to the average amount of time it takes for a floppy drive,
hard drive, CD drive or other drive to find any particular
piece of data on a disk.
The required time to read or write data to RAM or other
storage device. Since the operating environment and varying
conditions affect access time, this is usually given as an
(Advanced Configuration and Power Interface)
An industry specification for the efficient handling of power
consumption in desktop and mobile computers. ACPI specifies
how a computer's BIOS, operating system and peripherals communicate
with each other about power usage.
This refers to a response from a server to a network request
such as a PING. Basically, the server is saying, "I'm
here, and I saw your request!" This also refers to the
6th ASCII character.
LCD panels that are active matrix have a sharper, brighter
image than those with passive matrix screens. They can also
continue to be seen at much greater angles off of central
viewing and cost more to produce. The "active" part
of the word describes the use of a transistor or diode that
actively controls each pixel.
The term used to describe expansion cards that are inserted
into bus expansion slots.
A flat rectangular fiberglass board with electronic circuitry.
Inserted in an expansion slot on the computer main bus, it
provides additional system functions, such as device controllers
or video adapters. Also called an Add-In card.
Every memory location is numbered consecutively. This
number is the address of the memory location. An address can
be a label, number, or name that identifies a register, memory
location, or a location on a disk drive or external device
accessed via an I/O port.
AGP (Advanced Graphics Port)
An interface specification that enables 3-D graphics to display
quickly on ordinary personal computers. AGP is an interface
designed to convey 3-D images (for example, from Web sites
or CD-ROMs) much more quickly and smoothly than is possible
today on any computer other than an expensive graphics workstation.
The interface uses your computer's main storage (RAM) for
refreshing the monitor image and to support the texture mapping,
z-buffering and alpha blending required for 3-D image display.
The AGP main memory use is dynamic, meaning that when not
being used for accelerated graphics, main memory is restored
for use by the operating system or other applications.
A term used to describe any device that represents values
by a continuously varied physical property, such as voltage.
ANSI - American National Standards Institute
A standards-setting, non-government organization that
develops and publishes standards for voluntary use in the
USA. API Attachment Packet Interface. A standard hard disk
drive specification used for Integrated Drive Electronics
A process where devices compete for possession of the
channel on a prioritized basis.
Refers to the way a system is designed and how the components
are connected with each other. There are computer architectures,
network architectures and software architectures.
An array of disk drives combines the storage space on
the disk drives into a single segment of contiguous storage
space. MegaRAID can group disk drives on one or more SCSI
channels into an array. A hot spare drive does not participate
in an array.
Array Management Software
Software that provides common control and management for
a disk array. Array Management Software most often executes
in a disk adapter or intelligent host bus adapter, but can
also execute in a host server. When it executes in a disk
adapter or adapter, Array Management Software is often called
Array spanning by a logical drive combines storage space
in two arrays of disk drives into a single, contiguous storage
space in a logical drive. MegaRAID logical drives can span
consecutively numbered arrays that each consist of the same
number of disk drives. Array spanning promotes RAID levels
1, 3, and 5 to RAID levels 10, 30 and 50, respectively. ASCII
American Standard Code for Information Interchange. An industry
standard 7-bit code consisting of control, character, and
graphic codes (8 bits if the parity bit is included).
Advanced SCSI Programming Interface (ASPI)
developed by Adaptec so that different devices could be controlled
by different SCSI cards and therefore the SCSI card version
would not matter. Basically, if the correct ASPI driver is
being used, you can send the same command to any Adaptec SCSI
card to make something happen to a device. Most commonly,
ASPI is associated with CD-ROM drives.
Data transfer (usually at a low rate and independent of
any external timing constraints) performed by a SCSI device
involving the interlocking of a signal to the initiator (REQ)
and a signal to the target (ACK) such that each step of the
data transfer protocol must occur before the next step can
Asynchronous Event Notification
A process when a SCSI target can send unsolicited sense
information to an initiator using the SCSI SEND command.
Those operations that bear no relationship to each other
in time and can overlap. The concept of asynchronous I/O operations
is central to independent access arrays in throughput-intensive
Advanced Technology. Specifically refers to the IBM PC
AT incorporating the Intel 80286 processor. It is also used
as a reference of BIOS compatibility. AT refers to the original
IBM PC/AT computer architecture, more commonly known as ISA
ATA - AT Attachment
A disk drive implementation that integrates the controller
on the disk drive itself.
ATA: Known also as IDE, supports one
or two hard drives, a 16-bit interface and PIO modes 0, 1
ATA-2: Supports faster PIO modes (3
and 4) and multiword DMA modes (1 and 2). Also supports logical
block addressing (LBA) and block transfers. ATA-2 is marketed
as Fast ATA and Enhanced IDE (EIDE).
ATA-3: Minor revision to ATA-2.
Ultra-ATA: Also called Ultra-DMA,
ATA-33, and DMA-33, supports multiword DMA mode 3 running
at 33 MBps.
ATA/66: Also called Ultra-DMA/66 or
UDMA-66. A new version of ATA proposed by Quantum Corporation,
and supported by Intel, that doubles ATA's throughput to 66
ATA/100: Also called Ultra-DMA/100
or UDMA-100. A new version of ATA proposed by Quantum
Corporation, and supported by Intel, that doubles ATA's throughput
to 100 MBps.
(AT Attachment Packet Interface)
An interface between your computer and attached CD-ROM drives
and tape backup drives. Most of today's PC computers use the
standard IDE (Integrated Drive Electronics) interface to address
hard disk drives. ATAPI provides the additional commands needed
for controlling a CD-ROM player or tape backup so that your
computer can use the IDE interface and controllers to control
these relatively newer device types. ATAPI is part of the Enhanced
IDE (EIDE) interface (also known as ATA-2).
ATASPI - ATA Software Programming Interface
A specification for writing enhanced IDE drivers. It specifies
a standard low-level interface for all enhanced IDE functions.
The ATASPI driver acts as an I/O manager and will provide arbitration
and handshaking that will allow two different types of peripheral
devices to be attached to a single enhanced IDE connector.
ATX is an industry-wide open specification for a desktop computer's
motherboard. ATX improves the motherboard design by taking the
small AT motherboard that has been an industry standard and
rotating by 90 degrees the layout of the microprocessor and
expansion slots. This allows space for more full-length add-in
cards. A double-height aperture is specified for the rear of
the chassis, allowing more possible I/O arrangements for a variety
of devices such as TV input and output, LAN connection and so
forth. The new layout is also intended to be less costly to
manufacture. Fewer cables will be needed. The power supply has
a side-mounted fan, allowing direct cooling of the processor
and cards, making a secondary fan unnecessary. Version 2.0 incorporates
improvements suggested by chassis and power supply vendors.
A unit of measurement of the discrete number of signal elements
that can be transmitted per second by a device. It is not an
exact measure of the amount of information being transmitted
and is not the same as Bits Per Second.
The base two numbering system, where the only digits are
0 and 1. It is used by all computers.
Binary Coded Decimal
A method of encoding decimal digits into four binary bits.
BIOS (Basic Input/output system)
The program a personal computer's microprocessor uses to get
the computer system started after you turn it on. It also manages
data flow between the computer's OS and attached devices such
as the hard disk, video adapter, keyboard, mouse and printer.
When BIOS boots up the computer, it first determines whether
all of the attachments are in place and operational and then
it loads the OS (or key parts of it) into a computer's random
access memory (RAM) from a hard disk or diskette drive.
A binary digit that can take either the value 0 or 1. A
bit is the smallest unit of information that a computer can
An amount of data moved or addressed as a single unit; the
smallest amount of data that can be read or written at a time.
Blocks are separated by physical gaps, or identified by their
track/sector addresses or logical addresses.
The process of initializing, testing and configuring a computer
system blding higher-level services on top of lower-level primitive
B.P.S. - Bits per Second
The number of binary digits that can be transmitted in one
second. Generally, modem speeds are given in BPS, not baud rate.
Neither baud rate nor BPS take into account the gaps between
transmissions, so neither baud rate nor BPS accurately express
the amount of information being transferred.
An area of memory or storage that is temporarily reserved
for I/O processing.
A method of data transfer that allows a device to remain
inactive for long periods of time and then send large amounts
of data in a short time without interruption. Can be used for
DMA transfers on the EISA bus.
A collection of wires through which data is transmitted from
one part of a computer to another. You can think of a bus as
a highway on which data travels within a computer. When used
in reference to personal computers, the term bus usually refers
to internal bus. This is a bus that connects all the internal
computer components to the CPU and main memory. There's also
an expansion bus that enables expansion boards to access the
CPU and memory. All buses consist of two parts -- an address
bus and a data bus. The data bus transfers actual data whereas
the address bus transfers information about where the data should
go. The size of a bus, known as its width, is important because
it determines how much data can be transmitted at one time.
For example, a 16-bit bus can transmit 16 bits of data, whereas
a 32-bit bus can transmit 32 bits of data. Every bus has a clock
speed measured in MHz. A fast bus allows data to be transferred
faster, which makes applications run faster. On PCs, the old
ISA bus is being replaced by faster buses such as PCI. Nearly
all PCs made today include a local bus for data that requires
especially fast transfer speeds, such as video data. The local
bus is a high-speed pathway that connects directly to the processor.
Several different types of buses are used on Apple Macintosh
computers. Older Macs use a bus called NuBus, but newer ones
Bus Free Phase
The phase when no SCSI device is actively using the SCSI
bus and the bus is available for use.
Refers to a feature supported by some bus architectures that
enables a controller connected to the bus to communicate directly
with other devices on the bus without going through the CPU.
Most modern bus architectures, including PCI, support bus mastering
because it improves performance.
A unit of data made up of eight contiguous bits. A byte
is usually the smallest addressable unit of memory.
Pronounced cash, a special high-speed storage mechanism.
It can be either a reserved section of main memory or an independent
high-speed storage device. Two types of caching are commonly
used in personal computers: memory caching and disk caching.
A memory cache, sometimes called a cache store or RAM cache,
is a portion of memory made of high-speed static RAM (SRAM)
instead of the slower and cheaper dynamic RAM (DRAM) used for
main memory. Memory caching is effective because most programs
access the same data or instructions over and over. By keeping
as much of this information as possible in SRAM, the computer
avoids accessing the slower DRAM. Some memory caches are built
into the architecture of microprocessors. The Intel 80486 microprocessor,
for example, contains an 8K memory cache, and the Pentium has
a 16K cache. Such internal caches are often called Level 1 (L1)
caches. Most modern PCs also come with external cache memory,
called Level 2 (L2) caches. These caches sit between the CPU
and the DRAM. Like L1 caches, L2 caches are composed of SRAM
but they are much larger. Disk caching works under the same
principle as memory caching, but instead of using high-speed
SRAM, a disk cache uses conventional main memory. The most recently
accessed data from the disk (as well as adjacent sectors) is
stored in a memory buffer. When a program needs to access data
from the disk, it first checks the disk cache to see if the
data is there. Disk caching can dramatically improve the performance
of applications, because accessing a byte of data in RAM can
be thousands of times faster than accessing a byte on a hard
disk. When data is found in the cache, it is called a cache
hit, and the effectiveness of a cache is judged by its hit rate.
Many cache systems use a technique known as smart caching, in
which the system can recognize certain types of frequently used
data. The strategies for determining which information should
be kept in the cache constitute some of the more interesting
problems in computer science.
Coercion of Drives
The ability to coerce a drive into taking on a specific
size. At AMI, drive coercion refers to the ability of our controllers
to recognize the sizes of the drives connected, and to then
coerce the larger drives to use only that amount of space which
the smallest has available. So, if there is a 10GB drive connected
with a 15GB drive, and they are combined in a RAID configuration,
the 15GB drive will act as a 10GB drive, and will only have
10GB worth of usable space.
A device that controls the transfer of data from a computer
to a peripheral device and vice versa. For example, disk drives,
display screens, keyboards, and printers all require controllers.
In personal computers, the controllers are often single chips.
When you purchase a computer, it comes with all the necessary
controllers for standard components, such as the display screen,
keyboard, and disk drives. If you attach additional devices,
however, you may need to insert new controllers that come on
expansion boards. Controllers must be designed to communicate
with the computer's expansion bus. There are three standard
bus architectures for PCs -- the AT bus, PCI (Peripheral Component
Interconnect), and SCSI. When you purchase a controller, therefore,
you must ensure that it conforms to the bus architecture that
your computer uses.
Connecting two or more computers together in such a way that
they behave like a single computer. Clustering is used for parallel
processing, for load balancing and for fault tolerance. Clustering
is a popular strategy for implementing parallel processing applications
because it enables companies to leverage the investment already
made in PCs and workstations. In addition, it's relatively easy
to add new CPUs simply by adding a new PC to the network. Microsoft's
clustering solution for Windows NT systems is called MSCS.
CPU (Central Processing Unit)
Is an older term for processor and microprocessor, the central
unit in a computer containing the logic circuitry that performs
the instructions of a computer's programs.
DAC(Digital to Analog Converter)
This is an electronic device that converts digital signals into
analog signals. You can find DACs in a variety of devices, including
a sound card. The DAC would be used to convert digital signals
from your computer into analog signals that can be sent to your
speakers - assuming you use standard analog speakers.
Data Rate SDRAM)
This is a standard that is used to boost conventional SDRAM
memory up to speeds of 133MHz and beyond. Standard SDRAM tops
out around that speed, and DDR SDRAM will start at around 166MHz
speeds. A new standard called DDR II will push DDR SDRAM to
around 300MHz speeds.
(Dual In-Line Memory Module)
A double SIMM (Single In-Line Memory Module). Like a SIMM, it's
a module containing one or several random access memory (RAM)
chips on a small circuit board with pins that connect it to
the computer motherboard. A SIMM typically has a 32 data bit
(36 bits counting parity bits) path to the computer that requires
a 72-pin connector. For synchronous dynamic RAM (SDRAM) chips,
which have a 64 data bit connection to the computer, SIMMs must
be installed in in-line pairs (since each supports a 32 bit
path). A single DIMM can be used instead. A DIMM has a 168-pin
connector and supports 64-bit data transfer. It is considered
likely that future computers will standardize on the DIMM.
Also known as RAID Level 1. A form of RAID in which the Array
Management Function maintains two or more identical copies of
data on separate disks. Also known as RAID Level 1 and disk
Disk spanning allows multiple disk drives to function
like one big drive. Spanning not only overcomes disk space shortage;
it also simplifies storage management by combining existing
resources or adding relatively inexpensive resources. For example,
four 400 MB disk drives can be combined to appear to the operating
system as one single 1600 MB drive. Spanning alone does not
provide reliability or performance enhancements. Spanned logical
drives must have the same stripe size and must be contiguous.
For example Logical Drives 1 and 2 can be spanned; Logical Drives
1 and 3 cannot.
Also known as RAID Level 0. A mapping technique in which fixed-size
consecutive ranges of virtual disk data addresses are mapped
to successive array members in a cyclical pattern.
DMI (Desktop Management Interface)
Managing and keeping track of hardware and software components
in a system of personal computers from a central location. DMI
was created to automate system management and is particularly
beneficial in a network computing environment where dozens or
more computers are managed. DMI is hardware and operating system-independent,
independent of specific management protocols, easy for vendors
to adopt, mappable to existing management protocols such as
the Simple Network Management Protocol (SNMP) and used on network
and non-network computers.
When a drive is moved from one placement on a bus to another,
it is said to have roamed.
A program that extends the capabilities of a computer
by enabling the computer to operate peripheral devices, such
as WORM drives, CD-ROM drives, or tape drives.
ECC (Error Correction [or Correcting] Code;
Error Checking and Correcting)
Allows data that is being read or transmitted to be checked
for errors and, when necessary, corrected "on the fly." It differs
from parity-checking in that errors are not only detected but
also corrected. ECC is increasingly being designed into data
storage and transmission hardware as data rates (and therefore
error rates) increase.
EDO - Extended Data Output Dynamic Random Access Memory
A type of DRAM that is faster than conventional DRAM. Unlike
conventional DRAM which can only access one block of data at
a time, EDO RAM can start fetching the next block of memory
at the same time that it sends the previous block to the CPU.
EISA - Extended Industry Standard Architecture
A bus architecture designed for PCs using an Intel 80386,
80486, or Pentium microprocessor. EISA buses are 32 bits wide
and support multiprocessing. The EISA bus was designed by nine
IBM competitors (sometimes called the Gang of Nine): AST Research,
Compaq Computer, Epson, Hewlett-Packard, NEC, Olivetti, Tandy,
WYSE, and Zenith Data Systems. They designed the architecture
to compete with IBM's own high-speed bus architecture called
the Micro Channel architecture (MCA). The principal difference
between EISA and MCA is that EISA is backward compatible with
the ISA bus (see above), while MCA is not. This means that computers
with an EISA bus can use new EISA expansion cards as well as
old AT expansion cards. Computers with an MCA bus can use only
MCA expansion cards. EISA and MCA are not compatible with each
other. This means that the type of bus in your computer determines
which expansion cards you can install. Neither EISA nor MCA
has been very successful. Instead, a new technology called local
bus (PCI) is being used in combination with the old ISA bus.
Programming inserted into programmable read-only memory (PROM),
thus becoming a permanent part of a computing device. Firmware
is created and tested like software (using microcode simulation).
When ready, it can be distributed like other software and, using
a special user interface, installed in the programmable read-only
memory by the user. Is sometimes distributed for printers, modems
and other computer devices.
Hot Plug (Hot Swap)
The ability to add and remove devices to a computer while
the computer is running and have the operating system automatically
recognize the change. Two new external bus standards -- Universal
Serial Bus (USB ) and IEEE 1394 -- support hot plugging. This
is also a feature of PCMCIA. Hot plugging is also called hot
I20 - Intelligent I/O
A fairly new I/O architecture developed by a consortium
of computer companies called the I2O special Interest Group
(SIG). I2O is designed to eliminate I/O bottlenecks by utilizing
special I/O processors (IOPs) that handle the nitty gritty details
of interrupt handling, buffering and data transfer. In addition,
an I2O driver consists of an OS-specific module (OSM) that deals
with higher-level operating system details (such as accessing
files) and a hardware device module (HDM), that understands
how to communicate with specific devices. Because the OSM and
HDM are autonomous, they can perform a number of tasks independently,
without sending data over the I/O bus. I2O is designed to work
IDE (Integrated Drive Electronics)
A standard electronic interface used between a computer motherboard's
data paths or bus and the computer's disk storage devices. The
IDE interface is based on the IBM PC ISA 16-bit bus standard,
but it is also used in computers that use other bus standards.
Most computers sold today use an enhanced version of IDE called
EIDE. IDE gets its name because the disk drive controller is
built into the logic board in the disk drive. IDE was adopted
as a standard by ANSI in November 1990. The ANSI name for IDE
is Advanced Technology Attachment (ATA). The IDE (ATA) standard
is one of several related standards maintained by the T10 Committee.
IDE disks with Ultra DMA can now run at up to 33Mb/s, but because
they can not be daisy-chained in the same way as SCSI and Fibre
drives, they require a special RAID controller. The RAID controllers
uses Ultra-Wide SCSI for a host channel, because IDE has a very
limited cable length and less bandwidth, so although IDE disks
are used, the RAID appears to the host as a normal SCSI drive.
A new, very fast external bus * standard that supports data
transfer rates of up to 400 Mbps (400 million bits per second).
Products supporting the 1394 standard go under different names,
depending on the company. Apple, which originally developed
the technology, uses the trademarked name FireWire. Other companies
use other names, such as i.link and Lynx, to describe their
1394 products. A single 1394 port can be used to connect up
63 external devices. In addition t/o its high speed, 1394 also
supports isochronous data -- delivering data at a guaranteed
rate. This makes it ideal for devices that need to transfer
high levels of data in real-time, such as video devices. Although
extremely fast and flexible, 1394 is also expensive. Like USB,
1394 supports both Plug-and-Play and hot plugging, and also
provides power to peripheral devices. The main difference between
1394 and USB is that 1394 supports faster data transfer rates
and is more expensive. For this reason, it is expected to be
used mostly for devices that require large throughputs , such
as video cameras, whereas USB will be used to connect most other
(1) With hardware, initializing a disk means formatting it.
(2) In programming, initialize means to assign a starting value
to a variable. (3) Initialize can refer to the process of starting
up a program or system.
Describes any operation, program or device that transfers data
to or from a computer. A typical I/O device includes a printer,
hard disk, keyboard and mouse. Some devices are basically input-only
devices (keyboard and mouse); others are primarily output-only
devices (printer) and others provide both input and output of
data (hard disk, diskette, writeable CD-ROMs).
Intelligent Platform Management Interface (IPMI) is a hardware
level interface specification that defines a common, abstracted,
message-based interface to platform monitoring and control functions.
As a hardware-level interface, it sits at the bottom of a typical
management software stack. Thus, IPMI is “management software
neutral.” It can be exposed through any standard management
software interface, such as WMI, CIM, SNMP or DMI.
ISA (Industry Standard Architecture)
A standard bus (computer interconnection) architecture that
is associated with the IBM AT motherboard. It allows 16 bits
at a time to flow between the motherboard circuitry and an expansion
slot card and its associated device(s).
LAN (Local Area Network)
A network of interconnected workstations sharing the resources
of a single processor or server within a relatively small geographic
(Multibank Dynamic RAM)
A type of video RAM, developed by MoSys, that divides memory
into multiple 32 KB parts or "banks" that can be accessed individually.Having
individual memory banks allows accesses to be interleaved concurrently,
increasing overall performance. It's also cheaper since, unlike
other forms of video RAM, cards can be manufactured with just
the right amount of RAM for a given resolution capability instead
of requiring it to be in multiples of megabytes.
Short for disk mirroring; Also known as RAID Level 1. A form
of RAID in which the Array Management Function maintains two
or more identical copies of data on separate disks. Also known
as RAID Level 1 and disk shadowing.
The physical arrangement in a computer that contains the computer's
basic circuitry and components.
NVRAM - Non-Volatile Random Access Memory
A type of memory that retains its contents when power is turned
off. One type of NVRAM is SRAM that is made non-volatile by
connecting it to a constant power source such as a battery.
Another type of NVRAM uses EEPROM chips to save its contents
when power is turned off. In this case, NVRAM is composed of
a combination of SRAM and EEPROM chips.
OLCE - Online Capacity Expansion
Provides the capability of adding drive space to existing arrays
without the need for rebooting the system. When using OLCE,
the system dynamically reallocates the existing information,
while realigning the array to provide for the additional drive(s).
The quality of being either odd or even. The fact that all
numbers have a parity is commonly used in data communications
to ensure the validity of data. This is called parity checking.
PC100 compliant SDRAM is the latest memory standard. This
new memory is a new standard for SDRAM, capable of providing
memory access time by following the new JEDEC standard of SDRAMs
and is fully backward compatible with existing memory systems.
This new specification is the latest module design to fully
support the Intel 440BX AGPset and a new generation of mainboard
series power, PC100 SDRAM modules is strongly recommended. Due
to the strict timing issues involved when operating at 100MHz
frequency, using non-compliant memory modules may cause systems
to fail to boot. Even if the system does boot, changes to the
operating environment such as temperature or certain hardware
applications will severely impact memory reliability. PC100
compliant modules must be embedded with all PC100 compliant
SDRAM chips. Although most memory modules shipped today are
specified or marked as 10ns, which are often called 100MHz,
they don't actually perform at 100MHz all the time and, most
importantly, are not PC100 compliant. Some chip venders have
been developing special versions of PC100 compliant 8 ns modules,
but only a select group of them with very sophisticated test
capabilities will be able to meet the critical requirements.
PCI (Peripheral Component Interconnect)
An interconnection system between a microprocessor and attached
devices in which expansion slots are spaced closely for high-speed
operation. Using PCI, a computer can support both new PCI cards
while continuing to support ISA expansion cards, currently the
most common kind of expansion card. Designed by Intel, the original
PCI was similar to the VESA Local Bus. However, PCI 2.0 is no
longer a local bus and is designed to be independent of microprocessor
design. PCI is designed to be synchronized with the clock speed
of the microprocessor, in the range of 20 to 33 MHz.
PCI is now installed on most new desktop computers, not only
those based on Intel's Pentium processor but also those based
on the PowerPC. PCI transmits 32 bits at a time in a 124-pin
connection (the extra pins are for power supply and grounding)
and 64 bits in a 188-pin connection in an expanded implementation.
PCI uses all active paths to transmit both address and data
signals, sending the address on one clock cycle and data on
the next. Burst data can be sent starting with an address on
the first cycle and a sequence of data transmissions on a certain
number of successive cycles.
PCMCIA (Personal Computer Memory Card International Association)
An industry group organized in 1989 to promote standards for
a credit card-size memory or I/O device that would fit into
a personal computer, usually a notebook or laptop computer.
The PCMCIA 2.1 Standard was published in 1993. As a result,
PC users can be assured of standard attachments for any peripheral
device that follows the standard. The initial standard and its
subsequent releases describe a standard product, the PC Card.
RAID (Redundant Array of Independent Disks)
A way of storing the same data in different places (thus, redundantly)
on multiple hard disks. By placing data on multiple disks, I/O
operations can overlap in a balanced way, improving performance.
Since multiple disks increases the mean time between failure
(MTBF), storing data redundantly also increases fault-tolerance.
Random Access Memory. Any byte of RAM can be accessed
directly in a single memory cycle. Information can be read from
and written to RAM. RAM is volatile — when power is turned
off, it loses its memory.
RDRAM - Rambus DRAM
A type of memory (DRAM) developed by Rambus, Inc. Whereas
the fastest current memory technologies used by PCs (SDRAM) can
deliver data at a maximum speed of about 100 MHz, RDRAM transfers
data at up to 600 MHz. In 1997, Intel announced that it would
license the Rambus technology for use on its future motherboards,
thus making it the likely de facto standard for memory architectures.
However, a consortium of computer vendors is working on an alternative
memory architecture called SyncLink DRAM (SLDRAM). RDRAM is already
being used in place of VRAM in some graphics accelerator boards,
but it is not expected to be used commonly for the main memory
of PCs until late 2000 or 2001. Intel and Rambus are also working
a new version of RDRAM, called nDRAM, that will support data transfer
speeds at up to 1,600 MHz (1.6GHz).
RISC - Reduced Instruction Set Computer (Pronounced risk)
A type of microprocessor that recognizes a relatively limited
number of instructions. Until the mid-1980s, the tendency among
computer manufacturers was to build increasingly complex CPUs
that had ever-larger sets of instructions. At that time, however,
a number of computer manufacturers decided to reverse this trend
by building CPUs capable of executing only a very limited set
of instructions. One advantage of reduced instruction set computers
is that they can execute their instructions very fast because
the instructions are so simple. Another, perhaps more important
advantage, is that RISC chips require fewer transistors, which
makes them cheaper to design and produce. Since the emergence
of RISC computers, conventional computers have been referred to
as CISCs (Complex Instruction Set Computers). There is still considerable
controversy among experts about the ultimate value of RISC architectures.
Its proponents argue that RISC machines are both cheaper and faster,
and are therefore the machines of the future. Skeptics note that
by making the hardware simpler, RISC architectures put a greater
burden on the software. They argue that this is not worth the
trouble because conventional microprocessors are becoming increasingly
fast and cheap anyway. To some extent, the argument is becoming
moot because CISC and RISC implementations are becoming more and
more alike. Many of today's RISC chips support as many instructions
as yesterday's CISC chips. And today's CISC chips use many techniques
formerly associated with RISC chips.
RLM - RAID Level Migration
Allows a user to change RAID levels. Dynamic RLM allows the user
to do this without restarting the system.
SAF-TE - SCSI Accessed Fault-Tolerant Enclosure
Industry Standard for enclosure management support. SAF-TE
uses a common interface to continuously monitor elements such
as temperature, drive, power and fan status. In addition, SAF-TE
supported enclosures can provide status updates to LAN administrators
via any SAF-TE-compliant device
SCSI (Small Computer System Interface)
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 supported by all major OSes.
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 seven 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.
SDRAM (Synchronous Dynamic Random Access Memory)
A generic name for various kinds of DRAM that are synchronized
with the clock speed that the microprocessor is optimized for.
This tends to increase the number of instructions that the processor
can perform in a given time. The speed of SDRAM is rated in MHz
rather than in nanoseconds (ns). This makes it easier to compare
the bus speed and the RAM chip speed. You can convert the RAM
clock speed to nanoseconds by dividing the chip speed into 1 billion
ns (which is one second). For example, an 83 MHz RAM would be
equivalent to 12
SIMM - Single In-Line Memory Module
A small circuit board that can hold a group of memory chips.
Typically, SIMMs hold up to 8 (on Macintoshes) or 9 (on PCs) RAM
chips. On PCs, the ninth chip is often used for parity error checking.
Unlike memory chips, SIMMs are measured in bytes rather than bits.
SIMMs are easier to install than individual memory chips. The
bus from a SIMM to the actual memory chips is 32 bits wide. A
newer technology, called dual in-line memory module (DIMM), provides
a 64-bit bus. For modern Pentium microprocessors that have a 64-bit
bus, you must use either DIMMs or pairs of SIMMs.
SMBIOS - System Management BIOS
Allows manufacturers to develop structures to access attributes
that are known by the system BIOS, but have no standard interface
to management software.
UDMA (Ultra DMA or Ultra DMA/33)
A protocol for transferring data between a hard disk drive through
the computer's data paths (or bus) to the computer's random access
memory (RAM). The Ultra DMA/33 protocol transfers data in burst
mode at a rate of 33.3 MBps (megabytes per second), twice as fast
as the previous Direct Memory Access (DMA) interface. Ultra DMA
was developed as a proposed industry standard by the Quantum Corporation,
makers of hard disk drives and Intel, makers of chip sets that
support computer bus technology.
Ultra DMA support in your computer means that it will boot (start)
and open new applications more quickly. It will also help users
of graphics-intensive and other applications that require large
amounts of access to data on the hard drive. Ultra DMA uses Cyclical
Redundancy Checking (CRC), offering a new level of data protection.
Because the Ultra DMA protocol is designed to work with legacy
PIO and DMA protocols, it can be added to many existing computers
by installing an Ultra DMA/33 PCI adapter card. Ultra DMA uses
the same 40-pin IDE interface cable as PIO and DMA.
ULTRASCSI (Ultra160 / Ultra320)
A method that enables very fast data transfer rate on the SCSI
bus. The maximum UltraSCSI data transfer rates are 20 MBytes/second(160Mbits/second)
and 40 MBytes/second(320Mbits/second) for Wide SCSI host adapters.
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
USB supports a data speed of 12 megabits per second. This speed
will accommodate a wide range of devices, including MPEG-2 video
devices, data gloves and digitizers. It is anticipated that USB
will easily accommodate plug-in telephones that use ISDN and digital
Si dnce 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. Today, most new computers and peripheral devices are equipped
with USB. A different "plug-and-play" standard, Firewire/IEEE
1394, is designed to support much higher data rates and devices
such as video camcorders and digital videodisk (DVD) players.
Both standards are expected to exist together, serving different
WAN (Wide Area Network)
A geographically dispersed telecommunications network and the
term distinguishes a broader telecommunication structure from
a local area network (LAN). A wide area network may be privately
owned or rented, but the term usually connotes the inclusion of
public (shared user) networks. An intermediate form of network
in terms of geography is a metropolitan area network (MAN).
Write-Through / Write-Back Cache
When the processor writes to main memory, the data is first written
to cache memory, assuming that the processor will probably read
this data again soon. In write-through cache, data is written
to main memory at the same time it is written to cache memory.
In write-back cache, data is only written to main memory when
it is forced out of cache memory. Write-through caching is simpler
than write-back because an entry to cache memory that must be
replaced can be overwritten in cache memory because it will already
have been copied to main memory. Write-back requires cache memory
to initiate a main memory write of the flushed entry followed
(for a processor read) by a main memory read. However, write-back
is more efficient because an entry can be written many times to
cache memory without a main memory access.
XOR - eXclusive OR
A Boolean operator that returns a value of TRUE only if just one
of its operands is TRUE. In contrast, an inclusive OR operator
returns a value of TRUE if either or both of its operands are
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