thermal printers is fading since the special heat
sensitive paper is expensive and subject to random extraneous
marks and blurring.
The laser and ink jet printers are becoming more popular due to
rapid speed of printing and quiet mode of operation. They are
expensive with prices ranging from $600 to $2000. The ink jet
printer squirts individual dots of ink onto the paper to form
letters or other characters. A high quality paper is necessary
since the wet ink can smear if not carefully handled.
The laser printer is used for quickly producing one page of text
at a time. In operation, the laser scans a polished drum with an
image which is then dusted with dark toner particles which stick
to the exposed areas made sensitive by the laser. Paper is then
placed in contact with the drum and the toner is transferred to
the page and is finally fused with heat to "fix" or seal the
toner particles to the page.
Dot matrix and daisy wheel printers are common and affordable
alternatives for many small offices and home computer hobbyists.
The two differ in the sharpness and quality of the final printed
document.
Dot matrix printers produce letters via small pins which strike
the ink ribbon and paper to produce print which can be jagged
looking. Nine pin dot matrix printers produce somewhat rough
looking letters while 24 pin dot matrix printers produce
crisper, fully-formed letters. In many cases the 24 pin dot
matrix printer approaches the quality of the daisy wheel printer
which seems to be fading from the computer printer scene. Both
dot matrix and daisy wheel printers strike the paper through a
ribbon to transfer ink to the printed page.
Connecting a printer via a cable to the computer is always done
through one of two plugs (or interfaces) on the back of the
computer. One type of interface (computer plug) is serial, the
other called parallel. The most commonly used interface for
printers today is the parallel interface but serial interface
printers do exist. What is the difference? Recall that there are
eight bits (computer dots and dashes) to a byte (or computer
word). The serial interface has each bit sent one at a time to
the printer - like men in single file at the supermarket
checkstand. The parallel interface sends all eight bits at once
- like eight men all entering eight supermarket checkstands at
once. Each interface is different, the printer manufacturer will
tell you which interface to use. As a clue, frequently modems or
mouse devices use the serial interface leaving the printer to
the parallel interface.
We have talked about output to paper, next let's briefly discuss
output to a monitor or screen. The monitor or video display
works much like your television - some older home computers
still use a TV. Another term for a monitor is the cathode ray
tube or CRT. Monitors differ in the sharpness or resolution they
can display. On the low end of the resolution spectrum is the
monochrome (single color) monitor frequently available in either
green or amber screens. Next is the color RGB monitor (RGB
stands for Red, Green and Blue) which displays low resolution
color dots to make up an image. Higher resolution is obtained
with an EGA monitor (Enhanced Graphics Adapter) and still higher
with a VGA (Video Graphics Array) Monitor. Each monitor is mated
to work with a circuit card located within the body of the
computer. One way to upgrade a computer is to switch both the
monitor and display/graphics circuit card to produce a sharper,
more colorful image. The dots which make up all images on the
monitor screen are called pixels. The smaller the pixels, the
higher and sharper the image resolution.
What is the difference between computer hardware and software?
In simplest terms, hardware is the physical parts associated
with a computer - the circuit boards, floppy drives, printers,
cables and physical pieces of a system. Software is the
electronic instructions necessary to make the computer perform.
These instructions are usually stored inside a piece of hardware
(e.g., software instructions stored inside a circuit chip or
floppy drive) but they are nevertheless software. There are two
major types of software: operating system software and
applications software.
Operating system software (like DOS) performs very elemental
housekeeping instructions (e.g., where is monitor, how can I
keep track of what data is on which track or sector of a floppy
drive.)
Applications programs perform tasks on a higher level (e.g.,
word processing programs or database programs are applications.)
Generally an application software package uses the lower level
operating system (DOS) to do routine tasks (e.g., your word
processing application uses the lower level DOS operating system
frequently to write and store data on a disk.
We interrupt this tutorial for a brief reminder: be sure to
submit your registration fee to receive your BONUS DISKS!
Now back to our regularly scheduled tutorial . . .
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INTRODUCTION TO COMPUTER TECHNOLOGY - PROCESSING AND THE CPU
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You can pause for a while if you like or go onto to another
tutorial. But if you want delve into great complexity, read on.
Now it's time to delve deeper into the heart of the computer.
The central processing unit or CPU is the "brains" of every
computer. On the PC, the CPU is simply a tiny integrated
circuit. It is the control center and contains two circuit
elements to perform tasks plus several special locations or
memory areas called registers which hold instructions.
Registers, located within the CPU chip are temporary storage
locations which hold instructions. Secondly, the arithmetic
logic unit or ALU is the location within the CPU where seven
basic math and logic operations take place (such as addition and
subtraction.) Finally, the control unit is a portion of the CPU
which directs all elements of the computer. It does not add or
subtract like the ALU, it only directs the activity.
Let's first examine the registers within the CPU. Four registers
are present in the CPU - some computers contain more than four.
The storage register is simply a parking area for information
taken from or sent to memory. The accumulator register
accumulate the results of calculations. The address register
stores the location of where the information or instructions are
located. Finally, one or more general purpose registers are
usually available and have several functions which can
interchangeably include addressing (where is it?) or arithmetic
(add or subtract it.)
Registers can vary in size or bits with the variety of the
computer. 8-bit registers are common on small computers. 16-bits
for larger personal computers. And finally minis, mainframes and
supercomputers have 64-bit or larger registers. This length (8-
bit, 16-bit, etc) is called a word and frequently larger and
more powerful computers feature larger register size.
Despite this seeming complexity a basic fact remains: all
digital computers can only add and subtract two numbers: zero
and one! Let's back up a bit. For purposes of digital computer
electronics, internally a computer can only respond to two
things: on and off - just like a light switch. These electronic
states of being might actually be a positive and negative
voltage or a high and low voltage stored in a series of
transistors etched in silicon on a chip, but to the computer the
logic is on or off. Two conditions, that is all.
Back in the human world we can represent these as one and zero
(1 and 0). A special branch of mathematics deals with
calculations of numbers represented by 1 and 0 which is called
binary arithmetic.
Each one or zero is a pulse of electricity or magnetism
(electricity inside a chip, magnetism out on the surface of a
floppy disk.) Each pulse, either a 1 or 0 is called a bit. Whole
series of bits in a row can be used to represent numbers larger
than 9 in our human decimal system. Bits in strings of eight
units are called bytes. One byte represents a single character
of data in the computer. As a curious aside, a nibble is half a
byte or four bits.
We go back to our analogy of the light switch (on and off
representing one and zero to a computer.) In simplest terms, if
we have two light switches we have the following ideas:
OFF OFF = 0 0 = (human decimal number) zero = 0
OFF ON = 0 1 = (human decimal number) one = 1
ON OFF = 1 0 = (human decimal number) two = 2
ON ON = 1 1 = (human decimal number) three = 3
Notice something peculiar: in the above we find FOUR binary
numbers (0,1,2,3) but THREE human decimal numbers (1,2,3.) We
rarely think of 0 as a number since we consider it NOTHING.) To
computers ZERO is always a number!!!
Going a little further a single bit can only represent two
numbers: (ON or OFF = 1 or 0 ). Two bits (our above example can
represent four numbers (0,1,2,3). And four bits could represent
16 numbers. If you go all the way to a byte (eight bits) you
could get 256 numbers. The pattern is that each additional bit
doubles the quantity of possible numbers.
To a computer these binary numbers march together in a long
string, one after another. Remember, the CPU has only two
numbers to work with: 1 and 0.
This table should provide some help:
Human Computer
Decimals Binary
────────┬────────
0 │ 0
1 │ 1
2 │ 10
3 │ 11
4 │ 100
5 │ 101
6 │ 110
7 │ 111
8 │ 1000
9 │ 1001
10 │ 1010
11 │ 1011
12 │ 1100
13 │ 1101
14 │ 1110
15 │ 1111
Notice several eccentricities about this system. In computer
binary, you start on the right and keep adding digits to the
left. When you fill a space with all 1's, you zero out
everything, add one digit to the left, and start with "1" again.
When you reach binary 111 you start the WHOLE series over again
with a 1 in front of it. One bit counts two numbers, two bits
count four, three bits count eight and so on as we mentioned
earlier. When you add a binary digit to the growing string of
1's and 0's you double the number of total decimal digits you
can use!
These eccentricities appear odd, but to the computer they are
shortcuts which simplify calculations and keep things to 1's and
0's. It is this simple system of on and off (like light
switches) which make computers and their odd binary system so
FAST!
Now that we understand the basic binary arithmetic of a computer
we can say a few words about addressing. Simply put, each piece
of information in the computer lives in a little memory location
(like eggs in a carton -each egg is a piece of data, each carton
hole is an address or location.) Each address is unique, of
course. The first address, the second, and so on. How many
addresses can an 8-bit binary number describe? 256. A 16-bit
number can specify 65536 addresses or possible locations for
data.
As we finish our introduction to computer technology we should
briefly list a few terms. There are more in the glossary
contained elsewhere on this disk:
Kilo - Thousand units. Example: kilobyte. Because of the binary
math associated, this is actually 1024 bytes. Frequently
abbreviated as the simple letter "K".
Meg - Million. Example: 20 Meg hard disk which hold 20 million
bytes approximately.
Millisecond - One thousandth of a second.
Microsecond - One millionth of a second.
Nanosecond - One billionth of a second.
Picosecond - One trillionth of a second.