500 MB Nyheder Direkte fra Internet 2/500 MB nyheder direkte fra internet CD 2.iso/start/data/text/disks.txt

Text File | 1994-09-21 | 57KB | 1,055 lines

                  FUNDAMENTALS
  Memory comes in three popular forms: RAM chips, ROM chips, and
disks.
  You already learned about RAM chips and ROM chips. Let's
examine disks.
  A computer disk is round, like a phonograph record.

                   Three kinds
  You can buy three popular kinds of computer disks:
A floppy disk is made of flimsy material. It's permanently
encased in a sturdy, square dust jacket.
A hard disk is made of firmer material. It typically hides in
your computer permanently, unseen.
A CD-ROM is a compact disk. It's the same kind of CD compact disk
that plays music.
  Each kind has its own advantages and disadvantages.
  Floppy disks are the cheapest (about 50¢ per disk) and the
easiest to mail to your friends: just stick the floppy disk in an
envelope, perhaps with some padding. Unfortunately, floppy disks
work the most slowly, and they hold the least data: the typical
floppy disk holds about 1 megabyte, while the typical hard disk
or CD-ROM can hold many hundreds of megabytes.
  Hard disks work the fastest ___ over 20 times faster than the
other kinds! But hard disks are also the most expensive.
Moreover, they typically can't be removed from your computer and
therefore can't be mailed to your friends.
  CD-ROMs are the best value: CD-ROM disks cost less than 1¢ per
megabyte to manufacture. But they have a frustrating limitation:
the information on CD-ROM disks cannot be edited.
  Since each kind of disk has its own advantages and
disadvantages, you'll want to buy all three kinds.

                    Spelling
  Computer experts argue about spelling. Some experts write
``disk'', others write ``disc''.
  Most manufacturers write ``disk'' when referring to floppy
disks or hard disks, but write ``disc'' when referring to
CD-ROMs. That inconsistency annoys me.
  To be more consistent, I'll always write ``disk'', even when
referring to CD-ROMs. Most computer magazines (such as PC
Magazine and PC World) feel the same way I do: they always write
``disk''. The growing tendency is to always write ``disk''.
  For hard disks, IBM used to write ``disc'' but now writes
``disk''.

      FLOPPY DISKS
                                                     A floppy
disk (or diskette) is round but comes permanently sealed in a
square dust jacket. (Don't try to remove the floppy disk from its
square jacket.)
                                                     The floppy
disk is as thin and flimsy as a sheet of paper but is protected
by the sturdy, square jacket that encases it.

                                                          Three sizes
                                                     Floppy disks
come in three sizes.
                                                     The most
popular size is called a 3½-inch floppy disk, because it comes in
a square jacket that's about 3½ inches on each side. (Actually,
each side of the jacket is slightly more than 3½ inches, and the
disk's diameter is slightly less.)
                                                     An older
size, used mainly on older computers, is called 5¼-inch. It comes
in square jacket that's exactly 5¼ inches on each side.
                                                     An even
older size, 8-inch, is used just on ancient computers that are no
longer built.
                                                     Those three
sizes have nicknames:
An 8-inch floppy disk is called a large floppy.
A 5¼-inch floppy disk is called a minifloppy.
A 3½-inch floppy disk is called a microfloppy.

                                                         Jacket colors
                                                     The jacket
of a 5¼-inch or 8-inch floppy disk is usually black. The jacket
of a 3½-inch floppy disk is usually black, blue, white, or beige
(very light grayish brown).
                                                     If you pay a
surcharge, you can get jackets that have wilder colors.

                                                            History
                                                     8-inch
floppies were invented in the early 1970's by IBM. 5¼-inch
floppies were invented in the late 1970's by Shugart Associates,
which later became part of Xerox.
  3½-inch floppies were invented in the 1980's by Sony. They've
become the most popular size because they're the smallest,
cutest, and sturdiest. They're small enough to fit in the pocket
of your shirt, cute enough to impress your friends, and sturdy
enough to survive when you fall on your face. They're also easy
to mail, since they're small enough to fit in a standard white
business envelope and sturdy enough to survive the U.S. Postal
System. Yup, nice things come in small packages!

           Magnetized iron
  The round disk (which hides inside the square jacket) is coated
with rust, so it looks brown. Since the rust is made of iron,
which can be magnetized, the disk stores magnetic signals. The
pattern of magnetic signals is a code representing your data.

               Drives
  To use a floppy disk, you must buy a floppy-disk drive, which
is a computerized record player.
  If the drive is external, it's a box sitting near the computer.
If the drive is internal, it's built into the middle of the
computer.
  The drive has a slit in its front side. To use the drive, push
the disk (including its jacket) into the slit.
  When pushing the sheathed treasure into the box's slit, don't
shove too hard. Oooh! Please be gentle!
  When you push your disk into the slit, don't push the disk in
backwards or upside-down! Here's how to push the disk in
correctly. . . .
  First, notice that the disk's jacket has a label on it and also
has a big oval cutout. (If the disk is 3½-inch, the cutout is
covered by a metal slider.) Insert the disk so that the oval
cutout goes into the drive before the label does. If the drive's
slit is horizontal, make sure the label is on the top side of the
jacket; if the drive is vertical, make sure the label is on the
left side of the jacket.
  After putting the disk into the slit, close the latch to cover
the slit. (If the disk is 3½-inch, there is no latch.) Since the
slit and latch act as a door, closing the latch is called closing
the door.
  As soon as you close the door, the disk drive automatically
positions the disk onto the turntable that's hidden inside the
drive. The turntable's called the spindle. It can spin the disk
quickly.
  Like a record player, the disk drive contains an arm with a
``needle'' on it. The needle is called the read-write head,
because it can read what's on the disk and also write new
information onto the disk.
  Here's how to write new information onto the disk. Put your
fingers on the computer's keyboard. Type a command that tells the
computer you want to use the disk. Then type the information you
want to transfer to the disk.
  To transfer the information to the disk, the computer lowers
the read-write head onto the disk. An electrical charge passes
through the head. The charge creates an electromagnetic field,
which magnetizes the iron on the disk's surface. Each iron
particle has its own north and south pole; the patterns formed by
the north and south poles are a code that stands for the
information you're storing.
  Tracks As the disk spins, the head remains stationary, so that
the head draws a circle on the spinning disk's surface. The
circle's called a track. To draw the circle, the head doesn't use
ink; instead, it uses a pattern of magnetic pulses. Since your
eye can't see magnetism, your eye can't see the circle; but it's
there!
                                         When you start using a
blank disk, the arm puts the head near the disk's outer rim, so
that the head's track (circle) is almost as wide as the disk.
That track's called track 0.
                                         Then the arm lifts the
head, moves the head slightly closer to the virgin disk's center,
and puts the head back down onto the disk again. The head draws
another circular track on the disk, but this new circular track
is slightly smaller than the previous one. It's called track 1.
                                         Then the head draws
track 2, then track 3, then track 4, and so on, until the head
gets near the center of the disk, and draws the last circular
track (which is smaller than the other tracks).
                                         To organize the
information on a track, the computer divides the track into
sectors. Each ``sector'' is an arc of the circle.
                                         Single-sided versus
double-sided drives A modern disk drive has two read-write heads.
One head uses the disk's top surface, while the other head uses
the disk's bottom, so that the drive can use both sides of the
disk simultaneously. That's called a double-sided disk drive. The
drive puts information onto the disk by first using track 0 of
the main side, then track 0 of the flip side, then track 1 of the
main side, then track 1 of the flip side, etc.
                                         If a disk drive is not
modern ___ if it's ancient and primitive ___ it has just one
read-write head, which uses just one side of the disk. The flip
side of the disk is unused. That kind of drive is called a
single-sided disk drive. Which side of the disk does the drive
use? Though some drives use the side that has the label, other
drives (by other manufacturers) use the side opposite the label
instead.
                                         Double-sided is also
called DS and 2-sided and 2S. Single-sided is also called SS and
1-sided and 1S.
                                         Capacity How many
kilobytes can you fit on a floppy disk? The answer depends on
which kind of drive you have.
                                         The most popular kind of
drive is called a 3½-inch high-density floppy drive. Here's how
it works.
                                         It holds a 3½-inch
floppy disk. It writes on both sides of the disk simultaneously,
since it's a double-sided disk drive. It writes 80 tracks on each
side. It divides each track into 18 sectors. Each sector holds
``512 bytes'', which is half a kilobyte, ½K.
                                         Since the disk has 2
sides, 80 tracks per side, 18 sectors per track, and ½K per
sector, the disk's total capacity is ``2 times 80 times 18 times
½K'', which is 1440K. So altogether, the disk holds 1440K. That's
called 1.44M (where an M is defined as being 1000K). That's why a
3½-inch high-density floppy drive is also called a 1.44M drive.
                                         The kind of disk you put
into it is called a 1.44M floppy disk (or a 3½-inch high-density
floppy disk). Since the disk holds 1.44M (which is 1440K), and
since a K is 1024 bytes, the disk holds ``1440 times 1024''
bytes, which is 1,474,560 bytes altogether. That's a lot of
bytes!
                                         Although the disk holds
1440K, some of those K are used for ``bureaucratic overhead''
(such as holding a directory that reminds the computer which data
is where on your disk). A Mac uses just 1 sector (½K) for
bureaucratic overhead. An IBM-compatible computer uses 33 sectors
(16½K) for bureaucratic overhead, leaving just 1423½K (1,457,664
bytes) for your data.
  When you buy a blank disk to put in a 1.44M drive, make sure
the disk is the right kind. Make sure the disk is 3½-inch; and to
get full use of what the drive can accomplish, make sure the disk
is high-density! The abbreviation for ``high-density'' is HD. A
high-density 3½-inch disk has the letters HD stamped in white on
its jacket; but the H overlaps the D, so it looks like this: HD.
Also, a high-density 3½-inch disk has an extra square hole cut
through its jacket.
  Old computers use inferior floppy drives, whose capacities are
less than 1.44M.
A capacity that's less than 150K is called single-density (SD).
A capacity bigger than 150 but less than 1M is called
double-density (DD).
A capacity bigger than 1M is called high-density (HD).
Anything less than high-density is called low-density.
  Although the jacket of a high-density 3½-inch disk has ``HD''
stamped on them and an extra hole punched through it, the jackets
of other kinds of disks often lack any distinguishing marks. Too
bad!
  Popular IBM-compatible drives For IBM-compatible computers,
four kinds of floppy drives have been popular:
IBM drive's nameCapacity  Details
5¼-inch double-density 360K40 tracks per side,  9 sectors per
track
5¼-inch high-density1200K (which is 1.2M)80 tracks per side, 15
sectors per track

3½-inch double-density 720K80 tracks per side,  9 sectors per
track
3½-inch high-density1440K (which is 1.44M)80 tracks per side, 18
sectors per track
  Each of those IBM-compatible drives is double-sided and has ½K
per sector. They're manufactured by companies such as TEAC, NEC,
and Chinon.
  The fanciest drives (3½-inch high-density) used to be
expensive, but now you can buy them for just $49 from mail-order
discount dealers (such as Insight at 1912 W. Fourth St., Tempe AZ
85281, phone [phone] or 602-902-1176).
  Mac drives For Mac computers, three kinds of floppy drives have
been popular:
Mac drive's nameCapacity  Details
1-sided double-density 400K1 side, 8-12 sectors per track
2-sided double-density 800K2 sides, 8-12 sectors per track
high-density  1440K (which is 1.44M)2 sides, 18 sectors per track
Each Mac drive is 3½-inch and has 80 tracks per side, ½K per
sector. The Mac's high-density drive is called the Mac
Superdrive.
  On a disk, the inner tracks have smaller diameters than the
outer tracks. Most drives squeeze as many sectors onto an inner
track as onto an outer track, but the Mac double-density drives
puts fewer sectors onto the inner tracks and put extra sectors
onto the outer tracks. Specifically, the outer 16 tracks are
divided into 12 sectors, the next 16 tracks into 11 sectors, the
next 16 into 10, the next 16 into 9, and the inner 16 into 8.
  Drives for other computers For other computers, many kinds of
floppy drives have been invented:
Computer    Drive capacityDetails
Apple 2 family140K    5¼", 1 side,  35 tracks, 16 sectors, ¼K per
sector

Tandy Color Computer157½K5¼", 1 side,  35 tracks, 18 sectors, ¼K
per sector
Tandy Models 3, 4, 4P180K5¼", 1 side,  40 tracks, 18 sectors, ¼K
per sector
Tandy Model 4D360K    5¼", 2 sides, 40 tracks, 18 sectors, ¼K per
sector

Commodore 64170■K     5¼", 1 side,  35 tracks, 17-21 sectors, ¼K
per sector
Commodore Amiga880K   3½", 2 sides, 80 tracks, 11 sectors, ½K per
sector
  For the Commodore 64, the 17 outer tracks are divided into 21
sectors, the next 7 tracks into 19 sectors, the next 6 tracks
into 18 sectors, and the inner 5 tracks into 17 sectors.
  Speed In the disk drive, the disk spins quickly. The exact
speed depends on what size disk the drive uses.
Low-density 5¼-inch disks revolve 5 times per second. That makes
300 revolutions per minute, 300 rpm.
8-inch disks and high-density 5¼-inch disks revolve faster: 6
times per second (360 rpm).
3½-inch disks revolve even faster: between 6½ and 10 times per
second.

                  Buying disks
  When you buy a floppy disk, make sure its size matches the size
of the drive. For example, a 3½-inch disk will not work in a
5¼-inch drive.
                                                     If you buy a
blank 5¼-inch floppy disk, you can stick it into any normal
5¼-inch drive, regardless of who manufactured the drive and who
manufactured the computer. But after you've put information onto
the disk, that information is understandable only to your kind of
computer. For example, an Apple 2e cannot understand what an IBM
PC writes.
                                                     When you go
into a computer store to buy a disk that contains software, tell
the salesperson which kind of computer you have, so that the
salesperson can give you a disk containing information
understandable to your computer.
                                                     If your
drive is single-density or double-density, it cannot handle
high-density disks at all.
                                                     If your
drive is 5¼-inch and high-density, it can read single-density and
double-density disks, but it might have trouble writing new
information onto them. So when buying blank disks for your
5¼-inch high-density drive to write on, avoid buying
single-density or double-density disks.
                                                     The three
crummy kinds of 5¼-inch floppy disks (single-sided
single-density, single-sided double-density, and double-sided
double-density) are all manufactured by the same process as each
other. The only difference is the manufacturer's ``guarantee'': a
double-sided double-density disk is ``guaranteed'' to work on
both sides and hold lots of data; a single-sided or
single-density disk is not. Even if you buy a disk that has a
poor guarantee (just ``single-sided single-density''), it
typically works fine even if you use both sides and store lots of
data. The only difference is that the manufacturer hasn't
bothered testing the second side and hasn't bothered testing
double-density data. During the 1970's and 1980's, single-sided
single-density disks were significantly cheaper than double-sided
double-density, but now the prices are about the same.
                                                     Formatting
the disk Before you can use a blank floppy disk, its surface must
be formatted (divided into tracks and sectors). Buy a disk that's
been formatted already, or buy an unformatted disk and format it
by typing a command on your computer's keyboard.
                                                     After the
disk's been formatted, you can store whatever information you
wish onto the disk. Do not tell the drive to format that disk
again. If you accidentally make the drive format the same disk
again, the drive will create new tracks and sectors on the disk,
and erase the old tracks and sectors, and therefore erase all
your old data!
  Remember:
If a disk is blank, format it before you use it.
If a disk already contains info, do not format it; it's been
formatted already.
  Name brands The most famous manufacturers of floppy disks are
Verbatim and Maxell. But instead of buying those brands, buy
generic floppy disks instead. The generics cost less and
typically work just as well.
  Discount dealers To get the lowest prices on generic floppy
disks, contact MEI Micro Center (1100 Steelwood Rd., Columbus OH
43212, 800-634-3478) or Diskettes Unlimited (6206 Long Dr.,
Houston TX 77087, 800-DOG-DISK).
  For example, here are the prices from MEI Micro Center for
double-sided disks:
Kind of disk      100 disks1000 disks
5¼-inch double-density, unformatted$19+$1.20$160+$12
5¼-inch high-density,   unformatted$26+$1.20 $21+$12

3½-inch double-density, unformatted$35+$2.40$290+$24
3½-inch high-density,   formatted$41+$2.40$350+$24
Add up the prices of what you want, then add the handling charge
($3.25). For example, for 100 of the best disks (3½-inch
high-density, formatted), MEI charges you $41 (for the disks) +
$2.40 (shipping) + $3.25 (handling), which is $46.65. That's
about 47¢ per disk. For 1000 of the best disks, MEI charges you
$350 + $24 + $3.25, which is $377.25, which comes to about 38¢
per disk. Diskettes Unlimited charges even less but might give
you slightly lower quality; for details, phone them.
  What's a disk worth? Although you can buy a blank floppy disk
for under 50¢, a disk containing information costs much more. The
price depends on how valuable the information is. A disk that
explains to the computer how to play a game costs about $40. A
disk teaching the computer how to handle a general business task
(such as accounting, filing, or correspondence) usually costs
about $200.
  A disk containing intimate, personal data about your business's
customers, suppliers, employees, and methods is worth even more
___ perhaps thousands of dollars! To compute how much it's worth
to you, imagine that you've lost it, or that it fell into the
wrong hands!

         Protect your disks
  Most parts of a computer system are sturdy: even if you bang on
the keyboard and rap your fist against the screen, you probably
won't do any harm. Only one part of a computer system is
delicate: that part is the disk. Unfortunately, the magnetic
signals on your disk are easy to destroy.
  One way to accidentally destroy them is to put your disk near a
magnet; so keep your disks away from magnets! For example, keep
your disk away from paper clips that have been in a magnetized
paper-clip holder. Keep your disk away from speakers (such as the
speakers in your stereo, TV, and phone), because all speakers
contain magnets. Keep your disk away from electric motors,
because motors generate an electromagnetic field. So to be safe,
keep your disk at least six inches away from paper clips,
stereos, TV's, telephones, and motors.
  Keep your disk away from heat, because heat destroys the disk's
magnetism and ``melts'' your data. So don't leave your disk in
the hot sun; don't leave it on a sunny windowsill; don't leave it
in the back of your car on a hot day. If your disk drive or
computer feels hot, quickly lower the temperature, by getting an
air conditioner or at least a fan.
  3½-inch floppy disks come in strong jackets, but 5¼-inch and
8-inch floppy disks come in jackets that are too weak and
thin to protect disks from pressure. Don't squeeze your disk.
Don't put it under a heavy object, such as a paperweight or a
book. If you want to write a note on the disk's jacket, don't use
a ball-point pen (which crushes the disk); use a soft felt-tip
pen instead.
                                         Keep the disk away from
dust. For example, don't smoke cigarettes near the disk, because
the smoke becomes dust that lands on the disk and wrecks the
data.
                                         Keep the disk dry. If
you must transport a disk during a rainstorm, put the disk in a
plastic bag. Never drink coffee or soda near the disk: your drink
might spill.
                                         To handle the disk,
touch just the disk's jacket, not the brown disk itself. Holes in
the jacket let you see the brown disk inside; don't put your
fingers in the holes.
                                         Power surges in ancient
computers If your computer is an IBM clone or by Apple, skip
ahead to the next topic (``Write-protect notch'').
                                         If your computer is made
by Commodore or Radio Shack and is so ancient that it's not an
IBM PC clone, be careful: flipping the power switch on your
ancient computer creates an electrical surge that wrecks the
disk. On such a computer, don't flip the power switch when the
drive contains a disk. Flip the power switch just when the
drive's empty.
                                         To turn such a computer
on, make sure the drive's empty, then flip the power switch on.
After the power's come on, insert the disk.
                                         Before turning such a
computer off, remove the disk from the drive. When the drive's
empty, turn off the power.
                                         Write-protect notch When
you buy a blank 5¼-inch or 8-inch floppy disk, the disk comes in
a square black jacket. Since the jacket's square, it has four
sides; but one of the sides has a notch cut into it.
                                         You can cover the notch,
by sticking a plastic tab over it. The tab has a gummed back, so
you can stick it on the disk easily and cover the notch. You get
the tab free when you buy the disk.
                                         (For a 3½-inch disk, the
notch is different: it's a square hole near the jacket's corner
but not on the jacket's edge. To cover it, you use a black slider
instead of a tab. On old Apple Mac disks, the slider was red
instead of black.)
                                         Whenever you ask the
computer to change the info on the disk, the drive checks whether
you've covered the notch.
                                         For a 5¼-inch disk, the
normal situation is for the notch to be uncovered. For a 3½-inch
or 8-inch disk, the normal situation is for the notch to be
covered.
                                         If the situation's
normal, the computer will obey your command: it will change the
info on the disk as you wish. But if the situation's abnormal
(because the notch is covered when it should be uncovered, or is
uncovered when it should be covered), the computer will REFUSE to
change the disk's info.
                                         Suppose your disk
contains valuable info, and you're afraid some idiot will
accidentally erase or alter that info. To prevent such an
accident, make the situation abnormal (by changing whether the
notch is covered), so that the computer will refuse to change the
disk's info. It will refuse to erase the disk; it will refuse to
add new info to the disk; it will refuse to alter the disk; it
will refuse to write onto the disk. The disk is protected from
being changed; it's protected from being written on. The disk is
write-protected (or locked).
                                         Since the tab affects
whether the disk is write-protected, the tab is called a
write-protect tab, and the notch is called a write-protect notch.
  When you buy a disk that already contains info, the disk
usually comes write-protected, to protect you from accidentally
erasing the info. So if you buy a 5¼-inch floppy disk that
already contains info, it might come with a write-protect tab
already covering the notch, to write-protect the disk.
  Instead of creating a notch and then covering it with a tab,
some manufacturers save money by getting special disks that have
no notch. The computer treats a notchless disk the same way as a
disk whose notch is covered.
  Backup Even if you handle your disk very carefully, eventually
something will go wrong, and some of the info on your disk will
get wrecked accidentally.
  To prepare for that inevitable calamity, tell the computer to
copy all info from the disk onto a blank disk, so that the blank
disk becomes an exact copy of the original. Store the copy far
away from the original: store it in another room, or ___ better
yet ___ another building, or ___ better yet ___ another city. If
you're working in a country that's having a war, store the copy
in another country.
  The copy is called a backup. Use the backup disk when the
original disk gets wrecked.
  Making a backup disk is like buying an insurance policy: it
protects you against disasters.
  Every evening, make backup copies of all your disks ___ except
for disks containing the same info as the day before.
  Each week, I get phone calls from distressed business
executives whose disks got wrecked and who didn't make backups.
All I can offer them is sympathy. Their companies are ruined.
Remember: ``a backup a day keeps disaster away!''
  When you buy a floppy that already contains software, try
copying the floppy before you begin using it. If you're lucky,
the computer will make the backup copy without any hassles. If
you're unlucky, the software company has put instructions on the
floppy that make the computer refuse to copy the disk, because
the company fears that you'll illegally give copies to all your
friends for free. A floppy that the computer refuses to copy, and
which is therefore protected against illegal copying, is called
copy-protected. A floppy that you can copy is called copyable (or
unprotected).
  Short files The information on the disk is divided into files.
Each file has its own name and its own purpose. For example, one
file might be named JILL and consist of a memo that you wrote to
your friend Jill; another file might be named PAYROLL and consist
of information about your company's payroll. Each file consists
of many sectors.
  If one of the disk's sectors gets damaged, the computer might
get so confused that it handles the entire file incorrectly, and
so the entire file becomes unusable.
  To minimize such damage, avoid creating large files; create
many small files instead. For example, if you're writing a book
and want to store the book on your disk, do not make the entire
book be a single file; instead, split the book into chapters, and
make each chapter a separate file. That way, a damaged sector
will hurt at most one chapter, and can't hurt the entire book.
  Drive cleaners Don't bother trying to clean the heads of your
floppy drive. The heads don't collect much dirt anyway, since the
floppy disk's jacket has a cloth liner that traps most dirt. If
your disk ever starts to act unreliable, clean the heads if you
wish, but the culprit is more likely a misaligned head, a
brownout, overheating, defective software, or a mistyped command.

             HARD DISKS
                                         Hard disks are better
than floppy disks in three ways. . . .
Hard disks are sturdier than floppies.
Hard disks are hard and firm; they don't flop or jiggle.
They're more reliable than floppies.

Hard drives hold more information than floppy drives.
The typical floppy drive holds 360K, 720K, 1.2M, or 1.44M.
The typical hard drive holds 250M, 340M, or 420M.

Hard drives work faster than floppies.
The typical floppy disk rotates between 5 and 10 times per
second.
The typical hard disk rotates between 60 and 120 times per
second.
                                         Hard drives are more
expensive than floppy drives. The typical floppy drive costs
about $50; the typical hard drive costs about $300.
                                         Unfortunately, the
typical hard disk can't be removed from its drive: the hard disk
is non-removable, stuck inside its drive permanently. (Hard disks
that are removable are rare.)
                                         Since the typical hard
disk is stuck forever inside its drive, in one fixed place, it's
called a fixed disk.
                                         Though the typical
floppy-disk drive holds just one disk at a time, the typical
hard-disk drive holds a whole stack of disks and handles all the
stack's disks simultaneously, by using many arms and read-write
heads. For example, the typical 420M hard drive holds a
non-removable stack of disks, and the entire stack totals 420M.
Each disk in the stack is called a platter.
                                         If your hard drive is
the rare kind that holds a removable stack of disks, the stack
comes in a cartridge or pack that you can remove from the hard
drive.
                                         Back in 1977, the
typical hard disk had a 14-inch diameter and was removable. The
hard-disk drive was a big cabinet, the size of a top-loading
washing machine; it cost about $30,000 and held 100M. It required
a minicomputer or mainframe.
                                         Hard disks, drives, and
prices have all shrunk since then! Now the typical hard disk has
a diameter of just 3½ inches. The typical hard drive is just 1
inch tall, costs $279, and holds 420M. It fits in a desktop
microcomputer.
                                         Some notebook computers
use tiny hard disks whose diameter is just 2½ inches.

                                                 IBM drive letters
                                         The typical
IBM-compatible computer has both a floppy drive and a hard drive.
The floppy drive is called drive A; the hard drive is called
drive C.
                                         If the computer has two
floppy drives, the main floppy drive is called drive A, and the
other floppy drive is called drive B. If the computer has two
hard drives, the main hard drive is called drive C, and the other
hard drive is called drive D.

                                          Copy from floppy to hard & back
                                         When you buy a program,
it usually comes on a floppy disk. To use the program, put that
floppy disk into the floppy drive, then copy the program from the
floppy disk to the hard disk. (To copy the program onto an
IBM-compatible hard disk, type the word ``copy'' or ``install''
or ``setup''. To find out which of those three words to type and
when, follow the instructions in the manual that came with the
program.)
                                         Then use just the copy
on the hard disk (which is sturdier, holds more info, and works
faster than the floppy disk).
                                         Like floppy disks, hard
disks are coated with magnetized iron. Floppy disks and hard
disks are both called magnetic
disks. Like floppy disks, hard disks are in constant danger of
losing their magnetic signals ___ and your data!
  Protect yourself! Every day, take any new info that's on your
hard disk and copy it onto a pile of floppy disks, so that those
floppy disks contain a backup copy of what was new on your hard
disk.
  To avoid giant disasters, avoid creating giant files. If you're
writing a book and want to store it on your hard disk, split the
book into chapters, and make each chapter a separate file, so
that if you accidentally say ``delete'' you'll lose just one
chapter instead of your entire masterpiece.

         How the head works
  In a floppy drive, the read-write head (the ``needle'') touches
the spinning floppy disk. But in a hard drive, the read-write
head does not touch the spinning hard disk; instead, it hovers
over the disk.
  The distance from the read-write head to the hard disk is a
tiny fraction of an inch, and small enough so that the read-write
head can detect the disk's magnetism and alter it.
  Since the head doesn't actually touch the disk, there isn't any
friction, and so the head and the disk don't suffer from any
wear-and-tear. That's why a hard-disk system lasts longer than a
floppy-disk system and is more reliable.
  Winchester drives In all modern hard drives, the head acts as a
miniature airplane: it flies above the disk. It flies at a very
low altitude: a tiny fraction of an inch. The only thing keeping
the head off the rotating disk is a tiny cushion of air ___ a
breeze caused by the disk's motion.
  When you unplug the drive, the disk stops rotating, so the
breeze stops, and the head comes to rest on a landing strip,
which is like a miniature airport.
  Such a drive is called a flying-head drive. It's also called a
Winchester drive, because ``Winchester'' was IBM's secret
code-name for that technology when IBM was inventing it.
  The head flies at an altitude that's extremely low ___ about a
ten-thousandth of an inch! That's even smaller than the width of
a particle of dust or cigarette smoke! So if any dust or smoke
lands onto the disk, the head will smash against it, and you'll
have a major disaster.
  To prevent such a disaster, the entire Winchester drive is
sealed air-tight, to prevent any dust or smoke from entering the
drive and getting onto the disk. Since the drive is sealed, you
can't remove the disks (unless you buy an extremely expensive
Winchester drive that has a flexible seal).

                Speed
  Here's how the computer retrieves data from the drive.
  First, the drive's head moves to the correct track. The time
that the head spends moving is called the seek time. Since that
time depends on how far the head is from the correct track, it
depends on where the correct track is and where the head is
moving from.
  According to calculus, on the average the head must move across
a third of the tracks to reach the correct track. The time to
traverse a third of the tracks is therefore called the average
seek time.
  A millisecond (ms) is a thousandth of a second. In a typical
hard drive, the average seek time is 12 milliseconds. (In faster
hard drives, the average seek time is 9 milliseconds; in slower
hard drives, the average seek time is 28 milliseconds.)
  After the head reaches the correct track, it must wait for the
drive to rotate, until the correct sector reaches the head.
That rotation time is called the latency. On the average, the
head must wait for half a revolution; so the average latency time
is a half-revolution. The typical hard drive rotates 60 times per
second, so a half-revolution takes half of a sixtieth of a
second, so it's a 120th of a second, so it's about .008 seconds,
which is 8 milliseconds.
                                         If you add the average
seek time to the average latency time, you get the total average
access time. So for a typical hard drive, the average access time
= 12 milliseconds seek + 8 milliseconds latency = 20
milliseconds.
                                         During the last few
years, hard drive manufacturers have become dishonest: they say
the ``average access time'' is 12 milliseconds, when they should
actually say the ``average seek time'' is 12 milliseconds.
                                         After the head finally
reaches the correct sector, you must wait for the head to read
the data. If the data consumes several sectors, you must wait for
the head to read all those sectors.

                                                   Manufacturers
                                         Most hard drives for
microcomputers are manufactured by four companies: Seagate
Technology (ST), Conner Peripherals, Quantum, and Western
Digital.
                                         Seagate was the first of
those companies to make hard drives for microcomputers, and it
set the standard that the other companies had to follow. New
Seagate drives work fine, though Seagate's older models were
often noisy and unreliable.
                                         Conner was the first
company to invent hard drives tiny enough to fit in a laptop or
notebook computer. Seagate ignored the laptop/notebook
marketplace too long, and Conner's popularity zoomed up rapidly.
Conner became the fastest-growing company in the history of
American industry, though Conner's popular finally started to
level off.
                                         Quantex became famous by
manufacturing the hard drives that Apple buys to put in Mac
computers. Quantex also builds drives for IBM PC clines. Quantex
drives are excellent.
                                         Western Digital has
invented hard drives that cost less. They're popular in cheap
clones and discount computer stores.
                                         When buying a hard
drive, you might also need to buy a hard-drive controller.

                                                 How many sectors?
                                         Back in the 1980's, the
typical hard-drive controller for IBM-compatible computers put 17
sectors on each track. That scheme was called the Seagate
Technology 506 with Modified Frequency Modulation (ST506 MFM).
                                         An improved scheme,
which squeezed 26 sectors onto each track, was called the ST506
with Run Length Limited (ST506 RLL). A further improvement, which
squeezed 34 sectors onto each track, was called the Enhanced
Small Device Interface (ESDI).
                                         Squeezing extra sectors
onto each track increases the drive's capacity (total number of
megabytes) and also the transfer rate (the number of sectors that
the head reads per rotation or per second).
                                         All those schemes ___
MFM, RLL, and ESDI ___ have become obsolete.
                                         Now the most popular
scheme is called Integrated Drive Electronics (IDE). Like ESDI,
it squeezes 34 sectors onto each track; but it uses special
tricks to transfer data faster. It's used on all popular
IBM-compatible hard drives under 600M. It's starting to become
popular in the 600M-1100M range also.
  An even faster scheme is the Small Computer System Interface
(or SCSI, which is pronounced ``scuzzy''). It's used on all Mac
hard drives. It's also used on most IBM-compatible hard drives
over 600M.

               Discounts on drives
  You can buy a hard drive cheaply from a discount dealer called
Hard Drives International (HDI). It's a division of Insight,
which is at 1912 W. Fourth St., Tempe AZ 85281; phone
800-998-8028 or 602-902-1176. You get a 30-day money-back
guarantee and toll-free technical help. You can call sales and
technical support anytime (24 hours per day, 365 days per year).
  A smaller dealer, Mega Haus, usually charges even less. But
Mega Haus doesn't have a true 30-day money-back guarantee (you
must pay a 15% restocking fee), doesn't stay open late at night,
and doesn't ship outside the USA. Mega Haus is in Houston at
800-786-1153 (IBM), 800-786-1173 (Mac), or 713-333-1910.
  IBM-compatible drives For an IBM-compatible hard drive,
discount dealers such as HDI charge about 50 cents per megabyte,
plus $75. So here's how to estimate a hard drive's price.
Take the number of megabytes, divide by 2, then add 75. That's
the price in dollars.
  For example, here's how to find the price of a 540-megabyte
hard drive.
Take 540, divide by 2 (giving 270), then add 75. That makes 345,
so the drive's price is $345.
  That formula (price = megabytes/2 + 75) is just an
approximation. The exact price depends on the dealer, the drive's
speed, and which models are on sale this month.
  Besides buying the hard drive, you must also buy a card to put
in the computer's slot. For example, here are HDI's prices:
CapacitySeek timeTypeBrand  Model numberHDI's price
 340M   12 ms   IDE Western DigitalWD2    340 A $229 +  $20 card
 420M   12 ms   IDE Western DigitalWD2    420 A $279 +  $20 card
 540M   12 ms   IDE Western DigitalWD2    540 A $349 +  $20 card
 730M   12 ms   IDE Western DigitalWD2    700 A $469 +  $20 card
1080M   10 ms   IDE Western DigitalWD3   1000 A $679 +  $20 card
1800M   10 ms   SCSIQuantum QUPD  1800 S $939 + $100 card
4294M    8 ms   SCSISeagate ST1   5150 N$2399 + $100 card
9100M   10 ms   SCSIMicropolisMC19    91$4059 + $100 card
  For the IDE drives, the extra $20 is for a paddle board that
fits in an IBM PC AT slot. For the SCSI drives, the extra $100 is
for a hard-drive controller card that fits in an IBM PC AT slot.
  At the end of the model number, an A means AT-bus IDE; anything
else (S, N, or blank) means SCSI.
  For Western Digital and Quantum drives, the main part of the
model number is the capacity. For Micropolis drives, the main
part of the model number is the capacity divided by 100. For
Seagate drives, the main part of the model number is the
unformatted capacity, which is about 10% bigger than the usable
(formatted) capacity.
  Each price in that chart is close to the price predicted by the
formula ``price=megabytes/2+75''. How close? Within 13%.
  Each price in that chart went into effect in July 1994. By the
time you read this book, prices might be even lower! Prices
continually drop.
  Mac drives The price of a Mac hard drive depends on whether the
drive is internal (fits inside the Mac) or external (comes in a
separate box that you put next to the Mac). Internal drives are
cheaper; but if your Mac is small or filled up, you must buy an
external drive instead.
  For Mac drives, Mega Haus charges much less than HDI. Here are
the Mega Haus prices:
CapacitySeek timeRotationCacheBrandInternalExternal
 170M   17 ms   3600 rpm  32KQuantum $179  $239
 270M   12 ms   4500 rpm 128KQuantum $238  $298
 340M   12 ms   4500 rpm 128KQuantum $275  $335
 540M   12 ms   4500 rpm 128KQuantum $389  $449
1080M   10 ms   5400 rpm 512KQuantum $725  $785
1750M   10 ms   5400 rpm 256KMicropolis $999$1059
2148M    9 ms   5400 rpm 512KSeagate$1429 $1529
3020M   11 ms   5400 rpm 256KMicropolis$2139$2169

                                                     What size
hard drive to buy Back in the 1980's, a 40-megabyte drive was
considered ``big''. In the 1990's, a 40-megabyte drive is
considered ``too small''. Here's why.
                                                     MS-DOS 6.2
consumes over 5 megabytes. Windows 3.1 consumes over 10
megabytes. So far, we've consumed over 15 megabytes!
                                                     The typical
Windows word-processing program (such as such Word Perfect 6 or
Microsoft Word 6) consumes about 25 megabytes. So altogether, for
DOS plus Windows plus a word-processing program, we've consumed
over 40 megabytes already!
                                                     You'll need
additional megabytes for additional business programs (about 10
megabytes per program), plus additional megabytes to hold what
you type.
                                                     After buying
the computer, you'll probably spend the next several years
accumulating many programs (a few each year). After a year or
two, you'll accumulate over 170 megabytes, and you'll wish you'd
bought a bigger drive instead.
                                                     Buy at least
a 340-megabyte drive. A 340-megabyte drive costs just slightly
more than a 170M drive and will last you for many years. You're
buying peace of mind!
                                                     It's much
cheaper to buy a 340-megabyte drive now than to buy a
170-megabyte drive now and another 170-megabyte drives later.
Another reason for buying a 420-megabyte drive is that it will
act faster than a 170-megabyte drive.
                                                     For example,
suppose you want to store 170 megabytes of information, and
you're debating whether to buy a 170-megabyte drive or a
340-megabyte drive. Suppose each drive is advertised as having a
12-millisecond seek time. The 340-megabyte drive will
nevertheless act faster. Here's why. . . .
                                                     Suppose you
buy the 340-megabyte drive and use just the first 170 megabytes
of it. Since you're using just the first half of the drive, the
head needs to move just half as far as usual; so over the
170-megabyte part that you're using, the effective average seek
time is just half as much as usual: it's 6 milliseconds!

                     CD-ROMS
  Instead of buying a program on a floppy disk, you can buy a
program on the same kind of compact disk (CD) that holds music.
Since the CD cannot be erased, it's called a CD read-only memory
(CD-ROM).
  To make your computer read the CD-ROM disk, put the disk into a
CD-ROM drive, which is a souped-up version of the kind of CD
player that plays music.
  Like an ordinary CD player, a CD-ROM drive uses just optics. No
magnetism is involved. The drive just shines a laser beam at the
shiny disk and notices, from the reflection, which indentations
(pits) are on the disk; the pattern of pits is a code that
represents the data. So a CD-ROM drive is an example of an
optical disk drive.
  To put the disk into the drive, press a button on the drive.
That makes the drive stick its tongue out at you! The tongue is
called a tray. Put the disk onto the tray, so that the disk's
label is face-up. (If the drive is old-fashioned, you must put
the disk into a caddy first; but the most modern drives are
caddyless.) Then push the tray back into the drive. Finally, use
the keyboard or mouse to give a command that makes the computer
taste what you've put on its tongue.

                IBM drive letters
  In the most modern kind of IBM-compatible computer, drive A is
a 3½-inch floppy drive (1.44M), drive B is a 5¼-inch floppy drive
(1.2M), drive C is a hard drive (holding about 420M), and drive D
is a CD-ROM drive.
  But if your computer has two hard drives, here's what happens:
the first hard drive is C, the second hard drive is D, and the
CD-ROM drive is E.

                      Size
  CD-ROM disks come in two sizes:
The standard size has a diameter of 12 centimeters (which is
about 5 inches) and holds 540 megabytes.
The miniature size has a diameter of 8 centimeters (which is
about 3 inches) and holds 180 megabytes.
  Your CD-ROM drive can handle both sizes of CD-ROM disks. Each
CD-ROM disk is single-sided: all the data is on the disk's bottom
side ___ the side that doesn't have a label.
  Yes, a standard-size CD-ROM disk holds 540 megabytes, which is
a lot!
It's more than the typical hard drive.
It's 375 times as much as a high-density 1.44M floppy.
It's 1500 times as much as a 360K floppy.
  Because a CD-ROM disk holds so much, a single CD-ROM can hold a
whole library (including encyclopedias, dictionaries, other
reference materials, famous novels, programs, artwork, music, and
videos). It's the ideal way to distribute massive quantities of
information! Moreover, a CD-ROM disk costs just $1.50 to
manufacture (once you've bought the appropriate CD-ROM-making
equipment, which costs several thousand dollars).

                Speed
  When buying a CD-ROM drive, the most important factor to
consider is the drive's speed.
  Transfer rate The speed at which the drive spins is called the
transfer rate. The higher, the better!
On old drives, the transfer rate was 150 kilobytes per second.

Most new drives spin twice as fast: 300K per second.
That's called double spin or double speed or dual speed or 2X.

Some drives spin even faster: 450K per second.
That's called triple spin or triple speed or 3X.

The fastest drives spin at 600K per second.
That's called quad spin or quad speed or 4X.
  Seek time The average time it takes for the head to move to the
correct track is called the average seek time.
  The lower the average seek time, the better!
Under 200 milliseconds is great.
  200-300 milliseconds is good.
  300-400 milliseconds is typical.
  400-500 milliseconds is poor.
 Over 500 milliseconds is terrible.

           Buying a drive
  If you're looking for an IBM-compatible drive that's reasonably
fast at a low price, get the Sony CDU-33A or the Mitsumi FX001-D.
Each of those drives is double-spin.
  The Sony's average seek time is 320 milliseconds. The Mitsumi's
is better, 250 milliseconds.
  You can buy that Sony drive for $169, the Mitsumi for $139,
from discount dealers such as USA Flex in Illinois (phone
800-723-2261 or 708-582-6202).
  When buying those drives, make sure you get the right model!
For the Sony, make sure you get the CDU-33A, not the CDU-31A
(which is slower, at 490 milliseconds). For the Mitsumi, make
sure you get the FX001-D, not the FX001 (which is single-spin
instead of double-spin).
  Faster drives For even faster drives ___ at higher prices ___
consider the NEC 3Xi (triple-spin, 195 milliseconds, $424), the
Plextor PX-34CH (quadruple-spin, 220 milliseconds, $475), or the
NEC 4X Pro (quadruple-spin, 180 milliseconds, $929). Those NEC
prices are from USA Flex; the Plextor's price is from Insight, a
discount dealer in Arizona (phone [phone] or 602-902-1176).
  But if you buy one of those faster drives, you'll be
disappointed: those drives make the typical CD-ROM program run
just slightly faster. That's because most CD-ROM programs are
still designed under the assumption you're using a CD-ROM drive
that's slow. So save your money: buy just a cheap drive (the Sony
CDU-33A or Mitsumi FX001-D).
  External drives Those prices are for internal drives, which fit
inside the computer's system unit. If your system unit is filled
up and doesn't have any room left to insert an internal drive,
you must buy an external drive instead, which sits outside the
system unit and costs about $100 more.
  Multimedia kits If you buy a CD-ROM drive, you'll also want a
sound card, a pair of stereo speakers, and a few sample CD-ROM
disks (so you can admire all that equipment you bought). That
combo ___ a CD-ROM drive, sound card, pair of speakers, and
sample CD-ROM disks ___ is called a multimedia kit.
                                         For example, USA Flex
sells the Flex CD-Pro Classic, which is a kit including the Sony
CDU-33A drive and the Sound Blaster 16 sound card. That kit costs
$249; add $15 for a pair of speakers (batteries not included!),
plus $49 for a collection of 3 sample CD-ROM disks. To pay $50
less, get the Flex CD-Pro Value instead, which is similar but
includes imitations of the Sony and Sound Blaster.
                                         Insight sells the Reveal
Multimedia FX. It's a multimedia kit that includes the Reveal
CD-ROM drive (double-spin, 320 milliseconds, internal), sound
card, pair of speakers, headset (including a microphone), and
sample disks. . . .
The model 01, at $299, includes 5 sample disks.

The model 02, at $349, includes 16 sample disks and a second
microphone (which stands on your desk).

The model 04, at $479, includes 23 sample disks and a second
microphone.

The model 08, at $599, includes 35 sample disks, second
microphone, higher-quality speakers, higher-quality sound card,
and higher-quality headset.
                                         When you buy a new
computer, you can ask the salesperson to include a multimedia
kit. The computer, together with the multimedia kit, form a combo
that's called a multimedia computer system. For example, Quantex
sells multimedia systems based on the Sony CDU-33A drive; VTech
sells multimedia computer systems based on the Mitsumi FX001-D
drive.

                                                       Dirt
                                         A CD-ROM disk's main
enemy is dirt.
                                         When you buy a CD-ROM
disk, it comes in a clear square box, called the jewel box. To
use the CD-ROM disk, remove it from the jewel box and put the
disk into the drive. When you finish using the disk, put it back
into the jewel box, which keeps the dust off the disk.
                                         When putting the CD-ROM
disk into or out of a drive, don't put your fingers on the disk's
surface: instead, hold the disk by its edge, so your greasy
fingerprints don't get on the disk's surface.
                                         Once a month, gently
wipe any dust off the CD-ROM disk's bottom surface (where the
data is). While wiping, be gentle and don't get your greasy
fingerprints on the disk. Start in the middle and wipe toward the
outer edge.
                                         For example, my
assistant and I were getting lots of error messages when using a
sample CD-ROM disk we bought from Microsoft. I was going to phone
Microsoft to complain, but my assistant asked, ``What about
dust?'' I flipped the CD-ROM disk over and sure enough, a big
ball of dust was on the disk's bottom side, where the data is
recorded. I wiped it off. That CD-ROM disk has worked perfectly
ever since.
                                         I was so embarrassed! If
my assistant hadn't reminded me to wipe the dust off, I'd have
wasted hours of Microsoft's time hunting uselessly for a
high-tech reason my CD-ROM disk wasn't working.

                                                   Other dangers
                                         Don't put any fluids on
the disk. The fluids that clean phonograph records will wreck
CD-ROM disks.
                                         If you want to write on
the disk, use a felt-tipped pen (not a ballpoint or pencil).
Don't stick any labels on the disk.