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Socket 7
The form factor
for fifth-generation CPU
chips from Intel, Cyrix,
and AMD. All Pentium
chips, except Intel's Pentium
Pro (Socket 8)
and Pentium II (Slot
1), conform to the Socket 7 specifications. Intel has decided to phase out
Socket 7 and replace it with Slot 1. But Intel's competitors, such as AMD and
Cyrix, are sticking with Socket 7, and are developing an enhanced version.
The receptacle on the motherboard that holds a Pentium CPU chip. It is also
used to hold Pentium-compatible chips such as AMD's K5 and K6 CPUs.
Socket 7 Socket 7 uses a zero insertion force (ZIF) assembly.
After insertion of the chip, the lever is pulled down and the pins
are locked in.
Socket 370
A CPU plug and socket from Intel for Pentium CPUs that is more economical
than the elaborate Slot 1 system introduced with the Pentium II. Socket 370
accepts a 370-pin PPGA (plastic pin grid array) chip package, instead of the SEC
(single edge cartridge) Slot 1 package. Socket 370 chips and motherboards cost
less to manufacture.
Socket 8
The receptacle on the motherboard that holds a Pentium Pro CPU chip. The
form
factor for Intel's Pentium
Pro microprocessors.
The Pentium Pro was the first microprocessor not to use the venerable Socket
7 form factor. The Pentium II microprocessors use an even newer form
factor called Slot 1. Socket 8 is a 387-pin ZIF
socket with connections for the CPU and one or two SRAM dies for the Level
2 (L2) cache.
Slot 1
The form factor
for Intel's Pentium
II processors. The Slot 1 package replaces the Socket
7 and Socket 8
form factors used by previous Pentium
processors. Slot 1 is a 242-pin slot on the motherboard that holds Intel CPU modules including the
Intel Single Edge Contact Cartridge (SECC and SECC2) and Single Edge Processor
Package (SEPP). A
motherboard can have one or two Slot 1s. The Pentium II was the first to use Slot 1. Slot 1 is a narrow
slot like a PCI bus slot, not a small rectangular chip socket.
Slot 2
A 330-pin slot on the motherboard that holds an Intel Single Edge Contact
Cartridge (SECC). Intel's Xeon chips were the first to use Slot 2. Designed for
multiprocessing (SMP) systems, Slot 2 motherboards typically come with two slots
and require a termination card if only one CPU is plugged in.
Slot A
A receptacle on the motherboard for a K7 CPU chip from AMD. It is
physically similar to Slot 1, but has different electrical requirements.
Form Factor
The physical size and shape of a device.
It is often used to describe the size of circuit
boards.
CPU
Abbreviation of central processing unit, and
pronounced as separate letters. The CPU is the brains of the computer.
Sometimes referred to simply as the processor
or central processor, the CPU is where most calculations take place. In
terms of computing power, the CPU is the most important element of a computer
system.
On large machines, CPUs require one or more printed
circuit boards. On personal
computers and small workstations,
the CPU is housed in a single chip
called a microprocessor.
Two typical components of a CPU are:
Intel
The world's largest manufacturer of computer chips.
Although it has been challenged in recent years by newcomers AMD
and Cyrix, Intel
still dominates the market for PC microprocessors. Nearly all PCs are based on
Intel's x86
architecture.
Intel was founded in 1968 by Bob Noyce and Gordon Moore. Strategically, it
is closely allied with Microsoft
because the Windows
3.x and 95 operating
systems are designed for x86 microprocessors.
The popularity of Windows creates a demand for Intel or Intel-compatible
microprocessors. Many people refer to this alliance as Wintel (short
for Windows-Intel).
Pentium Class
Refers to a Pentium CPU chip or to a PC that uses the chip. The term is
also used for non-Intel CPUs that are Pentium compatible, such as the K5 from
AMD and the 6x86 from Cyrix. See
Pentium Clone
Refers to a Pentium-based computer system that is not made by a top-tier
vendor or to a Pentium CPU chip that is not made by Intel.
Pentium
A family of 32-bit CPU chips from Intel. The term may refer to the chip or
to a PC that uses it. Pentium chips and Pentium PCs are the most widely used
in the world for general-purpose computing.
The first Pentium chip was introduced in 1993 as the successor to the 486,
thus the Pentium began as the fifth generation of the Intel x86 architecture.
Numerous variations of the Pentium have been introduced with increased
performance. Each new line executes more instructions in the same clock cycle
as the previous, and clock speeds increase constantly. The Pentium uses a
64-bit internal bus compared to 32-bits on its 486 predecessor. Intel's
next-generation Itanium chip departs from the Pentium architecture
Pentium - 1993-1996 (60MHz-200MHz)
First Pentium CPU models. The Pentium has L2 cache from 256KB to 1MB, uses
a 50, 60 or 66MHz system bus and contains from 3.1 to 3.3 million transistors
built on 0.6 to 0.35 process. Chips were housed in PGA packages.
Pentium MMX - 1997-1999 (233MHz-300MHz)
Added MMX multimedia instructions to Pentium CPU and increased transistors
to 4.5 million. Desktop units used PGA package and 0.35 process while mobile
units used TCP and 0.25 process.
Pentium Pro - 1995-1997 (150MHz-200MHz)
Typically used in high-end desktops and servers, the Pentium Pro increased
memory from 4GB to 64GB. The Pentium Pro has L2 cache from 512KB to 1MB, uses
a 60 or 66MHz system bus, contains from 5.5 to 62 million transistors. It is
made with 0.35 process and is housed in a dual cavity PGA package. When
introduced, it was touted as being superior to the Pentium for 32-bit
applications.
The Pentium Pro is Intel's
sixth generation microprocessor
(P6). Although, it shares the same name as the fifth-generation Pentium
microprocessor, the Pentium Pro is architecturally quite different. Thanks to
modern design techniques, including superpipelining,
dynamic execution, and on-chip L2
cache, the Pentium Pro can perform at nearly twice the speed of previous
Pentium microprocessors.
Pentium II - 1997-1999 (233MHz-450MHz)
Added MMX multimedia instructions to Pentium Pro and introduced large
Single Edge Connector Cartridge (SECC) for Slot 1. The Pentium II uses a 66 or
100MHz system bus. Desktop models have 7.5 million transistors, 512KB L2 cache
and are housed in SECC packages. Mobile models have 27.4 million transistors,
256KB L2 cache and are housed in either BGA or Mobile Mini-Cartridge (MMC)
packages.
The Pentium II builds on the design of the Pentium
Pro, but adds an additional 2 million transistors
to bring the total up to 7.5 million. Current versions of the chip run at
speeds of 233, 266, 300, and 333 MHz.
In addition, the Pentium II features the following:
Pentium II Xeon - 1998-1999 (400MHz-450MHz)
Typically used in high-end and 2-way and 4-way servers, Xeon specs are like
Pentium II with L2 cache from 512KB to 2MB and 100MHz system bus.
Pentium III - 1999-2001 (500MHz-1.13GHz)
The Pentium III added 70 additional instructions to the Pentium II. The
Pentium III uses a 100 or 133MHz system bus and either a 512KB L2 cache or a
256KB L2 Advanced Transfer Cache. Depending on model, it contains from 9.5 to
28 millions transistors, uses the 0.25 or 0.18 micron process and comess in
SECC and SECC2 packages. Mobile units come in BGA and micro PGA packages.
Pentium III Xeon - 1999-2001 (500MHz-933MHz)
Typically used in 2-way to 8-way servers, Xeon specs are like Pentium III,
except both types of L2 cache can go up to 2MB. The Xeon uses the SECC2 and
SC330 chip packages.
Pentium 4 - 2000 (1.4-2GHz)
Latest Pentium architecture adds a 400MHz system bus and 256KB L2 Advanced
Transfer Cache. It contains 42 million transistors, uses the 0.18 micron
process and comes in a PGA422 chip package. Intel's i850 chipset for the
Pentium 4 supports only dual Rambus memory although DDR supports is expected.
Celeron
A family of lower-cost Pentium II chips from Intel that was introduced in
mid-1998. The first models (266 and 300MHz) did not include an external L2 cache
and were somewhat sluggish, relegating them to an entry-level or novice rating.
However, subsequent models added 128KB of L2 cache that runs at the full speed
of the CPU just like the high-end Xeon chips that contain up to 2MB of L2. The
chip yields are greater with smaller amounts of cache and can be sold at a
better price. Pentium III-based Celerons using the Coppermine technology were
introduced in 2000.
Celeron - 1998-2001 (266MHz-800MHz)
Typically used for lower-end PCs. Initially Celerons had no L2 cache, but
128KB on-die cache was added in 1999 making them competitive with Pentium IIs.
Celerons use a 66 to 100MHz system bus and 0.25 micron process. Desktop models
have from 7.5 to 19 million transistors and use SEPP or PPGA chip packages.
Mobile units have 18.9 million transistors and are housed in BGA packages.
Cyrix
A U.S. corporation founded in 1988 that manufactures Intel-compatible
microprocessors. Its 6x86 line of processors is comparable to Intel's
line of Pentium chips. Cyrix was
acquired by National Semiconductor in 1997 and then by VIA in 1999.
(Cyrix Corporation, Richardson, TX) Founded in 1988, Cyrix was a manufacturer
of x86-compatible CPU chips. Its first product was a math coprocessor. In
1992, it introduced a line of 486 CPUs, later followed by the 6x86
Pentium-class and 6x86MX Pentium II-class chips. In 1998, Cyrix was acquired
by National Semiconductor and operated as a wholly owned subsidiary. In 1999,
National Semi sold its Cyrix processor business to Via Technologies, Inc., a
leader in PC chipset design.
AMD
Short for Advanced Micro Devices, a
manufacturer of chips
for personal
computers. AMD is challenging Intel
with a set of Intel-compatible
microprocessors. AMD's latest chips, the Athlon, support MMX
instructions.
(Advanced Micro Devices, Inc., Sunnyvale, CA, www.amd.com) A manufacturer of
semiconductor devices including x86-compatible CPUs, embedded processors,
flash memories, programmable logic devices and networking chips. Founded in
1969 by W. J. (Jerry) Sanders III and seven other individuals, AMD was the
first to produce 386 and 486-compatible CPU chips in 1991 and 1993
respectively and compete head on with Intel.
AMD later introduced its K5 and K6 lines of Pentium-compatible chips and
subsequently its Athlon line, which was introduced in 1999 at 700MHz. Athlon
chips have since broken the 1GHz range. In the summer of 2000, it introduced
its Duron family of lower-priced CPUs for the value market. Over the years,
numerous PC vendors, both small and large, have successfully used AMD's CPU
chips in their PCs.
K5
A Pentium-class CPU chip from AMD. K5 chips are available in models that
rival a 166MHz Pentium chip.
K6
A Pentium II-class CPU chip from AMD. The first models were introduced in
1997 at 166MHz, 200MHz and 233MHz clock speeds. The K6 chip contains the MMX
instruction set used in Pentium MMX and Pentium II CPUs and plugs into the
Socket 7 processor socket on Pentium motherboards. The K6 was originally
engineered by NexGen, which AMD acquired in 1966.
The second-generation K6-2 chip, introduced in 1998, improves performance and
adds AMD's 3DNow 3-D instructions for faster rendering of games and animation.
The K6-3 was introduced in 1999 with improved performance and greater onboard
cache. The K6 series has been superseded by the Athlon.
K7 / Athlon
A Pentium III-class CPU chip from AMD. The first models were introduced in
1999 with clock speeds from 500MHz to 650MHz and a 200MHz system bus. Subsequent
models have exceeded 1GHz clock, and the bus was increased to 266MHz. The Athlon
contains the MMX multimedia instructions used in Pentium MMX and Pentium II CPUs
along with an enhanced version of AMD's 3DNow 3-D instruction set for faster
rendering of games and animation.
The Athlon plugs into a slot, known as Slot A, which is similar to the elongated
slot used by Pentium II's and III's. The Athlon is the successor to the K6
series and was formerly known as the K7.
Athlon XP
A family of CPU chips from AMD that was introduced in 2001. Departing from
the traditional MHz designation, Athlon XP chips use model numbers that
combine clock speed and architectural features into a numerical rating. For
example, the 1500+, the first XP model, has a clock speed of 1.33GHz, but
provides greater performance than the Athlon 1.4GHz chip.
Pentium Microprocessor
A 32-bit microprocessor
introduced by Intel
in 1993. It contains 3.3 million transistors,
nearly triple the number contained in its predecessor, the 80486 chip. Though
still in production, the Pentium processor has been superseded by the Pentium
Pro and Pentium
II microprocessors. Since 1993, Intel has developed the Pentium III and
more recently the Pentium 4 microprocessors.
Pentium Pro
The Pentium Pro is Intel's
sixth generation microprocessor
(P6). Although, it shares the same name as the fifth-generation Pentium
microprocessor, the Pentium Pro is architecturally quite different. Thanks to
modern design techniques, including superpipelining,
dynamic execution, and on-chip L2
cache, the Pentium Pro can perform at nearly twice the speed of previous
Pentium microprocessors.
AT
(Advanced Technology) IBM's first 286-based PC, introduced in
1984. It was the most advanced machine in the PC line and featured a new
keyboard, a higher-capacity 5.25" floppy (1.2MB) and a 16-bit data bus.
AT-class machines ran considerably faster than the 8088-based XT PCs).
AT motherboards also used a 5-pin keyboard connector that was a 1/2" in
diameter.
ATX
A motherboard that superseded the AT design. ATX rotates the CPU and memory
90 degrees, allowing full-length boards in all sockets. The power supply blows
air over the CPU rather than pulling air through the chassis. The Micro or Baby
ATX is a smaller version of the ATX with fewer slots. The fewer slots are
generally a result of integrated Video and sound eliminating the need for the
AGP or PCI slot.
Motherboard
Also called the "system board," it is the main printed circuit
board in an electronic device, which contains sockets that accept additional
boards. In a personal computer, the motherboard contains the bus, CPU and
coprocessor sockets, memory sockets, keyboard controller and supporting chips.
Chips that control the video display, serial and parallel ports, mouse and disk
drives may or may not be present on the motherboard. If not, they are
independent controllers that are plugged into an expansion slot on the
motherboard.
Motherboard This is a Baby AT style motherboard for a PC. The adapter
cards (expansion boards) plug into the expansion slots on the motherboard.
PCI Bus / Slot
(Peripheral Component Interconnect) A peripheral bus
commonly used in PCs, Macintoshes and workstations. It was designed primarily
by Intel and first appeared on PCs in late 1993. PCI provides a high-speed
data path between the CPU and peripheral devices (video, disk, network, etc.).
There are typically three or four PCI slots on the motherboard. In a Pentium
PC, there is generally a mix of PCI and ISA slots or PCI and EISA slots. Early
on, the PCI bus was known as a "local bus."
PCI provides "plug and play" capability, automatically configuring
the PCI cards at startup. When PCI is used with the ISA bus, the only thing
that is generally required is to indicate in the CMOS memory which IRQs are
already in use by ISA cards. PCI takes care of the rest.
PCI allows IRQs to be shared, which helps to solve the problem of limited IRQs
available on a PC. For example, if there were only one IRQ left over after ISA
devices were given their required IRQs, all PCI devices could share it. In a
PCI-only machine, there cannot be insufficient IRQs, as all can be shared.
PCI runs at 33MHz, supports 32- and 64-bit data paths and bus mastering. PCI
Version 2.1 calls for 66MHz, which doubles the throughput. There are generally
no more than three or four PCI slots on the motherboard, which is based on 10
electrical loads that deal with inductance and capacitance. The PCI chipset
uses three loads, leaving seven for peripherals. Controllers built onto the
motherboard use one, whereas controllers that plug into an expansion slot use
1.5 loads. A "PCI bridge" can be used to connect two PCI buses
together for more slots.
The PCI Bus The PCI bus provides a wider bandwidth than the
traditional ISA bus, allowing peripherals to transfer data at higher speed.
Types of Expansion Boards Except for ISA and AGP, all the
other boards in this illustration have given way to PCI. Although
most PC motherboards still have ISA slots, they too will give
way to PCI.
ISA Slot
(3) (Interactive Services Association) See Internet
Alliance.
(2) (The Instrumentation, Systems, and Automation Society, RTP, NC
27709, www.isa.org) A trade association dedicated to industrial control
systems which includes measurement and instrumentation, robotics, motion
control and process control. ISA supports ISA EXPO, the industry's premier
trade show for more than 50 years as well as the Directory of Instrumentation.
(1) (Industry Standard Architecture) Pronounced
"eye-suh." An expansion bus commonly used in PCs. It accepts plug-in
boards that control the sound, video display and other peripherals. Most PCs
today have a combination of ISA and PCI slots; however, many no longer support
ISA, and it is expected to be obsolete by the mid 2000s.
Originally called the "AT bus," it was first used in the IBM AT,
extending the 8-bit bus to 16 bits. Earlier ISA PCs provided a mix of 8 and
16-bit slots. Today, PCs have only 16-bit ISA slots.
ISA slots can be seen above. The three slots on the right are
examples of the early ISA slots. These can be found on old 286 to 486
mother boards. The two slots on the left are an example of the later ISA
slots that can be found on some currently produced mother boards.
AGP Slot
(Accelerated Graphics Port) A high-speed port developed
by Intel that is designed for the display adapter (video card) only. It provides
a direct connection between the card and memory, and only one AGP slot is on the
motherboard. AGP was introduced as a higher-speed alternative to the PCI-based
adapter, plus it freed up a PCI slot to be used for another peripheral device.
The brown AGP slot is slightly shorter than the white PCI slot and is located
about an inch farther back.
AGP uses a 32-bit bus. The original AGP standard (AGP 1x) provided a data
transfer rate of 264 Mbytes/sec. AGP 2x is 528 Mbytes/sec. AGP 4x is 1 Gbyte/sec.
AGP 8x is 2 Gbytes/sec.
RAM
(Random Access Memory) A group of memory chips,
typically of the dynamic RAM (DRAM) type, which function as the computer's
primary workspace. When personal computers first came on the market in the late
1970s, 64KB (64 kilobytes) of RAM was the upper limit. Today, 64MB (64
megabytes) of RAM is entry level for a desktop computer, a thousand times as
much.
The "random" in RAM means that the contents of each byte of storage in
the chip can be directly accessed without regard to the bytes before or after
it. This is also true of other types of memory chips, including ROMs and PROMs.
However, unlike ROMs and PROMs, RAM chips require power to maintain their
content, which is why you must save your data onto disk before you turn the
computer off.
RAM Module
A narrow printed circuit board that holds memory chips, typically dynamic
RAM (DRAM) or synchronous dynamic RAM (SDRAM). Earlier computers used SIMM
modules. Current-day machines use DIMMs for desktop computers and SODIMMs
for laptops. PCs use either nine-bit memory (eight bits and parity) or
eight-bit memory without parity. Macs use eight-bit memory without parity.
SIMMs (single in-line memory modules) evolved into DIMMs (dual in-line
memory modules), which double the number of paths between the module and
motherboard by using each side of the edge connector independently. SIMMs
are generally used in pairs, whereas DIMMs can be used one at a time. Rambus
modules must be used in pairs.
Common Memory Modules DIMMs are widely used in desktop computers and
servers. The smaller SODIMMs (Small Outline DIMMs) are used in laptops,
while SIMMs are typically found in older PCs. For identification purposes,
look at the pattern of the pins on the edge connector (bottom) and the
various notches between the pins and on the sides. The layout of the chips
is not important as they can differ signficantly.
To upgrade memory, read your motherboard or system manual.
It should show you all possible combinations of different-sized modules that
can be used in the available slots. With DIMMs and SODIMMs, there are
numerous chip configurations that yield the same total capacity. In some
cases, the motherboard is not sensitive to this; in other cases, it is. The
bottom line: read the documentation.
With DIMMs and 72-pin SIMMs, the module designation refers to the number of
data words on the module. For example, an 8x32 DIMM means that 8 million
32-bit words of memory are available to the system.
If another number follows the designation, such as 8x32-60, it refers to the
speed of the chips, in this case, 60 nanosecond 32MB DIMMs. Sometimes, there
is an "M" for "mega" after the first digit; for example,
8Mx32.
Cache
Pronounced "cash." A cache is used to speed up data
transfer and may be either temporary or permanent. Memory and disk caches are in
every computer to speed up instruction execution and data retrieval. These
temporary caches serve as staging areas, and their contents can be changed in
seconds or milliseconds (see below).
Browser caches and Internet caches hold popular Web pages long periods of time
and even for the duration, because caching servers constantly update the page
with the latest version from the Internet (see Web
cache and browser
cache). In these cases, the cache database is actually a folder on the disk.
See router
cache.
L2 Cache
A memory cache that is external to the CPU chip.
Memory Caches
A memory cache, or "CPU cache," is a memory bank that
bridges main memory and the CPU. It is faster than main memory and allows
instructions to be executed and data to be read at higher speed. Instructions
and data are transferred from main memory to the cache in blocks, using some
kind of look-ahead algorithm. The more sequential the instructions in the
routine being executed or the more sequential the data being read, the greater
chance the next required item will already be in the cache, resulting in better
performance.
A level 1 (L1) cache is a memory bank built into the CPU chip. A level 2 cache
(L2) is a secondary staging area that feeds the L1 cache. Increasing the size of
the L2 cache may speed up some applications but have no effect on others. L2 may
be built into the CPU chip, reside on a separate chip in a multichip package
module (see MCP)
or be a separate bank of chips. Caches are typically static RAM (SRAM), while
main memory is generally some variety of dynamic RAM (DRAM).
Disk Cache
A disk cache is a section of main memory or memory on the disk
controller board that bridges the disk and the CPU. When the disk is read, a
larger block of data is copied into the cache than is immediately required. If
subsequent reads find the data already stored in the cache, there is no need to
retrieve it from the disk, which is slower to access.
If the cache is used for writing, data is queued up at high speed and then
written to disk during idle machine cycles by the caching program. If the cache
is built into the hardware, the disk controller figures out when to do it.
Hard Drive / Fixed Disk
The primary computer storage device, which spins, reads and writes one or
more fixed disk platters. In practice, the terms "hard drive" and
"hard disk" are used synonymously. Hard drives are the storage medium
in desktop and laptop computers as well as all servers and mainframes throughout
the world. They are also used in printers for storing fonts and print jobs as
well as MP3 players and a myriad of other portable and stationary computer-based
devices. Although removable disks encased in cartridges use the same
"hard" disk media and a similar drive technology, they are mostly
called "removable drives" rather than hard drives.
The term "hard" differentiates high-capacity rigid disks made of
aluminum or glass from low-capacity floppy disks made of plastic.
IDE
(2) (Integrated Development Environment) A set of
programs run from a single user interface. For example, programming languages
often include a text editor, compiler and debugger, which are all activated and
function from a common menu.
(1) (Integrated Drive Electronics) A type of
hardware interface widely used to connect hard disks, CD-ROMs and tape drives to
a PC. IDE is very popular because it is an economical way to connect
peripherals. Starting out with 40MB capacities years ago, 20GB IDE hard disks
have become entry level, costing less than half a cent per megabyte. To learn
about the other major hardware interface used for disks, see SCSI.
With IDE, the controller electronics are built into the drive itself, requiring
a simple circuit in the PC for connection. IDE drives were attached to earlier
PCs using an IDE host adapter card. Today, two Enhanced IDE (EIDE) sockets are
built onto the motherboard, and each socket connects to two devices via a 40-pin
ribbon cable. Starting with ATA-66 drives, the cable uses 80 wires and 39 pins.
It plugs into the same socket with one pin removed.
The IDE interface is officially known as the ATA (AT Attachment) specification.
ATAPI (ATA Packet Interface) defines the IDE standard for CD-ROMs and tape
drives. ATA-2 (Fast ATA) defined the faster transfer rates used in Enhanced IDE.
ATA-3 added interface improvements, including the ability to report potential
problems (see S.M.A.R.T.).
Starting with ATA-4, either the word "Ultra" or the transfer rate was
added to the name in various combinations. For example, at 33 Mbytes/sec, terms
such as Ultra ATA, Ultra DMA, UDMA, ATA-33, DMA-33, Ultra ATA-33 and Ultra
DMA-33 have all been used.
EIDE
(Enhanced IDE) An extension to the IDE interface that supports
the ATA-2 and ATAPI standards. ATA-2 (Fast ATA) provides faster transfer rates
(see IDE
for details) and allows for multiple channels, each connecting two devices.
ATAPI supports non-hard disk devices such as CD-ROMs and tape drives. It also
specifies a new BIOS for supporting hard disks greater than 504MB. Since
mid-1994, PCs have shipped with EIDE interfaces, and most motherboards provide a
primary and secondary channel for a total of four devices. In practice, the
terms EIDE and IDE are synonymous.
SCSI
(Small Computer System Interface) Pronounced
"scuzzy." SCSI is a hardware interface that allows for the connection
of up to 15 peripheral devices to a single board called a "SCSI host
adapter" that plugs into the motherboard, typically using a PCI slot. SCSI
peripherals are daisy chained together. They all have a second port used to
connect the next device in line. SCSI host adapters are also available with two
controllers that support up to 30 peripherals.
Introduced in 1986 and originally developed by Shugart Associates (see SASI),
SCSI is widely used from desktop PCs to mainframes, although most desktop PCs
come with IDE drives. The advantage of SCSI in a desktop PC is that a scanner
and several other drives (CD-Rs, DVD-RAM, Zip drives, etc.) as well as hard
drives can be added to one SCSI cable chain. However, this has become less
important as alternate interfaces such as USB and FireWire have become popular.
Until the late 1990s, SCSI hard disks were the only ones used in RAID
configurations which provide improved performance and/or fault tolerance. Since
the advent of IDE RAID controllers, SCSI and IDE have become more equalized,
although SCSI continues to be the drive interface of choice in the server
market. See RAID.
Windows 95/98/NT/2000 and the Macintosh provide internal support for SCSI, but
Windows 3.1 and DOS did not. Installing SCSI in a Win 3.1 or DOS machine
required adding the appropriate SCSI driver.
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