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Acomputer is a machine that can manipulate
information (called data) according to a prepared
set of instructions called a program. In order
to do this, it has to have a place to store both the
data and the program. There are two places to store
things in a computer: memory and hard disk. If you
use a computer, you’ve probably heard these two
terms from time to time; but unless you’re a computer
specialist, you’ve probably never encountered
a clear explanation of the difference between
them. This paper describes what both of these parts
of a computer are in physical terms, how they
differ, and what they do.
The Central Processing Unit
The heart of your computer is called the central
processing unit, or CPU. The CPU does the actual
work of the computer. Specifically, it is designed to
carry out various simple operations on blobs of
data, in accordance with a sequence of instructions
contained in a program. Another name for the CPU
is the microprocessor.
In physical terms, the CPU is a tiny, flat, rectangular
chip of silicon, and is somewhat smaller than
a postage stamp. The chip contains nearly a hundred
million microscopic transistors. It is enclosed
in a matching flat ceramic box bristling with goldplated
connecting pins that lives deep inside the
case of your computer. The box is hidden beneath
the fins of a metal radiator and fan that are necessary
just to keep the CPU from melting (the CPU
consumes about as much electricity as a light
bulb—which is a lot for something smaller than a
postage stamp).
The data manipulated by the CPU, as well as the
program that provides it with instructions on which
manipulations to perfrom, are stored in the computer’s
electronic memory.
About Memory
Memory is a part of your computer that temporarily
stores information needed by the CPU.
The CPU cannot do its job without it. Physically,
memory consists of tiny flat boxes that look like
cyber-centipedes, arranged on rigid flat sticks that
are installed inside the computer. These boxes contain
computer chips similar to the chip that forms
the CPU, except that these chips contain memory
devices instead of CPU circuits. Virtually all computers
come with one or more memory sticks preinstalled
at the factory, but most computers let you
add more, if you wish.
Memory is just what its name implies: it is a part
of your computer that “remembers” things for the
CPU. The CPU needs something to work on while
it is running, including both the information it is
supposed to be manipulating (a business letter or
spreadsheet, for example) and instructions telling
the CPU what to do with that information (usually
referred to as a program or software). The memory
of your computer holds these things, and provides
them to the CPU (at blazing speed, of course)
whenever the CPU asks for them. In addition,
when the CPU calculates something new (such as
the result of some calculation in a spreadsheet), it
immediately stores it in the computer’s memory.
The memory of your computer has one annoying
characteristic: everything stored in that memory is
forgotten when you turn the computer off. This
characteristic makes a computer with just a CPU
and memory pretty useless, because it is impossible
to keep anything permanently (unless you
never, ever turn the computer off). Fortunately,
there is a remedy for this problem, and that remedy
is the hard disk.
About the Hard Disk
A hard disk is a part of every modern computer
that allows the computer to store information permanently.
A hard disk is like a tape recorder or dic-
Memory and Hard Disk:
What’s the Difference?
Copyright © 2005 by Anthony Atkielski. All rights reserved.
tating machine: you can store information on your
hard disk, and then get it back later, at any time.
You can also erase things on the hard disk and
replace them with something else.
The hard disk is essential to every computer
nowadays because of the above-mentioned nasty
little drawback of computer memory, namely, the
fact that computer memory is forgotten when the
computer is turned off. By recording things on the
hard disk before turning the computer off, it is possible
to retain them safely until the next time the
computer is turned on.
You might wonder why a computer even bothers
with memory if it can store things permanently on
the hard disk. The problem is speed, and space. The
computer’s memory isn’t very permanent and
doesn’t hold very much information, but it is blazingly
fast, and that’s what the CPU needs if it is
going to operate at anywhere near its maximum
speed. This being so, every computer uses memory
to remember things rapidly and temporarily, and
then puts them on the hard disk if they have to be
kept around permanently.
How They All Fit Together
The best way to describe how the CPU, memory,
and the hard disk work together is by way of analogy
with your own brain (which is a kind of computer,
although comparing a computer to a human
brain is rather like comparing a sleeping bag to
New York City). Think of the CPU as your own
brain, think of the computer memory as your own
memory, and think of the hard disk as a huge card
file on your desk.
Now, in order to think about anything, you need
to know about it first. This means that you have to
put some kind of information in your memory, just
like a computer. Similarly, whenever you do any
kind of calculation or solve any kind of problem,
the results of your efforts are in your memory.
Of course, your memory is neither infinitely large
nor perfect, so you need something to help you
keep track of things permanently. The card file on
your desk can serve this purpose. The card file is
your own “hard disk,” if you will.
Suppose you need to add a column of numbers
together, and suppose that these numbers are in
your card file. The first thing you need to do is
search through the card file and look up the numbers.
Next, you need to add the numbers together
using your head. Finally, you need to write the
results down somewhere (such as back in your card
file again) in case you forget them.
This process has an exact analogy inside your
computer. Looking up the numbers to be added
together is the equivalent of the computer looking
for something on the hard disk and putting it into
its memory. Adding the numbers together is the
equivalent of the computer’s CPU adding the numbers
in memory together and saving the result in
memory. Finally, writing the answers in the card
file is the equivalent of the computer writing the
results of its calculations onto the hard disk.
You may have realized by now that you have an
important advantage over the computer, in that you
are never “turned off” at the end of the day. This
means that your memory is never completely
erased. You can memorize some things permanently
as a result. However, a computer doesn’t have
this option; every time it is shut off, it forgets
everything in its memory, and so anything that it
hasn’t saved on its hard disk at that point is lost forever.
Thus, computers are much more diligent
about writing information on their hard disks than
you are about writing things in your card file.
You’ll probably never experience total amnesia, but
a computer experiences this every time you flip the
power switch (in fact, just pressing the reset button
on the computer, if there is one, will also produce
total amnesia in the computer’s memory). If your
own brain and body worked like a computer, you’d
have to learn how to walk and talk all over again
every morning after waking up.
By now it should be clear that memory and the
hard disk each have their places. Memory is essential
in order to allow the computer to “think”; and
the hard disk is essential if the computer is to recall
what it was doing the last time it was used.
The functioning of your computer’s memory is
invisible to you; the computer can manage that all
on its own. However, you do have to advise the
computer about what information to save permanently
on the hard disk, and you also have to decide
how you want things organized on the hard disk.
This is what we will discuss next.
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Organizing Your Hard Disk
Now that we’ve covered the details of both memory
and the hard disk, it should be easier to understand
how it is possible to save your work for later,
and how it is possible to accidentally lose your
work, if you aren’t careful.
Whenever you open a document or start a new
document or project on your computer, the computer
will automatically set up all the necessary
things in memory, without any help from you.
While you are working on your document, all the
changes, deletions, and additions you make are
stored in the computer’s memory automatically.
You don’t have to worry about any of this.
However, at some point, you’ll usually want to save
all your work somewhere so that you can return to
it hours, days, or even years later. At that point,
then, you need to know how to tell the computer to
save your work permanently on the hard disk. If
you just turn the computer off without saving your
work, it’s gone—you’ll have to start everything
over from scratch the next time you use it. But if
you tell the computer to save your work on the hard
disk, it will be there the next time you need it, and
you can just tell the computer to open it again.
The magic concepts you need to remember when
saving and looking for things on your hard disk are
those of files and folders.
Everything on your computer’s hard disk is
organized in files. Files are so called because they
are very much like electronic versions of the paper
files you keep in your file cabinet. For example, a
hand-typed annual report might be stored in a file
in your file cabinet, whereas an annual report that
you prepare with a word-processing program in
your computer will be stored as a file on the computer’s
hard disk.
Unlike memory, the hard disk isn’t automatically
managed by the computer. It’s up to you to decide
how you want to organize things on your hard disk.
When you create a document and save it on your
hard disk, you save it as a file, and you give the file
a name (which you choose), so that the computer
can identify it later on. You also need to tell the
computer where to put the file on the hard disk.
This latter task involves the concept of folders, so
we need to explain those.
A folder on your hard disk is very much like a
folder in your file cabinet. It can hold one or more
files. Every folder has a name (which you choose
yourself). You can have as many folders and files as
you want, as long as you have space left on your
hard disk. You can create folders (and files, for that
matter) whenever you want, and you can erase
them, too, at your discretion. If you erase a folder,
though, everything inside of it disappears as well.
(If you erase a file, that file disappears, but other
files in the same folder are unaffected.)
Folders on the hard disk of your computer are a
little fancier than folders in a file cabinet, because,
unlike a file cabinet, a hard disk can hold folders
inside folders. In other words, you can create a
folder and put a couple of files in it, but you can
also create another folder inside the first folder, and
put more files inside of that. This is a bit like putting
a manila folder inside another manila folder,
except that it’s hard to fit manila folders inside each
other, whereas putting folders inside other folders
on a computer is easy. In fact, you can create as
many levels of folders as you want on your computer.
How you organize the folders is up to you.
As an example, you might create a folder called
Payroll, and put spreadsheets or other documents
related to payroll inside of it (see Figure 1).
However, if you had twenty employees, you could
also create a subfolder for each employee within
the Payroll folder, and give each folder the name of
the employee. So, inside the Payroll folder, you’d
not only have documents related to payroll, but
you’d also have more folders called John Smith,
Jane Doe, etc., for each employee, in which you
could save other files on a per-employee basis. Of
course, this is just an example, but you get the idea.
How you actually set up folders on your hard disk
is entirely up to you. The computer doesn’t really
care.
There is one folder, called the root folder, that
exists on every hard disk. The computer creates
this for you. It doesn’t have a name. You create all
your folders and files inside this root folder. You
can also create additional levels of folders inside
folders, if you wish. You can delete all the folders
and files you create yourself, but you cannot delete
the root folder. The computer needs that folder so
that it can find all the other folders and files that
you create.
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Using and Saving Files
Now that you understand how things are organized
on the hard disk, it’s important to understand
what causes things to be saved in files, and what
doesn’t. Remember, when something is saved in a
file on the hard disk, it’s safe—it won’t be lost
when the power is turned off. Conversely, anything
that hasn’t been saved will disappear as soon as
you shut off or reset your computer. This is why it
is very important that you know what causes things
to be saved on the hard disk.
When you open a new document (a letter, a
spreadsheet, whatever) it is stored in your computer’s
memory. It stays in the computer’s memory
until you tell the computer to save the document in
a file, at which point the computer copies the document
in memory to the hard disk, with the file
name that you specify, in the folder that you select.
At that point, you have a copy of the document
open in memory, and a copy saved on your hard
disk. If you don’t ever save the document in a file,
it will never exist outside of the computer’s memory,
so it will disappear as soon as you turn the computer
off.
If you make changes or additions or any kind of
modification to an open document, those changes
are made to the copy in memory only, and they are
not saved to the hard disk until you tell the computer
to save the document. Furthermore, after
you’ve saved the document, any additional changes
are still kept in memory only, so each time you
make new changes to a document, you need to save
it again when you are finished with the changes, or
they will be lost when you turn off the machine. If
you make changes to an open document after saving
it, and you then close the document, and you
say “no” when the computer asks if you want to
save the document, the changes you made after the
last time you saved the document will be lost.
When you first create a new document, there is
no file on the hard disk to hold it. When you try to
save the new document for the first time, the computer
will ask you to tell it what file name you want
to use to create a file on the hard disk to hold it, and
it will give you the opportunity to select the folder
in which you want to create the file. Thereafter, the
computer will know where the file is, and when
you save the document again, it will automatically
save the document in the same file.
When you open an existing document, the computer
reads the document from the file that contains
it on the hard disk, and puts a copy of the document
in memory so that you can view it and change it
(remember that the computer’s CPU can’t do anything
with a document unless it is in memory). Any
changes you make are made in memory only; you
must tell the computer to save the changes if you
want them to become permanently recorded in the
document’s file on the hard disk.
When you close a document, if you have made
any changes to the document since you last told the
computer to save it to the hard disk, the computer
will ask you if you want to save the changes. If you
say “yes,” the changes will be recorded in the document
on the hard disk; if you say “no,” the
changes you made since the last save will disappear.
If you haven’t made any changes since the
last save, the computer won’t ask anything, since
there is nothing new that has to be permanently
recorded. If you close a brand new document that
you have never saved before, the computer will not
only ask if you want to save the changes, but it will
also ask you to select the file name and folder in
which you want to save your document (if you
answer “yes” to the computer’s question).
Deleting Files and Folders
Just as you can create and modify files, you can
delete them, erasing them forever from the hard
disk of your computer. If you delete a file, its contents
are gone forever; you cannot bring the folder
back into existence once it is gone. Furthermore, if
you delete a folder, not only is the folder erased,
but so are any files or folders it contains. Because
of this, you should never delete a file or folder
unless you are absolutely certain that you never
want to see it again.
Preventing Disasters
Hard disks are extremely reliable, but not perfect.
Every once in a while, they fail. If they fail, all the
information you have stored on them is lost. For
this reason, you really need to take backups of the
files on your hard disk if you want to be completely
protected.
A backup is simply a copy of important information
on your hard disk. For example, you can copy
a file that contains an important document from the
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hard disk to a floppy disk inserted into your computer;
in this way, if the hard disk ever breaks,
you’ll still have a copy of the document on the
diskette.
In practice, most backups involve lots of files, not
just one. For example, you could create a folder on
your hard disk in which you store your most important
documents. At regular intervals, you could
copy all the files in the folder to diskettes, and put
the diskettes in a safe place. In this way, if your
computer were ever to burn down or explode or
something, you’d still have safe copies of all your
most important documents. It’s really no different
from making photocopies of important paper documents
and putting them in a safe place.
In its most advanced form, a backup involves
copying the entire contents of the hard disk to some
other place, such as diskettes, a cassette tape, or
something similar. Since modern hard disks hold
the equivalent of hundreds or thousands of
diskettes, most backups of this kind are carried out
with a special program that writes all the information
to a cassette tape, a CD, a special kind of optical
diskette, or something similar.
The details of backup methods are outside the
scope of this paper; however, backups are very
important, and they are your only protection
against a failure of your hard disk. If you are a
home user, it may be sufficient to just back up the
most important files you have on a few diskettes. If
you are an office user in a large company, your
local computer support group may be able to back
up your machine automatically for you at regular
intervals, or they may be able to advise you on the
best way to perform a backup. Finally, some people
prefer to take their chances and never back up anything,
but we cannot recommend this approach.
One final note: If you are running your own small
business and your computer is important to that
business, you must take backups regularly. Many
small businesses (and not-so-small businesses)
have failed only months after a computer failure,
simply because they never bothered to back up the
essential data they kept on their computers. If you
are running a small business, back up everything
on your computer frequently—you will never
regret it. !
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