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Byte
In most
computer systems, a byte is a unit
of data that is eight binary
digits long. A byte is the unit most
computers use to represent a
character such as a letter, number,
or typographic symbol (for example,
"g", "5", or "?"). A byte can also
hold a string of bits that need to
be used in some larger unit for
application purposes (for example,
the stream of bits that constitute a
visual image for a program that
displays images or the string of
bits that constitutes the machine
code of a computer program).
In some
computer systems, four bytes
constitute a word, a
unit that a computer processor can
be designed to handle efficiently as
it reads and processes each
instruction. Some computer
processors can handle two-byte or
single-byte instructions.
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A byte is abbreviated with a "B". (A
bit is abbreviated with a small
"b".) Computer storage is usually
measured in byte multiples. For
example, an 820 MB hard drive holds
a nominal 820 million bytes - or
megabytes - of data. Byte multiples
are based on powers of 2 and
commonly expressed as a "rounded
off" decimal number. For example,
one megabyte ("one million bytes")
is actually 1,048,576 (decimal)
bytes. (Confusingly, however, some
hard disk manufacturers and
dictionary sources state that bytes
for computer storage should be
calculated as powers of 10 so that a
megabyte really would be one million
decimal bytes.)
Some language scripts require two
bytes to represent a character.
These are called double-byte
character sets (DBCS).
According to Fred Brooks, an early
hardware architect for IBM, project
manager for the OS/360 operating
system, and author of The
Mythical Man-Month, Dr. Werner
Buchholz originated the term byte
in 1956 when working on IBM's
STRETCH computer.
Diskette (3.5 Inch Floppy Disk)
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A diskette is a
random access, removable data
storage medium that can be used with
personal computers. The term usually
refers to the magnetic medium housed
in a rigid plastic cartridge
measuring 3.5 inches square and
about 2 millimetres thick. Also
called a "3.5-inch diskette," it can
store up to 1.44 megabytes (MB) of
data. Although most personal
computers today come with a 3.5-inch
diskette drive pre-installed, some
network computer
now omit them.
Some older computers provide drives
for magnetic diskettes that are 5.25
inches square, about 1 millimetre
thick, and capable of holding 1.2
megabytes of data. These were
sometimes called "floppy disks" or
"floppies" because their housings
are flexible. In recent years,
5.25-inch diskettes have been
largely replaced by 3.5-inch
diskettes, which are physically more
rugged. Many people also call the
newer hard-cased diskette a
"floppy."
Magnetic diskettes are convenient
for storing individual files and
small programs. However, the
magneto-optical is more popular for
mass storage, backup, and archiving.
An MO diskette is only a little
larger, physically, than a
conventional 3.5-inch magnetic
diskette. But because of the
sophisticated read/write technology,
the MO diskette can store many times
more data.
1)
In computers, dots per inch (dpi) is
a measure of the sharpness (that is,
the density of illuminated points)
on a display
screen. The
dot pitch
determines the absolute limit of the
possible dots per inch. However, the
displayed
resolution
of
pixels
(picture elements) that is set up
for the display is usually not as
fine as the dot pitch. The dots per
inch for a given picture resolution
will differ based on the overall
screen size since the same number of
pixels are being spread out over a
different space. Some users prefer
the term "pixels per inch (ppi)"
as a measure of display image
sharpness, reserving dpi for use
with the print medium.
2) In printing, dots per inch (dpi)
is the usual measure of printed
image quality on the paper. The
average personal computer printer
today provides 300 dpi or 600 dpi.
Choosing the higher print quality
usually reduces the speed of
printing each page.
FireWire
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FireWire is
Apple Computer's version of a
standard,
IEEE 1394,
High Performance Serial Bus, for
connecting devices to your personal
computer. FireWire provides a single
plug-and-socket connection on which
up to 63 devices can be attached
with data transfer speeds up to 400
Mbps (megabits
per second). The standard describes
a
serial
bus or
pathway between one or more
peripheral devices and your
computer's
microprocessor.
Many peripheral devices now come
equipped to meet IEEE 1394. FireWire
and other IEEE 1394 implementations
provide:
-
A simple common plug-in serial
connector on the back of your
computer and on many different
types of peripheral devices
-
A thin serial cable rather than
the thicker parallel cable you now
use to your printer, for example
-
A very high-speed rate of data
transfer that will accommodate
multimedia applications (100 and
200 megabits per second today;
with much higher rates later)
-
Hot-plug and
plug and play
capability without disrupting your
computer
-
The ability to chain devices
together in a number of different
ways without terminators or
complicated set-up requirements
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In time, IEEE
1394 implementations are expected to
replace and consolidate today's
serial and parallel interfaces,
including Centronics
parallel,
RS-232C,
and Small Computer System Interface
(SCSI).
The first products to be introduced
with FireWire include
digital cameras,
digital video disks (DVDs),
digital videotapes, digital
camcorders, and music systems.
Because IEEE 1394 is a
peer-to-peer
interface, one camcorder can dub to
another without being plugged into a
computer. With a computer equipped
with the socket and bus capability,
any device (for example, a video
camera) can be plugged in while the
computer is running.
Briefly How It Works
There are two
levels of interface in IEEE 1394,
one for the
backplane
bus within the computer and another
for the point-to-point interface
between device and computer on the
serial cable. A simple bridge
connects the two environments. The
backplane bus supports 12.5, 25, or
50 megabits per second data
transfer. The cable interface
supports 100, 200, or 400 megabits
per second. Each of these interfaces
can handle any of the possible data
rates and change from one to another
as needed.
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The serial bus functions as though
devices were in slots within the
computer sharing a common memory
space. A 64-bit device address
allows a great deal of flexibility
in configuring devices in chains and
trees from a single socket.
IEEE 1394
provides two types of data transfer:
asynchronous and
isochronous.
Asynchronous is for traditional
load-and-store applications where
data transfer can be initiated and
an application interrupted as a
given length of data arrives in a
buffer.
Isochronous data transfer ensures
that data flows at a pre-set rate so
that an application can handle it in
a timed way. For multimedia
applications, this kind of data
transfer reduces the need for
buffering and helps ensure a
continuous presentation for the
viewer.
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The 1394 standard requires that a
device be within 4.5 meters of the
bus socket. Up to 16 devices can be
connected in a single chain, each
with the 4.5 meter maximum (before
signal attenuation begins to occur)
so theoretically you could have a
device as far away as 72 meters from
the computer.
Another new
approach to connecting devices, the
Universal Serial Bus (USB),
provides the same "hot plug"
capability as the 1394 standard.
It's a less expensive technology but
data transfer is limited to 12 Mbps
(million bits per second). Small
Computer System Interface offers a
high data transfer rate (up to 40
megabytes per second) but requires
address preassignment and a device
terminator on the last device in a
chain. FireWire can work with the
latest internal computer
bus
standard, Peripheral Component
Interconnect (PCI),
but higher data transfer rates may
require special design
considerations to minimize undesired
buffering for transfer rate
mismatches.
Gigabyte
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A gigabyte (pronounced GIG-a-bite
with hard G's) is a measure of
computer data storage capacity and
is "roughly" a billion bytes. A
gigabyte is two to the 30th power,
or 1,073,741,824 in decimal
notation.
Hard Disk Drive
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In a personal
computer, a hard disk drive (HDD) is
the mechanism that controls the
positioning, reading, and writing of
the
hard disk,
which furnishes the largest amount
of data
storage for
the PC. Although the hard disk drive
(often shortened to "hard drive")
and the hard disk are not the same
thing, they are packaged as a unit
and so either term is sometimes used
to refer to the whole
As a measure of
computer
memory or
storage, a kilobyte (KB or Kbyte*)
is approximately a thousand
bytes
(actually, 2 to the 10th power, or
decimal 1,024 bytes).
Note:
In international English outside the
U.S., the equivalent usage is
sometimes "kbyte".
Megabyte
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1) As a measure
of computer processor storage and
real and virtual memory, a megabyte
(abbreviated MB) is 2 to the 20th
power
byte, or
1,048,576 bytes in decimal notation.
2) According to the IBM
Dictionary of Computing, when
used to describe disk storage
capacity and transmission rates, a
megabyte is 1,000,000 bytes in
decimal notation.
According to the Microsoft Press
Computer Dictionary, a megabyte
means either 1,000,000 bytes or
1,048,576 bytes.
According to Eric S. Raymond in
The New Hacker's Dictionary, a
megabyte is always 1,048,576 bytes
on the argument that bytes should
naturally be computed in powers of
two.
Iomega
Corporation uses the decimal
megabyte in calling the
Zip drive
disk a "100MB disk" when it actually
holds 100,431,872 bytes. If Iomega
used the powers-of-two megabyte, the
disk could be said to hold only 95.8
megabytes (if you divide 100,431,872
by 1,048,576
Pages Per Minute
(PPM)
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Usually used to
measure the speed of laser (and
other) printers. Note that ppm
typically means text and often is
used to mean the speed the printer
outputs pages once the page is
loaded into the printer's memory.
Graphics on a page will almost
always greatly slow a printer well
below its advertised ppm rating.
Parallel Ports
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Parallel ports
were originally developed by
IBM,
as a way to connect a printer to
your PC. When IBM was in the process
of designing the PC, the company
wanted the computer to work with
printers offered by Centronics,
a top printer manufacturer at the
time. IBM decided not to use the
same port interface on the computer
that Centronics used on the printer.
Instead, IBM engineers coupled a
25-pin connector, DB-25, with
a 36-pin Centronics connector to
create a special cable to connect
the printer to the computer. Other
printer manufacturers ended up
adopting the Centronics interface,
making this strange hybrid cable an
unlikely de facto standard.
When a PC sends
data to a printer or other device
using a parallel port, it sends
eight
bits
of data (one byte) at a time. These
eight bits are transmitted
parallel to (beside) each other,
as opposed to the same eight bits
being transmitted serially
(all in a single row) through a
serial port. The standard parallel
port is capable of sending 50 to 100
kilobytes of data per second.
A computer can
send data faster than a printer can
accept and process the data. A print
spooler or print buffer acts like a
dam, holding the data and then
releasing it at a speed the printer
can handle.
Processor
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A processor is
the logic circuitry that responds to
and processes the basic
instruction
that drive a computer.
The term
processor has generally replaced the
term central processing unit (CPU).
The processor in a personal computer
or that is embedded in small devices
is often called a
microprocessor
RAM
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RAM (random
access memory) is the place in a
computer where the
operating system,
application programs, and data in
current use are kept so that they
can be quickly reached by the
computer's
processor.
RAM is much faster to read from and
write to than the other kinds of
storage in a computer, the hard
disk, floppy disk, and CD-ROM.
However, the data in RAM stays there
only as long as your computer is
running. When you turn the computer
off, RAM loses its data. When you
turn your computer on again, your
operating system and other files are
once again loaded into RAM, usually
from your
hard disk.
RAM can be compared to a person's
short-term memory and the hard disk
to the long-term memory. The
short-term memory focuses on work at
hand, but can only keep so many
facts in view at one time. If
short-term memory fills up, your
brain sometimes is able to refresh
it from facts stored in long-term
memory. A computer also works this
way. If RAM fills up, the processor
needs to continually go to the hard
disk to overlay old data in RAM with
new, slowing down the computer's
operation. Unlike the hard disk
which can become completely full of
data so that it won't accept any
more, RAM never runs out of memory.
It keeps operating, but much more
slowly than you may want it to.
RAM is small,
both in physical size (it's stored
in
microchips)
and in the amount of data it can
hold. It's much smaller than your
hard disk. A typical computer may
come with 32 million bytes of RAM
and a hard disk that can hold 4
billion bytes. RAM comes in the form
of "discrete" (meaning separate)
microchips
and also in the form of
modules
that plug into holes in the
computer's
motherboard.
These holes connect through a
bus or set
of electrical paths to the
processor.
The hard drive, on the other hand,
stores data on a magnetized surface
that looks like a phonograph record.
Today's
personal computers come with 16 or
more
megabytes
of RAM, usually increasing in
multiples of 8 megabytes. Users of
graphic applications usually need 32
or 64 megabytes of memory. Most
personal computers are designed to
allow you to add additional RAM
modules up to a certain limit (for
example, up to 64 or 128 megabytes).
Having more RAM in your computer
reduces the number of times that the
computer processor has to read data
in from your
hard disk,
an operation that takes much longer
than reading data from RAM. (RAM
access time is in nanoseconds; hard
disk access time is in
milliseconds.)
RAM is called "random access"
because any storage location can be
accessed directly. Originally, the
term distinguished regular core
memory from offline memory, usually
on magnetic tape in which an item of
data could only be accessed by
starting from the beginning of the
tape and finding an address
sequentially. Perhaps it should have
been called "nonsequential memory"
because RAM access is hardly random.
RAM is organized and controlled in a
way that enables data to be stored
and retrieved directly to specific
locations. A term IBM has preferred
is direct access storage or
memory. Note that other forms of
storage such as the hard disk and
CD-ROM are also accessed directly
(or "randomly") but the term
random access is not applied to
these forms of storage.
In addition to
disk, floppy disk, and CD-ROM
storage, another important form of
storage is read-only memory (ROM),
a more expensive kind of memory that
retains data even when the computer
is turned off. Every computer comes
with a small amount of ROM that
holds just enough programming so
that the operating system can be
loaded into RAM each time the
computer is turned on.
ROM is
"built-in" computer memory
containing data that normally can
only be read, not written to. ROM
contains the programming that allows
your computer to be "booted up" or
regenerated each time you turn it
on. Unlike a computer's random
access memory (RAM),
the data in ROM is not lost when the
computer power is turned off. The
ROM is sustained by a small
long-life battery in your computer.
If you ever do the hardware setup
procedure with your computer, you
effectively will be writing to ROM.
SCSI
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SCSI (pronounced
SKUH-zee
and sometimes colloquially known as
"scuzzy"), the Small Computer System
Interface, is a set of evolving
ANSI
standard electronic interfaces that
allow personal computers to
communicate with peripheral hardware
such as disk drives, tape drives,
CD-ROM drives, printers, and
scanners faster and more flexibly
than previous interfaces. Developed
at Apple Computer and still used in
the Macintosh, the present set of
SCSIs are
parallel
interfaces. SCSI ports are built
into most personal computers today
and are supported by all major
operating systems.
In addition to
faster data rates, SCSI is more
flexible than earlier parallel data
transfer interfaces. The latest SCSI
standard, Ultra-2 SCSI for a 16-bit
bus can
transfer data at up to 80 megabytes
per second (MBps). SCSI allows up to
7 or 15 devices (depending on the
bus width) to be connected to a
single SCSI port in daisy-chain
fashion. This allows one circuit
board or card to accommodate all the
peripherals, rather than having a
separate card for each device,
making it an ideal interface for use
with portable and notebook
computers. A single host adapter, in
the form of a
PC Card,
can serve as a SCSI interface for a
"laptop," freeing up the parallel
and serial ports for use with an
external modem and printer while
allowing other devices to be used in
addition.
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Although not all devices support all
levels of SCSI, the evolving SCSI
standards are generally
backwards-compatible. That is, if
you attach an older device to a
newer computer with support for a
later standard, the older device
will work at the older and slower
data rate.
The original
SCSI, now known as SCSI-1, evolved
into SCSI-2, known as "plain SCSI."
as it became widely supported.
SCSI-3 consists of a set of primary
commands and additional specialized
command sets to meet the needs of
specific device types. The
collection of SCSI-3 command sets is
used not only for the SCSI-3
parallel interface but for
additional parallel and serial
protocols, including Fibre Channels,
Serial Bus Protocol (used with the
IEEE 1394
Firewire
physical protocol), and the Serial
Storage Protocol (SSP).
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A widely
implemented SCSI standard is Ultra-2
(sometimes spelled "Ultra2") which
uses a 40
MHz clock
rate to get maximum data transfer
rates up to 80 MBps. It provides a
longer possible cabling distance (up
to 12 meters) by using Low Voltage
Differential (LVD) signalling.
Earlier forms of SCSIs use a single
wire that ends in a terminator with
a ground. Ultra-2 SCSI sends the
signal over two wires with the data
represented as the difference in
voltage between the two wires. This
allows support for longer cables. A
low voltage differential reduces
power requirements and manufacturing
costs.
The latest SCSI
standard is Ultra-3 (sometimes
spelled "Ultra3") which increases
the maximum burst rate from 80 Mbps
to 160 Mbps by being able to operate
at the full clock rate rather than
the half-clock rate of Ultra-2. The
standard is also sometimes referred
to as Ultra160/m. New disk drives
supporting Ultra160/m will offer
much faster data transfer rates.
Ultra160/m also includes cyclical
redundancy checking (CRC)
for ensuring the integrity of
transferred data and domain
validation for testing the SCSI
network.
Universal Serial
Bus(USB)
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Universal
Serial Bus (USB) is a
plug-and-play
interface between a computer and
add-on devices (such as audio
players, joysticks, keyboards,
telephones, scanners, and printers).
With USB, a new device can be added
to your computer without having to
add an adapter
card or
even having to turn the computer
off. The USB peripheral
bus
standard was developed by Compaq,
IBM, DEC, Intel, Microsoft, NEC, and
Northern Telecom and the technology
is available without charge for all
computer and device vendors.
USB supports a
data speed of 12
megabits
per second. This speed will
accommodate a wide range of devices,
including
MPEG video
devices, data gloves, and
digitizers. It is anticipated that
USB will easily accommodate plug-in
telephones that use
ISDN and
digital
PBX.
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Since October,
1996, the Windows operating systems
have been equipped with USB
drivers or
special software designed to work
with specific I/O device types. USB
is integrated into
Windows 98
and later versions. Today, most new
computers and peripheral devices are
equipped with USB.
A different
plug-and-play standard,
IEEE 1394,
supports much higher data rates and
devices such as video camcorders and
digital video disk (DVD)
players. However, USB and IEEE 1394
serve different device types.
USB
|
FireWire
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One IT issue guaranteed to cause
confusion is the conflict between
connection technologies,
particularly the two apparently
competing industry standards - USB,
developed by Intel, and Apple's
brainchild, FireWire.
USB is an industry standard for
connecting peripherals to computers
to support plug-and-play
applications. It offers increased
bandwidth, data transfer at 12 Mbps
(Megabits per second), and the
ability to connect up to 127 devices
to a single computer, using external
hubs.
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USB ports are now very common on
both PC and Apple computers, aimed
mainly at low to medium speed
devices such as scanners, printers,
mice, keyboards and other
peripherals with low data loads. USB
has only one type of cable, which
means it is impossible to connect
wrongly - surprisingly valuable in
the average office.
FireWire is a faster standard, used
most commonly in audio/visual
consumer electronic devices such as
DV (Digital Video) Camcorders/VCRs.
It can transfer data directly from
one device to another without having
to go through a PC at all, which
gives it the edge for audio/visual
applications.
FireWires more advanced
specification has cornered the
market for high-bandwidth
peripherals, and comes as standard
on all new Apple computers, although
it still only features on a limited
number of higher end PCs.
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Both standards are 'hot-swappable',
meaning peripherals can be unplugged
at will without having to restart
your machine - ideal in an office
situation where you may share a
printer or scanner.
However, don't be tempted to invest
in one or the other on speed grounds
alone. USB 2.0 is looming, promising
speeds of 480 Mbps, and whilst
FireWire's maximum speed is 393
Mbps, there are future plans to even
double this.
Their strengths really lie in
different areas and they are likely
to continue to serve different
purposes. USB is more widespread and
versatile, and will dominate the
market for connecting desktop PC
peripherals requiring only moderate
bandwidth - the keyboard, mouse,
printer, scanner and floppy drive.
FireWire's point to point transfer,
along with its current speed, seems
set to maintain its position as the
preferred technology for digital
video and audio users requiring high
bandwidth.
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