Glossary

Address

Computers use addresses to keep track of information in much the same way as the post office uses them to find residences and businesses. The bigger the number in an address, the more locations it can refer to. Most current computers use a 32-bit "address space" for memory, which means that there can be over four billion separate locations to hold information. See Chapter 5 for more about addresses.

AGP

In an Intel system, the Advanced Graphics Port is a high speed bus that connects the graphics processor to the CPU/memory system.

Algorithm

An algorithm is another word for "formula." It is a set of rules and/or computations that is intended to be used over and over, like a recipe.

Analog

An analog system uses a representation of information, rather than a numerical version, in its processing. For example, traditional radio sends and receives sound with an electronic wave that is an analog of the voice, music, or noise waves that enter a microphone. Analog systems are well suited to carrying information, but not to modifying it.

Applet

In Java programming, an "applet" is a small special purpose application program that is designed to be downloaded over a network for use on a client machine.

Application program

A program that is at the command of a user is normally called an "application." By contrast, the operating system and other programs are usually known as "systems software."

Areal density

The number of bits that a disk drive can hold in a given space. The first generation of drives used in microcomputers had an areal density of 15 megabits per square inch; current drives have about 5,000–10,000 megabits per square inch (5–10 Gigabits).

ASCII

ASCII stands for "American Standard Code for Information Interchange." It uses 7 bits to reference 128 character letters, numbers, punctuation marks and some symbols used for system control. A version using 8 bits, "Extended ASCII," provides the same 128 plus another 128 special characters.

ATM

Asynchronous Transfer Mode (ATM) is a protocol stack developed as an international standard by the telephone companies. It uses packet switching technology (it's packets are called "cells"), but also creates virtual circuits that are effective in supporting streaming data. The small size of its cells is good for reducing jitter in voice and video, but inefficient for data.

Bandwidth

The capacity of a communications channel is its bandwidth. In computers, this is measured in bits per second. See also Chapter 10 for a primer on digital communications.

Batch processing

means that a program runs until it is complete with no user interaction. Also known as "batch mode."

Binary

Binary mathematics is a form that uses only two digits vs. the ten in our normal decimal system. Binary numbers are used in computing because they conform very well to the "on" and "off" states of electronic devices.

Biometrics

The processing of using some element of human anatomy—a fingerprint, voice pattern, etc.—to identify a person is called biometrics.

BIOS

The Basic Input/Output System (BIOS) refers to a chip in some types of microcomputers, especially those that use Microsoft operating systems. The BIOS, which can be thought of as an extension of the operating system, holds information about attached devices such as disk drives, external buses, etc. The BIOS also helps systems boot up. See Chapter 7 for more detail.

Bitmap

Bitmap has two meanings. In one, it simply refers to an image that comprises a pattern of dots. Using this definition, essentially all computer graphics—monitor or printer —are bitmaps. In the other definition, bitmap refers to how an image is made. A bitmapped image is created dot by dot for a specific size and resolution. Contrast with "vector."

BLOB

A Binary Large Object (BLOB) refers to a collection of data, usually video or sound information, that a relational database is able to access but not manipulate.

Booting

Computers are described as "booting up" after the observation of early designers that the system should "pull itself up by its bootstraps."

Border gateway protocol

Used in wide area networks, BGP is a router to router protocol that allows exterior routers to communicate with each other. More efficient than its predecessor, Exterior Gateway Protocol (EGP), because routers exchange only changes to their tables, not the entire table.

Branch

A point in a program where the CPU may have to switch to a different stream of instructions. A conditional branch is where the stream chosen depends on the result of some computation.

Bridge

A Layer 2 device that connects two local area networks and provides a filter so that only packets with appropriate addresses cross the bridge.

Buffer

Generally, a buffer is an area of a computer or other electronic system that holds data temporarily in order to match different device speeds or to compensate for the fact that one part is busy and unable to accept data. In the case of a network switch, a buffer holds incoming packets if an output port is busy.

Bus

A bus is a pathway that carries information between two or more parts of a computer. There are internal buses, such as the system (memory) and I/O buses, and external buses, such as USB or FireWire.

Cache

Cache refers to a special high-speed storage area. Memory cache is made from SRAM instead of DRAM. Disk cache refers to the use of memory to hold frequently used information that would take much longer to retrieve if secured directly from the disk.

Chipset

The chipset refers to a group of chips that provides an intelligent interface for the core components of a computer—CPU, memory, graphics, I/O system. Described as "core logic" or "glue logic."

Circuit

A circuit is a specific path from one point to another in a network.

Class

In Object-Oriented Technology, a "class" is a general category of objects that share many important characteristics.

Clock doubling

As CPU speeds increased in the mid-1980s, it quickly became clear that the memory bus could not keep up. This started the practice known as "clock doubling" because in the first iteration the processor's clock was twice as fast as that of the bus. Since then, however, the difference has gotten wider. For example, a 600 MHz Pentium III runs six times faster than it's 100 MHz bus.

Clock speed

CPUs (and other devices) are controlled by quartz crystal clocks. The consistent timing provided by the clocks helps to keep operations synchronized. Clock speed is usually measured in millions of cycles per second; abbreviated MHz.

Clone

Historically, "clone" has meant two different things when applied to CPUs. One is a chip that is "instruction set compatible," meaning that it can directly execute software designed for the CPU that it is cloning. The other meaning is "pin compatible," which means that the clone can fit into the same physical socket and make the same electrical connections as the target CPU. Contemporary clones are instruction set compatible, but pin compatibility is now less common—if one machine fails, the others take over for it.

Cluster

A cluster is a group of independent computers, typically 2–8, that are linked by high speed networks. Special software extends the operating system such that the cluster functions as one computer. Clusters are used in part to get more processing power and in part to provide extended reliability—if one machine fails, the others take over for it.

CMOS

Complementary Metal Oxide Semiconductor (CMOS) refers to an approach to making transistors that has proven to be incredibly productive. The big advantage of CMOS (pronounced "see-moss") is that it has significantly lower power consumption than rival technologies.

Compiled language

A programming language whose code is run through a compiler in order to create machine language directly is "compiled." Contrast with Interpreted language.

Computer graphics

The term "graphics" actually means "drawing," in both the computer and the real world. In the computer context, however, it has an additional meaning— "graphics" are images that are not limited to the small set of alphanumeric characters (e.g., ASCII) of the original mainframe and personal computers.

Connectionless

An unmanaged network link is called connectionless. "Manage" in this sense means that, unlike connection-oriented links, the two ends don't talk to each other about how to handle errors or the speed at which packets are sent. A sends packets to B, but doesn't make any attempt to communicate with B about the status of the packets (though B might ask to have one or more resent).

CPU

The Central Processing Unit (CPU) is the part of the computer that carries out the actual computation—arithmetic and logical operations. In the earliest computers the CPU used many chips, but today's computers nearly all use microprocessors, in which all functions are on one chip.

CRT

A Cathode Ray Tube is the principal type of display now used for computer monitors and televisions. The CRT uses scanning electron beams to draw an image on the surface of a tube. It is likely to be replaced in the near future for both types of applications by one or more of the flat panel technologies.

Cyclical redundancy check

A form of error detection that improves on parity checking. It uses a more complex calculation that is much less prone to error and more efficient because it can operate effectively on a block of data rather than just a byte at a time.

DAC

A digital to analog converter takes a computer's digital information and translates it to the format that can be used by an analog display such as a CRT.

Database index

In a database, those fields that are specially designated for fast sorting and retrieval are the indexed fields. Normally, the index is kept in memory for the fastest retrieval.

Database tuning

Deciding how a particular database should be organized for a certain machine—e.g., whether some or all indexes should be held in memory—is called "tuning" the database. For complex systems, especially those with multiple processors, careful tuning can make a huge difference in performance.

DBASE

One of the first DBMS for the PC was a program called DBASE. Published by a company called Ashton Tate, DBASE created a standard both for a data storage format and for a database programming language. DBASE survived into the mid-1980s, but its developers were too slow to adapt to the graphical environment presented by the Macintosh and Microsoft's Windows.

DBMS

A database management system (DBMS) is a program, or more likely a group of programs, that are responsible for data storage. Additional DBMS functions include a programming language to facilitate data entry and retrieval, a report writer, analysis tools, and soon.

Debugging

The process of testing computer code to find out if there are errors is called debugging.

DES

The Data Encryption Standard (DES) is a widely used secret key method of encryption.

Device driver

A software module that attaches to the OS and allows programs to use an external device such as a printer, a display system, a disk drive, etc. is called a device driver.

Dictionary-based compression

A form of lossless compression in which sender and receiver build dictionaries that allow them to send short symbols in place of long, frequently recurring strings of information. Often called LZW compression after its originators.

Digital

A digital system is one that translates all of the information it works with to numbers—binary numbers in the case of computers. Compare to "analog."

Digital certificate

A digital certificate is a means of verifying that a digital signature belongs to a specific person or organization. Digital certificates are maintained by Certifying Authorities (CAs).

Digital envelope

A digital envelope uses a secret key to encode a message, then uses a public key to carry the secret key. This is more efficient than using a public key for everything.

Digital signature

The digital signature is a means of using public key encryption to verify the identity of a sender.

Direct 3D

A 3D graphics language that is similar to Open GL, but limited to Microsoft operating systems.

Domain

In general networking usage, a domain, also called an "autonomous system," is a network administered by a single business or organization. When used with the Internet, "domain" also refers to the organization of addresses—for example ".com" is a "top level domain" in Internet parlance.

DRAM

Dynamic Random Access Memory (DRAM) is a type of chip-based storage that has the advantage of being reasonably fast and inexpensive, but the disadvantage of being "volatile"—losing its contents when electrical power is cut off.

DTD

XML uses a Data Type Definition (DTD) to allow one Web source to tell another how to read and work with specialized information.

Dynamic HTML

A variation of HTML, Dynamic HTML, allows designers considerable additional flexibility in page layout as well as the opportunity to add things like animation.

Embedded processor

An embedded processor is a CPU that is built into a device, usually a single purpose one such as a cell phone or an automotive engine management system. Many embedded processors are stripped down versions of standard desktop CPUs.

Encryption

The process of converting text and/or numbers to a disguised format is called encryption.

Enterprise

Software systems that are used across a large, complex business are called "enterprise" systems. For example, Oracle's software works at the enterprise level, but Microsoft's Access database is a workgroup product.

ERP software

Database software that is designed to integrate the diverse functions of a large organization (sales, payroll, inventory, marketing, etc.) is called Enterprise Resource Planning (ERP) software. Vendors such as SAP and PeopleSoft have made fortunes helping businesses replace a heterogenous mix of databases with a single interface and a high level of interactivity.

Error detection/ correction

Since most computer connections require that the same data be received as is sent, error detection and correction information is added to a packet or group of packets so that the receiver can be sure that it has accurate information. If not, it can ask that the packets or group of packets be resent.

Fabless

A semiconductor design firm that has another company fabricate (manufacture) its chips is called "fabless." Since there is a lot of foundry capacity out there, this is a comparatively low risk way to get into the chip business.

FAT

The File Allocation Table (FAT) is a small database kept on a disk. The FAT contains information about all the files on the disk, including where the various parts of the files are kept.

Fault tolerant

A computer that can suffer failure from one or more parts and keep on running is called "fault tolerant." For the most part, this ability is created by the presence of redundant parts in the computer (e.g. one hard drive will "shadow" another).

Firewall

A firewall is a server that sits at the interface of a local and a wide area network in order to provide some security function. Typically, a firewall hides the computers on a LAN so that outsiders cannot see and attack them.

Flash memory

Flash is a type of memory that is non-volatile. Flash cells are less dense than DRAM, so they are more expensive.

Flow control

When two computers are connected, they can use flow control information to manage the data rate to a level that the receiver can handle.

Font

A font is a collection of letters (and associated characters) in the same typeface, style, and point size.

Forward error correction

Systems like voice or video that can't resend bad information without a penalty in quality add redundant information to the bitstream. If the receiver finds an error, it can use the redundant data it has received, as opposed to a new transmission, to correct it. FEC is not flawless, but it is highly effective.

FTP

The File Transfer Protocol (FTP) is one of the original applications that were specified for the Internet Protocol suite (usually known as TCP/IP). FTP, as the name suggests, is responsible for transferring files over an IP network.

Garbage collection

Garbage collection refers to the need for a programmer to make sure that code that has been loaded into memory is removed when no longer needed. If this isn't done, the "garbage" will soak up memory and possibly cause conflicts. Software tool kits for developers provide code and techniques for dealing with this problem, but some programmers manage to leave garbage behind anyway.

Geometry

In 3D graphics, "geometry" refers to the work the CPU does in calculating the size and place of objects that are themselves made up of many polygons.

Grayscale

Refers to an image that can show shades of black (gray), though the term is also used for color. In a true grayscale image, each dot can be a number of different shades. Most printing approaches simulate this with "halftoning" or "dithering." A color (including black) that can be shown in 256 shades has "256 grayscale definition" and requires 8 bits of memory.

Halftoning

Refers to the process in printing of mixing white areas in with color to get different shades. Also called "dithering." In effect, this approach simulates grayscaling.

Hardware abstraction layer

The HAL (Hardware Abstraction Layer) is the equivalent of the microkernel in Microsoft's Windows NT/ 2000. Its principal purpose is to make it easier to port the OS to a variety of CPUs/ hardware platforms.

Hertz

The term "Hertz" (abbreviated Hz) is a measure used to describe the number of cycles per second in an activity. Examples: the CPU's clock speed is given in millions (MHz) or billions (GHz) of Hertz; the number of times a monitor image is redisplayed is described in Hz, etc. Also, the frequency of electromagnetic waves, the number of wave crests passing a point in a second, is given in Hz. For example, the frequency used by digital cellular telephones is 1.9 GHz (see Chapter 10).

HTML

The Hyper Text Markup Language (HTML) is the programming language used to create Web documents. HTML uses as system of tags (visible in most browsers by clicking on something like View-Page Source on the menu bar), that define a page's layout.

HTTP

The Hypertext Text Transfer Protocol (HTTP) is responsible for moving documents across the Web. The Universal Resource Locater (URL) in your Web browser, e.g. www.prenhall.com, is part of HTTP.

Hub

A hub is a network device that receives a signal from an attached computer and sends it on to other attached machines. A hub is a relatively unsophisticated device.

I/O

The term "input/output" (I/O) refers generally to movement into and out of the computer's CPU/memory system. Since this is frequently to and from disks, it is often called "disk I/O," although I/O can also refer to printers, to network connections, etc.

IEEE

The Institute of Electrical and Electronics Engineers sets international standards for many of the devices used in computing.

IETF

The Internet Engineering Task Force (IETF) is responsible for setting many Internet-related standards, for example the new version of the Internet Protocol, IPv6.

Inheritance

One way in which Object-Oriented Technology simplifies program development is through "inheritance." This simply means that, in creating a sub-class of a larger class, a programmer can have the sub-class inherit some of the characteristics of the class rather than restating them. This saves time and reduces the possibility of error.

Instance

In Object-Oriented Technology, an "instance" is an individual object within a specific class.

Instruction set

The instruction set refers to both the instructions that a CPU can execute and the way in which they are organized. CPUs from different vendors have different instruction sets—unless one is a "clone" of the other.

Interpreted language

A programming language that uses a program, called an interpreter, to create machine language only while the interpreter is running on the computer, is called "interpreted." Contrast with "compiled."

Intranet

A private network (separated from the Internet) that uses the Internet's TCP/IP structure, is called an Intranet. This term is used very loosely.

IP

The Internet Protocol, IP, is a Layer 3 routable protocol that can deal with very large networks. It carries either TCP or UDP and can ride on a range of lower level protocols.

IPv6

The current version of the Internet Protocol (IP) is version 4, written IPv4. The next generation, now being introduced, is IPv6. IPv6 has many advantages, including the capacity for far more separate numbers, and the ability to maintain quality of service (QoS).

ISP

An Internet Service Provider (ISP) provides the link from a residence or business to the regional or backbone levels of the Internet. ISPs range from small mom and pop operations to giant companies like AT&T.

Kernel

The kernel is the component ("layer") of the operating system that works directly with the hardware (especially the CPU and memory).

Label switching

An effort to combine the speed of switching with the flexibility of routing is know as label switching. The idea is that, once a routing decision is made, packets can flow through a switched virtual circuit instead of requiring continual packet by packet routing decisions.

LAN

A local area network (LAN) usually connects up to around 50 or so computers within a limited physical space—such as a small building or a floor of a larger one. The computers on a LAN are connected through a hub or a switch. LAN protocols, almost always Ethernet or Token Ring, operate at layer 2 of the protocol stack.

Latency

In communications, latency refers to delay in transmission of packets. While data connections can normally tolerate a fair amount of latency, anything more than a very small quantity will degrade the quality of voice and video connections.

Leased line

A connection, sometimes a physical wire but more often a channel in a wire or cable, that a telephone company makes available on a rental basis for data communications.

Logic gate

A logic gate is a series of transistors, connected in a way that allows it to carry out a mathematical or logical operation such as addition. Logic gates are grouped into electrical circuits that execute the CPU's instructions such as to "add" two numbers or "compare" two values.

Loop

In programming, a loop is code that performs some repetitive chore, repeating a computation until a specific event occurs. For example, a loop might go through a list of a company's sales staff, calculating bonuses according to sales information. The loop would end when the entire list had been evaluated. Writing efficient loops is a central part of programming.

Lossy compression

A compression technique that can't precisely recreate the original is called lossy. These approaches are obviously not used for sending spreadsheets or other computer data, but are widely employed for the transmission of voice and video. In many cases, a great deal of such information can be "lost" without humans being able to discern the difference between the source and the copy.

lpi

Printers measure the quality of graphics output in lines per inch (lpi). This takes into account the clusters of dots needed to simulate grayscaling.

Memory pages

memory is organized into blocks called "pages" that are typically 2–4 Kb in size. Rather than move an individual word from disk to memory (or vice versa), the entire page is moved.

Method

In Object-Oriented Technology, a "method" is program logic that is a part of an object and acts on its data.

Microkernel

The microkernel is a subset of an operating system's kernel that contains only those elements that interact directly with the local hardware. Separating these parts from the rest of the kernel simplifies "porting" the OS from one CPU/hardware platform to another.

Mission critical

The term "mission critical," borrowed from NASA and with obvious meaning, is used to refer to database systems that, should they fail, would bring the business or organization down with them.

Modular programming

A hot idea in programming is to segregate code into discrete units or modules, each of which maintains responsibility for specific functions. The approach is really a more structured extension of earlier ideas; object-oriented technology is inherently modular.

Modulation

The process of modifying a wave so that it can carry information is known as modulation. There are three ways of modulating a wave: changing its amplitude, frequency, or phase.

Moore's Law

Intel pioneer Gordon Moore stated that the number of transistors on a chip would double every eighteen months, and that their cost would fall by 50% during the same time.

MPEG

The Motion Picture Experts Group (MPEG) has developed a series of techniques for compressing streaming digital information so that it can be used more efficiently—employing less bandwidth in transmission or less capacity on a disk (DVD disks use MPEG coding). MPEG-1, which can deal with low to medium resolution video, has been replaced in most uses by MPEG-2, which can accommodate the original standards for high definition television. The most recent standard, MPEG-4, is being designed to get even greater efficiency through the use of vector graphics.

ms

A millisecond, abbreviated "ms," is one-thousandth of a second. The speed of access to data on disk drives is normally measured in milliseconds.

Multicasting

Sending a single stream of data to multiple receivers is called multicasting. The objective is to conserve network bandwidth by waiting until the last possible link to create a duplicate stream.

Multihoming

A network or computer that has two or more connections to the Internet or other wide area network is said to be "multihomed."

Network "pipe"

Network terminology often refers to wires or wireless channels as "pipes" because the concept of a pipe, with the size of its diameter regulating flow, is so easy to understand. Broadband access, usually meaning anything over about 1.5 MB/s, is often called a "fat pipe."

Network loop

To illustrate a network loop, consider the case of router A sending a packet to router B, thinking that it is on the path to network node C. But router B has a damaged routing table, which tells it that the way to get to C is through A. So it sends the packet to A, which consults its routing table again and sends it to B....

Object

In Object-Oriented Technology (OOT), an object is an element that includes both program logic (a "method") and data in a consistently structured way.

OLE

Microsoft's Object Linking and Embedding (OLE) allows a user to create an object (e.g. a table or a graphic) and embed it in another. For example, a table in Word could actually be a section of a spreadsheet created in Excel.

Open GL

A 3D graphics language that is available for a variety of hardware and software platforms. One advantage of the language is that it allows programmers to write software that is independent of the hardware. Open GL adapts the program to the capabilities of the system.

Open shortest path first (OSPF)

The most popular router to router communication protocol for interior networks. OSPF, which uses a link/state algorithm, is more efficient than the older RIP (Routing Information Protocol) because routers exchange only a subset, the active links, of their tables.

Overhead

In communications, everything that is sent that is not a part of the original data is overhead. ED/C adds to overhead, as does the various control information that goes with the use of packets (see Chapter 11).

Packet

In communications terms, a packet is a discrete collection of information that includes user data plus protocol information such as destination and source addresses, and controls for error detection/correction, sequencing, flow control, etc.

Parity checking

The simplest form of error detection is called parity checking. In this approach, the bits in a byte are summed—if the result is even, a "0" bit is added to the byte and if it is odd a "1" bit is added. Parity checking is used in the relatively interference free inside of computers, but not beyond that—it is prone to errors in high interference environments and adds a lot of overhead (12.5% more bits are required)

PGP

Pretty Good Privacy (PGP) combines public key and secret key cryptography.

Photolithography

The process of creating patterns on a wafer is known as photolithography. The technology is very similar to that employed in a photographic enlarger. Current processes use visible light, but alternative approaches, using x-rays or electron beams, are in development.

Physical Memory

is the amount of memory that can be directly addressed by a computer's hardware. It is limited by the maximum size and number of DRAM chips that can be accommodated in the system.

Pipeline

Engineers describe the path that instructions follow through a CPU—"fetch,""decode," "execute," "store," and variations—as its "pipeline." In a "pipelined" processor, every stage of the pipeline is doing work on every clock cycle.

Point of Presence (POP)

A point of presence (POP) is the location where one network touches another. The term is borrowed from telephone company jargon, in which a POP is the place where the local network connects to the long distance system.

Port

A port is a place where data can enter or leave the computer through some attached device. The term "port" is used to refer to both physical connections and their software address. The OS manages the use of ports.

Port cost

Devices such as switches and routers come with physical ports that are used for external connections (the points where cables snap in or screw on). As a way of providing consistent comparison, the cost of the various technologies is often given as the cost "per port."

Postscript

A page description language that describes how objects should be located on a printed page and how they should be scaled. Postscript uses vectors to create images and fonts and for layout commands.

Predicted frame

In a coding scheme like MPEG, some frames shown by the computer or television are created by the receiver based on information from a previously received frame. This saves a huge amount of bandwidth.

Private key

In public key cryptography, the private key is the part of the key pair that is not published but used to decrypt data encoded with the public key.

Protocol

A protocol is an agreed upon set of rules for the transmission of data. In packet networks, protocols usually have a variety of layers—"stacks."

Proxy server

A server that functions in place of another is called a proxy server. They can have many functions, including security and serving as a network cache.

Public key

In the public key approach to encryption, the public key is the part of a key pair that is given to others to use to encrypt data. The other part of the pair, the private key, is used for decryption.

QoS

While most data connections can tolerate a fair amount of latency, voice and video cannot. Quality of Service, QoS, refers to the process of providing bandwidth guarantees that keep latency within the limits specified for voice and video.

Rapid application development

An approach to programming that builds on a series of prototypes is called Rapid Application Development (RAD).

Raster

The term "raster," as in "raster image," is sometimes used as a synonym for any kind of bitmap. But "rasterization" also has the more specific meaning of using a scanning motion to create a pattern of dots—whether the original is a bitmap or vector based.

Ray casting

Since ray tracing is so computationally intensive that it can rarely be done in real time, most games employ a subset, called "ray casting." Instead of tracing lighting relationships for every pixel, ray casting does calculations for only a subset and typically limits the kinds of changes that are to be considered. The quality of the image is normally quite a bit lower as a result. Think of ray casting as the lite version of ray tracing.

Ray tracing

Ray tracing calculates the effects of light sources on objects in a 3D image.

Register

A register is an on-CPU storage space where instructions and data can be transferred and held temporarily for fast retrieval.

Relational database (RDBMS)

A relational database is one in which separate tables (usually in separate files), can be linked through common fields. Keeping information in separate, logically related groups most of the time, while retaining the ability to create multiple links when needed, provides a great deal of flexibility.

Rendering

The process of converting 3D information to a 2D screen image is called rendering. It is normally done by the graphics processor.

RGB

Color pixels in most types of monitors and televisions are built from the three primary colors of red, green, and blue. Displays using this approach are called "RGB."

RISC

"RISC" stands for "reduced instruction set computer" and refers to a CPU that is structured to gain efficiency through circuits designed to execute relatively few instructions at high speed.

ROM [vs. RAM]

Read Only Memory (ROM) is a kind of chip that has information permanently burned into it. Since ROM is non-volatile, this was a handy way to provide the computer with information that had to be readily accessible but didn't need to change. Early BIOSes used ROM, though Flash memory, which is also non-volatile but is faster and can be rewritten, is now preferred.

Router

A Layer 3 network device that analyzes incoming packets in order to make decisions as to where they should be sent. Routers are really special purpose computers; depending on the specific design, they can connect different physical networks and deal with multiple protocols.

Routing

The process of making decisions about where a packet or group of packets should go in a network is called routing.

Sampling

In order to make a digital representation of analog information, the original analog wave is sampled—checked at regular intervals. Each sample is given a numerical (binary) value. The result is that the digital system sends a stream of bits rather than an analog wave. The original wave can then be reproduced at the end. The more frequent the sampling, and the greater the range of values each can contain, the more accurately the wave can be reproduced.

SCSI

The Small Computer Systems Interface, a parallel I/O bus that has been used for both internal and external devices, has been popular, but may be yielding to USB and/or FireWire.

Shell

In operating systems such as Unix and its variants, the shell is a layer of the OS that sits between the user and the kernel. The user interacts only with the shell.

Sole sourcing

When a vendor is the only one to provide a component, it is the sole source. Manufacturers try to avoid this, since multiple suppliers can be less risky to deal with.

Source code

The written commands in a programming language, together with the comments that describe what they do, comprise a program's "source code."

Spaghetti code

Second generation programmers derisively referred to much of the work of their predecessors as "spaghetti code." Their criticism was that the code was written in a meandering way, including the frequent use of "goto" statements. This approach made it very difficult to understand the program's flow and therefore very difficult to maintain.

Spatial compression

Compression that operates within a given space, e.g. a video frame, is called "spatial."

Spectrum

The electromagnetic spectrum includes all forms of radiation from radio to gamma waves. Spectrum is often described in terms of frequency "bands" or ranges. The information carrying capacity of all of this is vast. The Federal Communications Commission has recently had a series of "spectrum auctions" to sell some of this capacity for wireless telephone services.

Speculative execution

A CPU that has circuits designed for speculative execution will execute the instructions after a branch just in case the program needs to go that way.

SRAM

"Static Random Access Memory" chips are similar to DRAM. They are faster, but use more space per bit of storage, so are more expensive.

Statistical compression

A form of compression that restructures the elements of a file or data stream such that those elements which are used most frequently get short symbols and those used least often get long ones. Often called Huffman coding after its inventor.

Structured programming

As an antidote to what they considered the unreadable (and hence difficult to maintain) "spaghetti code" of first generation programmers, second generation programmers advocated more carefully organized and documented code writing, which they called "structured programming."

Structured Query Language

Structured Query Language, abbreviated SQL, is a language that is employed for querying relational databases. Most implementations of SQL are specific to a particular database package.

Substrate

The surface on which something is placed is called its "substrate." Most CPUs, memory chips, etc., use silicon as the substrate, though Gallium Arsenide (GaAs) and Silicon Germanium (SiGe) are gaining somewhat. Magnetic disks uses aluminum or glass as the substrate.

Superscalar

A CPU that is "superscalar" has more than one pipeline.

Switch

A Layer 2 device that connects networks of a specific kind—e.g. all Ethernet. Switches operate faster than routers, but unlike routers, they can't see beyond other switches of the same kind.

Switched circuit

A dedicated path through a network that is not permanent, but can be established "on the fly" by a switching device.

TCP/IP

The TCP/IP stack (Transmission Control Protocol/Internet Protocol) is usually considered to be synonymous with the Internet. In fact, while all Internet links use IP at Layer 3, many transmissions use TCP's sister protocol, UDP, in Layer 4.

TCP

The Transmission Control Protocol, TCP, rides on top of IP and manages a connection for purposes of flow control, etc. Its sister Layer 4 protocol, UDP, is connectionless.

Temporal compression

This is compression that works on a moving or "streaming" image by eliminating redundancy from one frame to the next in the stream.

Texture

A texture is a predrawn pattern, stored and retrieved from memory as needed. A texture can convey complexity in an image that would otherwise require a great deal of computation. Heavily used in games.

Thrashing

A poorly written program can find itself constantly moving pages back and forth between memory and disk. Called "thrashing," this makes systems very inefficient.

Thread

A thread is a sub-process (program). It has no independent memory space, using instead resources assigned by its parent process.

Thunking

"Thunking" is a term used by programmers to describe the process of converting 16-bit addresses to 32-bits. The technique is widely used in Microsoft's Windows family to provide compatibility with older software.

Time sharing

The ability of a computer to run more than one program at a time was originally called "time sharing." The term "multitasking" is now used instead.

Trace

The wires that connect devices within a chip are so tiny that they are called "traces."

Transactional database (OLTP)

Transaction processing, also known as On Line Transaction Processing (OLTP), refers to a database that is accessed for the purpose of making changes to its records. For example, a library circulation system, which changes the status of a book from checked in to checked out (or vice versa) is responsible for transactions.

TrueType

A vector font (font scaling) technology developed by Microsoft and Apple and used in their operating systems.

UDP

The User Datagram Protocol, UDP, is an alternative to TCP. Like TCP, it rides on top of IP, but does not manage a connection for purposes of flow control, etc. It is truly connectionless.

Unicode

Unicode uses 16 bits for a character instead of the 8 bits employed with the original software systems. Because Unicode makes possible support for a whole range of languages at once, it is likely to become standard for documents in the near future.

Variable

In programming, a variable holds data in place and with a name that has been specifically designated by the programmer. The user or the program's logic can change the value as required.

Vector image

A vector image is built from a formula and can be scaled to different sizes and for different resolutions. Contrast with a bitmap, which is created dot by dot for a specific resolution. The final product of both methods is a bitmap, however.

Virtual circuit

Unlike a regular circuit, which refers to a physical path, a virtual circuit is a logical connection that has the characteristics of a circuit but can span a variety of physical links.

Virtual Memory

is an extension to a computer's physical memory that is provided by the OS. It allows the system to more easily manage multiple programs.

Virtual private network

A virtual private network (VPN) is one that uses the Internet protocols and connections, but encrypts packets at the entry and exit points to provide increased security.

VLIW

"Very Long Instruction Word" refers to a technique that is similar to SIMD, but supports the equivalent of multiple instructions in one word. In this manner, multiple instructions are feteched at the same time, alleviating the bottleneck of superscalar SIMD.

Wafer

A wafer is a slice of extremely pure crystalline silicon on which chips are fabricated. Current wafers are about 8″ in diameter, but 12″ wafer production is just beginning. Once the 12″ processes are perfected, the price of chips should fall even faster. Some, more exotic, processes use wafers made from other substances, such as Gallium Arsenide (GaAs) and Silicon Germanium (SiGe).

Wait state

A "wait state" occurs when the CPU has to wait for the system to fetch the next instruction or data that it needs

Wavelets

A very powerful approach to compressing graphical images uses a technique called wavelets. Wavelet-based compression builds on a complex mathematical technique originally applied to the analysis of music.

Word

A computer's "word" length is the amount of information that it can process at one time. Current desktop computers use 32-bit words; 64-bit systems will be commonplace soon.

Workgroup

The term "workgroup" is used for software that supports a fairly small collection of users who are working on the same project or who are responsible for a common area of a business or organization. A workgroup is usually connected with a local area network (LAN).

XML

The eXtensible Markup Language (XML) offers the possibility of creating Web documents that are far more powerful and flexible than HTML. XML documents can provide specialized commands that ease the manipulation of data sets.