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 Computer Hardware

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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:

  • The arithmetic logic unit (ALU), which performs arithmetic and logical operations.
  • 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:

  • A Singled Edge Contact (SEC) cartridge that fits into Slot 1
  • Dual Independent Bus (DIB) architecture
  • 512K L2 cache
  • 32K L1 cache
  • MMX support
  • 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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