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Pascal
A high-level programming language developed by Swiss professor Niklaus Wirth in the early 1970s and named after the French mathematician, Blaise Pascal. It is noted for its structured programming, which caused it to achieve popularity initially in academic circles. Pascal has had strong influence on subsequent languages, such as Ada, dBASE and PAL.

Pascal is available in both interpreter and compiler form and has unique ways of defining variables. For example, a set of values can be stated for a variable, and if any other value is stored in it, the program generates an error at runtime. A Pascal set is an array-like structure that can hold a varying number of predefined values. Sets can be matched and manipulated providing powerful non-numeric programming capabilities.

The following Turbo Pascal example converts Fahrenheit to Celsius:

   program convert;
   var
   fahr, cent : real;
   begin
    write('Enter Fahrenheit ');
    readln(fahr);
    cent := (fahr - 32) * 5 / 9;
    writeln('Celsius is ',cent)
   end.
passive matrix
A common LCD technology used in laptops. Passive matrix displays (DSTN, CSTN, etc.) are not quite as sharp and do not have as broad a viewing angle as active matrix (TFT) displays, but they have improved dramatically over the years. Looking head on into a passive matrix screen is not all that different than looking at an active matrix (TFT) screen. The difference is more noticeable with the viewing angle. A person looking from the side sees a dimmer image with passive matrix. See LCD.
patch cord
A short length of cable used to connect ports in patch panels or in expansion boards and systems that are in close proximity. It is typically a telephone, data communications, audio or video cable that is generally no longer than 10 feet in length. Both ends of the cable are terminated with plugs or sockets. See patch panel.
patch panel
A group of sockets used to connect incoming and outgoing lines in communications and electronic systems. Patch panels allow for manually wiring the connections with small cables (patch cords), rather than automatic switching. Wireless patch panels are also available that provide the cross connections by flipping a switch rather than plugging in wires. See wiring rack and patch cord.



Rack Mounted Patch Panels Patch panels come small and large. In large datacenters such as this one, patch panels are often rack mounted. Not remarkably high tech, they are simply used to manually connect one line to another.
path
(1) In communications, the route between any two nodes. Same as line, channel, link or circuit.

(2) In database management, the route from one set of data to another, for example, from customers to orders.

(3) A selected line or area in an image.

(4) The route to a file on a disk. In DOS, Windows and OS/2, the path for file MYLIFE located in subfolder STORIES within folder JOE on drive C: looks like:

            c:\joe\stories\mylife
PC
(3) (Printed Circuit)

(2) (Personal Computer) Any laptop or desktop computer such as Windows machine or a Macintosh.

(1) (Personal Computer) A stand-alone laptop or desktop computer running Windows (or DOS for earlier applications). PC hardware and operating systems are primarily governed by Intel and Microsoft respectively. The PC is the world's largest computer base.

PCs are also widely used as clients and servers in a local area network (LAN). PC clients predominantly run under Windows, but PC servers (x86-based servers) run under Windows, NetWare or a variation of UNIX such as Linux or UnixWare. PC servers may use Windows 95/98, but Windows NT and 2000 are more likely choices.

Although there are literally thousands of PC vendors, from mom and pop shops to large mail order houses (Dell, Gateway, etc.) to the major computer companies (Compaq, HP, etc.), and of course IBM, still one of the world's largest PC makers, all PCs use an Intel x86 or compatible CPU.

After IBM introduced the PC in 1981, the first attempts at cloning it were mostly unsuccessful. Except for Compaq's first PC, from 1982 to 1985, there were a lot of "almost compatible" PCs. However, as soon as the part of the operating system known as the BIOS was successfully cloned and made commercially available, true compatibles appeared in abundance.

Before Windows 95, adding another peripheral device to a PC was often an exercise in trial and error. Modifying DOS's infamous configuration files (AUTOEXEC.BAT and CONFIG.SYS) caused many a user to give up. Windows 95, 98 and 2000 added Plug and Play, which means for the most part, you can replace hard disks and display adapters, as well as add a scanner, CD-ROM or other device without difficulty.

The PC has become a commodity item, winding its way onto the shelves of retail outlets worldwide. This is a testimonial to the power of a computer standard, even one fraught with loopholes and inconsistencies.

Today, most PCs run most software and work with most plug-in boards. However, with the myriad of adapters and applications available, one device, application or utility can always conflict with another. The way to guarantee that something works is to try it. This has been true since day one in the computer business.
PC data buses
The bus in a PC is the common pathway between the CPU and peripheral devices. Parallel buses use slots on the motherboard and provide multiple lines for data (32 bits, 64 bits) between the CPU and peripheral card. Cards plug into the bus inside the cabinet. Serial buses have external ports, and the cable that plugs into them can connect to multiple devices. Following are the buses used in the PC:

Parallel Buses (Current)

PCI - Peripheral Component Interconnect
The PCI bus, available in 32- and 64-bit versions, is the most popular bus architecture. It is used in PCs as well as many other platforms. In 2002, PCI Express was introduced, providing greatly enhanced speeds. See PCI and PCI Express.

AGP - Accelerated Graphics Port
The 32-bit AGP bus was designed for faster screen display. If used, there is just one AGP slot on the motherboard, and only the display adapter plugs into it. See AGP and display adapter.

Serial Buses (Current)

USB - Universal Serial Bus
One USB port connects up to 127 peripherals. The first version of USB was designed for low-speed peripherals, while USB 2.0 increased speed significantly. See USB.

FireWire (IEEE 1394)
FireWire connects up to 63 peripheral devices and has been mostly used for digital camera connections. See FireWire.

Parallel Buses (Earlier)

ISA - Industry Standard Architecture
Pronounced "eye-suh," ISA stems from the original PC. It was an 8-bit bus originally known as the PC bus and then the XT bus. It was later extended to 16 bits and became the AT bus and eventually the ISA bus. See ISA.

Micro Channel (MCA)
IBM switched to the 32-bit Micro Channel with its PS/2 line in 1987, then later added back ISA. Eventually, it gave up Micro Channel for PCI. See Micro Channel.

EISA - Extended ISA
Pronounced "ee-suh," this bus was a 32-bit extension of ISA created by major vendors to counter IBM's Micro Channel. EISA slots accepted both EISA and ISA cards, but clock speed was still at the slow ISA rate. EISA was used in servers but later abandoned for PCI. See EISA.

VL-bus - VESA Local Bus
The 32-bit VL-bus was introduced during the 486 era and offered higher speed than ISA. It then gave way to PCI. See VL-bus.




Types of Expansion Boards Of the six parallel bus types in this illustration, all have disappeared from new systems with the exception of PCI and AGP.

PC keyboard
(1) Any keyboard made for the PC.

(2) The keyboard introduced with the first IBM PC in 1981 that provided a dual-function keypad for numeric entry and cursor movement. It was severely criticized for its non-standard shift key placement, which was corrected with the AT keyboard in 1984. Regardless of key placement, users have always loved the feel of IBM keyboards.
PCI
(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.
PCI Express
A high-speed peripheral interconnect from Intel introduced in 2002. Note that although sometimes abbreviated "PCX," PCI Express is not the same as "PCI-X" (see PCI-SIG and PCI-X for comparison). As a result of the confusion, "PCI-E" is the accepted abbreviation.

Initially used for high-speed display adapters, and intending to eventually replace the PCI and AGP buses entirely, PCI Express was designed to match the higher speeds of today's CPUs. It can accommodate Gigabit and 10 Gigabit Ethernet and even support chip-to-chip transfers.

Switched Architecture
Rather than the shared, parallel bus structure of PCI, PCI Express provides a high-speed, switched architecture. Each PCI Express link is a serial communications channel made up of two differential wire pairs that provide 2.5 Gbits/sec in each direction. Up to 32 of these "lanes" may be combined in x2, x4, x8, x16 and x32 configurations, creating a parallel interface of independently controlled serial links. The bandwidth of the switch backplane determines the total capacity of a PCI Express implementation.

Internal First, External Later?
Similar in architecture to InfiniBand, PCI Express was designed for internal connections, whereas InfiniBand provides a true fabric architecture for external networks. However, enhancements that extend PCI Express outside of the box for several meters are expected in time.

Different Slots
Originally called "Third Generation I/O" (3GIO), PCI Express is software compatible with PCI, but not plug compatible. It also uses slots of different lengths because of the combined lanes (see below). For a comparison of all PCI technologies, see PCI-SIG. See ExpressCard and PCI-X.




PCI Express Slot PCI Express slots are both shorter and longer than PCI and AGP. The x1 slot is a single-lane implementation, while the x16 slot combines 16 lanes for peripherals such as the display adapter (video card).




PCI-Express Vs. AGP This display adapter from NVIDIA was one of the first x16 PCI Express cards on the market. PCI Express, AGP and PCI cards all have different edge connectors to ensure that they fit into the appropriate slots. (Image courtesy of NVIDIA Corporation.)
PCI-X
(PCI eXtended) An enhanced PCI bus technology originally developed by IBM, HP and Compaq that is backward compatible with existing PCI cards. PCI and 32-bit PCI-X slots are physically the same, and PCI cards can plug into PCI-X slots. PCI-X cards will run in PCI slots, but at the slower PCI rates. The 64-bit PCI-X slots are longer.

First introduced in 1999, PCI-X offered increased speed over PCI and has steadily increased to more than 30 times that of the original PCI bus. Do not confuse PCI-X with PCX, which is the abbreviation of PCI Express. For a comparison of all PCI technologies, see PCI-SIG.




PCI-X Slots The two long green slots on this Gigabyte motherboard are 64-bit PCI-X slots, which will accept all 32-bit and 64-bit PCI-X and PCI cards. (Image courtesy of Giga-byte Technology Company Ltd., www.giga-byte.com)
PCI-SIG
(PCI-SIG, www.pci-sig.com) The PCI Special Interest Group (SIG) is an industry consortium that owns and maintains all PCI technologies. Volunteers from many organizations are involved. Following is a comparison of PCI bus speeds. See PCI, PCI-X and PCI Express.


 BANDWIDTH OF PCI-SIG TECHNOLOGIES
 This data courtesy of PCI-SIG.


       Year  Bus Size Clock       Date Rate
 Ver.  Intro (bits)   Speed       MBytes/sec
 
 PCI - Parallel Shared
 1.0  1992  32          33Mhz       133
 2.0  1993  32/64     33Mhz       133/266
 2.1  1995  32/64     66Mhz       266/533.3
 2.2  1998  32/64     66Mhz       266/533.3
 2.3  2002  32/64     66Mhz       266/533.3


 PCI-X 1.0 Parallel Shared
 66   1999  32/64     66Mhz       533


                           Data Rate
                           Gbytes/sec


 PCI-X 1.0 Parallel or Shared or Point-to-Point
 133  1999  32/64     133Mhz      .5/1.06


 PCI-X 2.0 Parallel or Point-to-Point
 266  2002  16/32/64  266MTs      .5/1.06/2.13
 533  2002  16/32/64  533MTs      1.06/2.13/4.26


 PCI Express (PCX) - Serial Switched Point-to-Point
      2002      x1   "2.5 Gbps      .5
      2002      x2    per              1.0
      2002      x4    lane            2.0
      2002      x8    per             4.0
      2002     x16    direction"   8.0
      2002     x32                    16.0


 MTs = megatransfers per second (see MT/sec).
 Megatransfers are used when megahertz is not
 accurate (not actual clock cycles).
 PCI-X 266 is running at 133MHz, but data is
 double clocked.  PCI-X 533 is quadruple clocked.
PCMCIA
(Personal Computer Memory Card International Association, San Jose, CA, www.pc-card.com) An international standards body and trade association that was founded in 1989 to establish a standard for connecting peripherals to portable computers. PCMCIA created the PC Card. See PC Card.
PC Card
A credit-card sized, removable module for portable computers standardized by PCMCIA. PC Cards are also known as "PCMCIA cards." PC Cards are 16-bit devices that are used to attach modems, network adapters, sound cards, radio transceivers, solid state disks and hard disks to a portable computer. The PC Card is a "plug and play" device, which is configured automatically by the Card Services software (see below).

All PC Cards are 85.6 mm long by 54 mm wide (3.37" x 2.126") and use a 68-pin connector. The original Type I card is 3.3 mm thick and is typically used to hold memory.

Type II cards (5.0 mm thick) are commonly used for memory, modems and LAN adapters in laptops. Type III cards (10.5 mm thick) are used to hold a hard disk, wireless transceiver or other peripheral that needs more space. The Type III slot can hold two Type II cards.

Toshiba introduced a 16 mm Type IV card, but this has not been officially adopted by the PCMCIA. Smaller cards will work in a Type IV slot.

Card and Socket Services
 In order to use a PC Card slot in the computer, Card and Socket services must be loaded, typically at system startup. Card and Socket Services software is generally included with laptops that have PC Card slots. It also comes packaged with PC Cards.

Card Services manage system resources required by the PC Card, and, on PCs, determines which IRQs and memory and I/O addresses are assigned. They also manage hot swapping and pass changes in events to higher-level drivers written for specific cards.

Card Services talk to Socket Services, which is the lowest level of software that communicates directly with the PC Card controller chips. Socket Services can be built into the system BIOS or added via software.

CardBus
 In early 1995, PCMCIA introduced the 32-bit CardBus standard. Although electrically different, the CardBus is architecturally identical to the PCI bus. The CardBus supports bus mastering and accommodates cards operating at different voltages. Its advanced power managment features allows the computer to take advantage of CardBus cards designed to idle or turn off in order to increase battery life. The CardBus specification allows data transfer up to 132 Mbytes/sec over a 33MHz, 32-bit data path. For more information, visit www.pc-card.com. See PCMCIA.



The PC Card The PC Card (left) on laptops is the equivalent expansion mechanism as the printed circuit board (right) is on desktop computers.
printer
A device that converts computer output into printed images. Following is an overview of the various technologies.

For more details, look up the individual entries.

GENERAL CATEGORIES
SERIAL PRINTERS (CHARACTER PRINTERS)
Serial printers print one character at a time moving across the paper. Electrosensitive, direct thermal, older daisy wheel and even ink jet printers could be cataloged in this group; however, the primary desktop serial printer is the serial dot matrix printer, with speeds ranging from 200 to 400 cps, which is about 90 to 180 lines per minute (lpm).

LINE PRINTERS
Line printers print a line at a time from approximately 400 to 2,000 lpm and are commonly found in datacenters and industrial environments. Earlier technologies included drum, chain, train and dot band matrix technologies. The surviving technologies use band and line matrix mechanisms.

PAGE PRINTERS
Page printers print a page at a time from four to more than 800 ppm. Laser, LED, solid ink and electron beam imaging printers fall into this category. All of these printers adhere toner or ink onto a drum which is transferred to the entire page in one cycle for black and white and multiple cycles for color.


IMPACT PRINTERS
SERIAL DOT MATRIX
A desktop printer that uses a moving printhead of wire hammers. It forms characters and graphics by impacting a ribbon and transferring dots of ink onto the paper. See dot matrix printer.

LINE MATRIX
A type of line printer that uses an oscillating row of print hammers. The hammers form characters and graphics by impacting a ribbon and transferring dots of ink onto the paper. See line matrix printer.

BAND (LINE CHARACTER)
A type of line printer that uses a fixed set of characters attached to a continuously revolving metal band. A set of hammers (one for each column) hit the paper, pushing it into the ribbon and against the character image on the band. See band printer.

EARLIER IMPACT TECHNOLOGIES
Impact printers were developed for the first computers, and several earlier technologies have gone by the wayside.

Chain, train and drum printers were precursors to band printers. They all used actual shaped characters, or type slugs, to print a fixed size and style of letter and digit. Daisy wheel printers were desktop impact printers used in the 1970s and 1980s. Dot band matrix printers used a combination of band printer and dot matrix methods.


NON-IMPACT PRINTERS
LASER & LED
Laser printers and LED printers employ the electrophotographic method used in copy machines. Both technologies are available from small desktop units to high-speed digital printing presses, ranging in speed from four to more than 700 ppm, and color units from three to 75 ppm. See laser printer, LED printer and electrophotographic.


INK JET
Ink jets have become the most popular form of desktop, personal printer. Most all units can print in color or have a color option. Ink jets propels droplets of ink directly onto the paper. See ink jet printer.

IRIS
IRIS printers use ink jet technology, but are in a class by themselves. They achieve a perceived 1,800 dpi resolution and can print on fabric as well as paper.

SOLID INK
Solid ink printers use sticks of wax ink that are melted into a liquid. The ink is directed onto a drum, similar to a laser printer, and then transferred onto the paper to produce high-quality output.

ELECTRON BEAM IMAGING
A technology somewhat similar to a laser printer, except that electricity is used to create the image instead of light. This evolved from ion deposition and is used in very high-speed page printers exceeding 800 ppm.

THERMAL WAX TRANSFER & DYE SUBLIMATION
Dots of ink or dye are transferred from a ribbon onto paper by passing the ribbon and the paper across a line of heating elements. Thermal wax is used for bar code and other types of labels as well as medium-resolution graphics. Dye sublimation is used for photorealistic color output. See thermal wax transfer printer and dye sublimation printer.

ELECTROSENSITIVE
A dot matrix printhead charges dots on aluminum-coated silver paper, usually in a serial fashion. The charge removes the coating, leaving a black image.

DIRECT THERMAL
Used in bar code and other specialty printers as well in earlier fax machines, dots are burned onto a type of coated paper that darkens when heat is applied to it. See direct thermal printer.

ELECTROSTATIC
Dots are charged onto a coated paper, typically a line at a time. A toner is attracted to the paper and made permanent by pressure or heat. See electrostatic plotter.





The Paperless Office? Based on the projections above, it seems the paperless office is hardly around the corner. Even though more documents will be maintained electronically in the future, more documents will still be printed.
PRISM
(1) (PhotoRefractive Information Storage Materials Consortium) A collaboration of IBM, Stanford University, GTE, Hughes Research Labs, Optitek, SRI International and Rockwell Science Center that is funded by the U.S. government's Advanced Research Projects Agency for the purpose of researching holographic storage.

(2) (PRogrammable Integrated Scripts for Mirror) The programming language for the Mirror communications programs.
PROM
(Programmable ROM) A permanent memory chip in which the content is created (programmed) by the customer rather than by the chip manufacturer. It differs from a ROM chip, which is created at the time of manufacture. PROMs are used for storage when their content is not expected to change, but in many applications, they have given way to EPROMs and EEPROMs, which can be reprogrammed. See memory types and EEPROM.
proxy server
Also called a "proxy," it is a computer system or router that breaks the connection between sender and receiver. Functioning as a relay between client and server, proxy servers are used to help prevent an attacker from invading the private network. Proxies are one of several tools used to build a firewall.

The word proxy means "to act on behalf of another," and a proxy server acts on behalf of the client and the server. All requests from the clients to the Internet go to the proxy server first. The proxy evaluates them, and if allowed, re-establishes the requests on the outbound side to the Internet. Likewise, responses or initial requests coming from the Internet go to the proxy server to be evaluated. The proxy then talks to the client. Both client and server think they are communicating with one another, but, in fact, are dealing only with the proxy.

Address Translation and Caching
The proxy server is a dual-homed host with two network interfaces and two IP addresses. The IP address on the outbound side of the proxy is the one the Internet sees, and the address of the machine making the request is hidden to the outside world. Proxies are often used in conjunction with network address translation (NAT), which hides all the IP addresses of the client machines on the internal network. Proxy servers may also cache Web pages, so that the next request for that same page can be obtained much faster locally.

Application Level and Circuit Level
Proxy servers are available for common Internet services; for example, an HTTP proxy is used for Web access; an FTP proxy is used for file transfers. Such proxies are called "application-level" proxies or "application-level gateways," because they are dedicated to a particular application and protocol and are aware of the content of the packets being sent. A generic proxy, called a "circuit-level" proxy, supports multiple applications. For example, SOCKS is a generic IP-based proxy server that supports TCP and UDP applications (see SOCKS).

Other Proxies
Without being called a proxy specifically, the Internet's e-mail system (SMTP) is an example of a proxy server because it stores and forwards messages. E-mail is not sent directly from client to client without going through the mail server. Likewise, the Internet's Usenet news system (NNTP) forwards messages to neighboring servers. See firewall.



A Proxy Server in a LAN In this LAN server illustration, the proxy server sits between two routers in what is known as a "demilitarized zone." See DMZ.
PS/1
An early IBM home computer series introduced in 1990. The original models featured an integrated monitor and easy-to-open case. The first PS/1 was a 286 with an ISA bus. See PC.
PS/2
An IBM personal computer series introduced in 1987, superseding the original PC line. It introduced the 3.5" floppy disk, VGA graphics and Micro Channel bus. The 3.5" disks and VGA are common in all PCs, but the Micro Channel has since given way to the PCI bus. See also PS/2 connector.



PS/2 Model 50 The 286-based Model 50 was one of IBM's early PS/2 models. The PS/2 introduced the Micro Channel bus, VGA graphics and the 3.5" floppy disk. (Image courtesy of International Business Machines Corporation. Unauthorized use not permitted.)
PS/2 connector
A 6-pin Mini-DIN plug and socket used to connect a keyboard and mouse to a computer. First introduced on IBM's PS/2 desktop PC, the port was later used by everybody else, first on laptops, then on desktops. See DIN connector.




PS/2 Connector The PS/2 connector uses a 6-pin Mini-DIN plug and socket.

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