Phil Storrs PC Hardware book

Computer Glossary

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ABT - Advanced BiCMOS Technology , a high speed BiCMOS logic family with traditional TTL pinouts but superior performance characteristics.

AC/ACL - Advanced CMOS logic, a high speed CMOS logic family.

Address Space - The amount of memory a CPU is designed to access. Address space is limited by the the size of the address bus available from the processor. 8088/8086 CPUs havd a 20 bit address bus and could access one megabyte (220 power) of memory. The 80286 and 80386SX CPUs have a 24 bit address bus and can access 16 megabytes (224) of memory. The 80386DX, the 80486 and Pentium processors have a 32 bit address bus and can access more than 4 gigabytes (232) of memory.

ACCESS.bus - A standard desktop high speed four-wire serial bus that was intended to link computers to a wide variety of input and output devices, such as keyboards, mice, bar-code readers, magnetic card readers, scanners and joysticks. As of 1998 it looks as though this technology has died without gaining any significant support and has been replaced by USB or FireWire.

A/D Converter (A to D) - A device that takes analog inputs and converts them to digital signals. For example, a sine wave input can be converted into a series of numbers that represents the value of the input curve at any given point on the curve. The resolution of the converter is determined by the number of bits (the more the better) and the sample rate (the faster the better).

Asynchronous - Transmission in which each data byte is preceeded by a start bit and followed by one or more stop bits. Data transmission is intermittent with an irregular time interval between data bytes.

AT computer (PC/AT) - This is a computer based on the 80286 processor. This processor 16 bit internal and external data buses and a 24 bit address bus. This processor can address up to 16Meg byte of memory. The external bus first used in this computer is called the ISA bus and is still the most common PC bus.

ATX Format (System Board) - A System Board form factor that has the common I/O port connectors on the back edge of the System Board. See also NLX and Baby AT format.

Baby AT Format (System Board) - A compact System Board format first used in the late 1980's. This format does not have the common I/O port connectors on the board itself, they must be mounted on the back plane. Like very old PC System Boards, only the Keyboard connector is on the board.

Backside bus - A special microprocessor bus inside the processor package that connects the CPU to a Level 2 cache. Typically, a backside bus runs at a faster clock speed than the frontside bus that connects the CPU to the main memory. For example, the Pentium Pro microprocessor actually consists of two chips, one contains the CPU and the primary level 1) cache, and the second contains the secondary (level 2) cache. A backside bus connects the two chips inside the processor package, at the same clock rate as the CPU itself (at least 200 MHz). In contrast, the frontside bus connects the the processor to the outside world and hence the RAM and runs at only a fraction of the CPU clock speed.

Baud - the rate of data transmission speed. The number of discrete signal events per second. In asynchronous transmission, baud is the unit of signaling speed corresponding to 1 unit interval per second. Baud is the same as bits per second, if and only if each signal event (1 or 0) represents one data bit exactly. The term Bits Per Second is used today as it is more correct.

BIOS - The BIOS (Basic Input Output System) routines are stored in ROM memory and provide instructions for system startup and communication with hardware devices. BIOS routines are stored on a chip and are non-volatile. BIOS routines, like other programs, use memory address locations. Because many of the BIOS routines are not needed at the time DOS is operating, there is the potential to use some of these BIOS addresses for additional high DOS memory.

BIOS Extension ROM - These are ROMs located on plug in I/O cards inserted into bus slots. They are used to supply BIOS support for devices the computers built in BIOS does not know about. The most common example of a BIOS Extension ROM is fitted to EGA or VGA video interface cards. The BIOS ROM fitted to all DOS computers does not have routines to handle the more advanced video standards supplied by these cards, the BIOS only knows about CGA and MDA video systems.

Bisync - An IBM defined byte controlled communications protocol using control characters and synchronized transmission of binary coded data for half-duplex communication between stations. Bitstream - Refers to the 1-bit pulse density modulated (PDM) code output from the quantizer in the noise-shaping stage.

BTL - Backplane Transceiver Logic. Differs from TTL logic levels in that logic High voltage is 2.1 volts and a logic Low voltage is 1 volt, a 1.1 volt swing, versus the 2.4 V TTL minimum swing. Used in Futurebus+ to increase bus speeds and reduce output capacitance and improve noise margin.

Bus - a common pathway or channel between hardware devices. Can be serial (information travels one-bit at a time) or parallel (information travels in groups of bits simultaneously moving along multiple parallel paths).

Bus topology (also called Daisy Chain topology): All devices are connected to a central cable, called the bus or backbone. Ethernet systems use a bus topology and Floppy drive and IDE interfaces use this topology as does the SCSI interface.

Cache - A 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.

CAN - Control Area Network. A two-wire serial bus designed to reduce the size of the wiring harness in automobiles. Also used in industrial applications.

Cards or Boards - These are plug in boards that are used to provide I/O functions in a DOS computer. They are plugged into the Bus Slots in the computers System Board. The most common "cards" found in DOS computers today are the Video Interface card and the IDE/FDC/SPG interface card. This card supplies the hard drive interface (IDE), the floppy disk drive interface (FDC), and the Serial, Parallel and Games ports (SPG).

Compact Disc ReWriteable (CD-RW) - A new type of CD disk that enables you to write onto it in multiple times. With CD-RW drives and disks, you can treat the optical disk just like a floppy or hard disk, writing data onto it multiple times. The capacity is 650 Meg byte. The first CD-RW drives became available in mid-1997. They are able to read CD-ROMs and can write onto today's CD-R disks, but not on CD-ROMs. Many experts believe that they will be a popular storage medium until DVD devices become widely available.

Coaxial Cable - A two-conductor (center conductor and shield) concentric, constant-impedance transmission cable, used for CATV installations and Local Area Networks. Coaxial cable gives very good protection against interference because of its heavy shielding.

CPU - Central Processing Unit. The functional unit in a processor that does the computing. It reads instructions and data from memory and performs the arithmetic and logic operations defined by the instruction. A complete computer system includes CPU, main memory, clock circuits, and input and output devices.

Conventional Memory - The first 640 K byte of the addressable memory space, usually occupied by RAM memory. This is where the operating system (DOS) and the application programs operate. The address range is from 00000 to 9FFFF hex.

Daisy Chain - A method of connecting devices on a bus or interface connection. An example is the way IDE (EIDE) devices are connected to the interface, the cable starts at the interface and goes from there to each device in turn.

Device Driver - An operating system enhancement program loaded during boot up by CONFIG.SYS and remaining in memory. Such programs perform communication functions between the computer and attached hardware devices such as hard disks, scanners, or local area networks.

Disk Caching - This 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 improves the performance of the system because accessing data in Cache RAM can be much faster than accessing data on a hard disk drive.

DMA - Direct Memory Access. A method in which a device other than the main processor can access main memory directly. It must first obtain control of the bus from the processor or other DMA devices. Then it can read and write to memory directly. This method is a fast method of transferring large amounts of data from a peripheral devoce to the main memory. Using the main processor to handle the data transfer requires more software overhead and reduces system throughput.

Duplex - Simultaneous two-way independent transmission in both directions. Also known as Full Duplex.

Digital Versatile Disc (DVD) - The next generation of CDROM device with capacities from 8.4GByte to 17GByte and data transfer rates as high as 10MB/s. A full length movie can be stored on a single DVD disk with several sound tracks and subtitle tracks.

Expanded Memory - Also called EMS (Expanded Memory Specification) memory, this uses a bank switching technique that allows up to 32 MB of memory to be accessed in 16 KB portions, by switch it in and out of a 64K Page Frame area addressed within the Reserved memory area. Some DOS programs could use Expanded Memory for temporary storage to improve performance. EMS memory can be implemented through memory board hardware using a device driver to provide the EMS services, or by using a memory manager device driver that uses Extended Memory to emulate EMS memory.

Extended Memory - Memory addressed above the 1 Meg Byte limit of the DOS operating System, imposed by the 20 address lines available to the 8088 processor. This memory is physically addressable by 80286 or later generation CPUs when operating in protected mode. Extended memory is not normally directly accessible to DOS. Applications and advanced operating systems can make use of memory above one Meg if they are aware of it.

EEPROM - Electrically Erasable Programmable Read-Only Memory. Special ROM memory that can be erased and programmed electrically, often the original system socket. Very useful in applications where control bits or small amounts of data must be updated without physically removing the EEPROM device. EPROM - Erasable Programmable Read-Only Memory. A special ROM memory that can be erased when exposed to strong doses of ultraviolet (UV) light. After erasing, EPROM memory can be reprogrammed. Erasing requires physically removing the device and placing it under a strong UV light for several minutes. Programing requires placing it into a special PROM programmer.

Ethernet - A baseband local area network to interconnect computer equipment using coaxial cable and transceivers. Became the basis of IEEE 802.3 baseband 10 Mb/s standard. Can also be supported using twisted-pair media.

FAST - An industry standard TTL logic family first introduced by Fairchild in 1978.

Fat Client - Term used to describe a Networked Computer. Not to be confused with a Network Computer or Thin Client. The alternative to a Fat Client is a Thin Client.

FCT - A high speed CMOS logic family with traditional corner pinouts for power and ground.

Fibre Channel - A serial data transfer architecture developed by a consortium of computer and mass storage device manufacturers and now being standardized by ANSI. Using optical fiber to connect devices, FC-AL (the most prominent Fibre Channel standard), supports full-duplex data transfer rates of 100MBps. FC-AL is compatible with, and is expected to eventually replace, SCSI for high-performance storage systems.

FireWire - Also called by the name of its specification number, IEEE 1394. An advanced serial bus system designed by Apple with a transfer rate of 400 Megbits/sec, and plans for an even faster version with a 1 Gbyte transfer rate. FireWire allows for 63 devices on a single bus. More advanced than the USB interface.

Form Factor - Term used to describe the dimensions of particular PC hardware. Used when describing Disk Drives and System Boards.

Frontside bus - The bus within a microprocessor that connects the CPU to the ouside world and the main memory. In contrast, a Backside bus connects the CPU to a Level 2 cache inside the processor case.

Full Duplex - Simultaneous two-way independent transmission in both directions.

Futurebus+ - The IEEE896.1 standard bus, a scalable, high performance bus designed for high bus-transfer speeds and distributed multiprocessing.

GAL (Generic Array Logic) - generic term for all programmable logic devices that have a fixed (hard wired) OR array. Similar to PAL.

Half Duplex - An alternating one way at a time transmission method.

HDLC - High Level Data Link Control. An ISO version of a bit-oriented data link control protocol.

High Level Language - Writing a High Level Language program requires an editor and a Compiler program. The programmer writes instructions using high level instructions such as DO... WHILE and IF... THEN... ELSE. The compiler program takes the instructions and translates them into sequences of 1's and 0's that the CPU can understand. The final output is machine code or binary executable code. For microcontrollers like the 80C51, this code is then placed in ROM or EPROM to control operation of some sysem or device.

High Memory Area - The HMA is a 64 KB region of extended memory starting at 1 MB. The HMA is made available to DOS by using the DOS device driver, HIMEM.SYS. this memory is made available to DOS by a trick in the way the processor accesses this area of memory.

Hot docking - Aother name for Live Insertion

ISA Bus (Industry Standard Architecture) - This is the 16 bit bus first introduced by IBM with the AT computer. It provides 16 data lines and 24 address lines.

I2C or Inter-Integrated Circuit bus - a simple, inexpensive two-wire serial bus used to control IC components in a system. One wire, SDA, carries the data between components and the other wire, SCL, carries the clock timing information that synchronizes the data transmission at speeds of up to 100 kilobits per second. Up to 40 components can be controlled on a single I2C bus at lengths of up to 25 feet. Philips manufacturers over 100 I2C bus components including microcontrollers, LCDs, I/O expanders, RAM memories, clock/calendars, video and audio devices.

L1 cache (Level one cache) also called the Primary Cache - Cache memory that is built inside the processor chip. This technology started with the 80486 processor.

L2 cache (Level two cache) also called the Secondary Cache - Cache memory that was external to the microprocessor. Until recently the L2 cache memory resided on the System Board in separate chips. The Pentium Pro and then the Pentium II processors have the L2 cache built into the processor package to give faster access to this memory.

LAN - Local Area Network. A communications network limited in size (typically 1 to 20 km), such as would be used in a single office building, a warehouse or campus.

LIM Memory - Another name used for EMS (expanded) memory, referring to the companies Lotus, Intel, and Microsoft that originated the specification.

Live Insertion - A feature that permits users to insert or remove a device from a socket on the backplane (a bus socket) while power is on and the operating system is operating. The operating system will detect the device and make it available. See also Hot Docking.

LM 78 - This is an integrated data acquisition system for monitoring the hardware in PC Computers. It can be used to monitor Power Supply voltages, temperatures (including Processor temperature) and fan speed.

LS-120 - A high capacity Floppy Disk system with a storage capacity of 120 MByte. LS-120 can also read and write 720 K and 1.44 Meg Floppy Disks. An alternative name of Super Disk is also being used.

Machine Code - The binary (1's and 0's) instructions that are readable by the CPU. The lowest level of instructions.

Memory Cache - Often simply called a Cache or RAM Cache. This is a 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 the cache, the computer avoids accessing the slower DRAM.

Microcontroller - A complete microprocessor system on a chip. Includes on-chip the CPU, local RAM, local ROM or EPROM, clock and control circuits, and serial and parallel I/O ports.

Microprocessor - The computing heart of a system placed on a single chip. Includes CPU, internal registers, control logic and bus interfaces to external memory and I/O ports. Some advanced systems also include floating point processors and some memory.

Mother Board - See System Board.

NLX Format (System Board) - A low profile System Board form factor that uses a riser card to support Bus Slots parallel to the System Board, and has the common I/O port connectors on the back edge of the System Board, like the ATX Format. Designed to reduce the overall height of the PC. See also ATX and Baby AT format.

PAL - generic term for all programmable logic devices that have a fixed (hard wired) OR array. Trademark of AMD.

Parallel Interface - a multi-line channel that transfers several bits, Eight parallel bits controlled and accessed simultaneously are considered a single 8-bit parallel port.

PC or PC/XT Computer - These are first generation DOS computers and use the 8088 processor. This processor has a 16 bit internal data bus and an 8 bit External data bus - these are often referred to as an eight bit DOS computer because the bus slots only provide an eight bit data path. The Address bus is 20 bit, limiting the addressable memory to One Meg byte.

PC96, PC97, PC98, PC99 - A series of definitions from Microsoft that represents the requirements of a generation of PC hardware that meets the requirements for wearing the "Designed for Windows" logo. As PC hardware is changing so rapidly the specifications change at least yearly.

PLA (Programmable Logic Array) - generic term for devices that have 2 programmable arrays and no registered elements. Note: many of Philips' sequencers are PLAs, with both buried and output registers added.

PLD - Generic term for all Programmable Logic Devices. PLDs are sometimes referred to as Sequencers.

Primary storage - This term is often used to refer to system RAM See also Secondary storage

Protected Mode - A mode of operation for the 80286 and above CPUs, which allows software running on them to access memory beyond 1 megabyte. Multi-tasking operating systems run in protected mode.

RAM - Random Access Memory. A memory that stores data and instruction in static or dynamic cells. Contents can be read and written freely. Static RAMs can be faster and require only that power be applied to maintain the memory contents. Dynamic RAMs require additional refresh circuitry that provides an electrical refresh pulse on a regular basis.

Real Mode - A mode of operation of an 80286 and above CPU that causes it to operate like the 8088/8086, and restricts its memory addressing capacity to 1 MB. DOS and most application programs must operate in real mode.

Reserved Memory Area - The memory between the 640K byte Conventional Memory area and the one Meg limit originally imposed on DOS by the design of the 8088 processor used in the original PC. This area is used for the Video Memory, the BIOS ROM, and by BIOS Extension ROMs. The address range of this area is A0000 to FFFFF hex.

Resolution - Refers to the number of pixels displayed on a video display and is quoted as X by Y figures. The CGA video system had a max. resolution of 640 by 200 and VGA has a basic resolution of 640 by 480.

Ring topology - All devices are connected to one another in the shape of a closed loop so that each device is connected directly to two other devices, one on either side of it.

ROM - Read Only Memory. A memory that stores data and instructions permanently. Contents are placed into memory at the time of manufacture and cannot be altered. Widely used in microcontroller applications.

SDLC - Synchronous Data Link Control. An IBM computer networking protocol associated with IBM's SNA architecture. The protocol provides for control of a single communications link or line, accomodates a number of networking arrangements, and operates in half- or full-duplex modes over private or switched lines.

Secondary storage - This term is often used to refer to magnetic media storage devices like Hard Drives and Magenetic Tape. See also Primary storage

Serial Interface - A data channel that transfers digital data (1's and 0's) in a serial fashion, one bit after another. Serial interfaces save conductors by requiring few lines compared to parallel interfaces but at the sacrifice of data transfer speeds.

Super Disk - See LS-120

Synchronous - Transmission where the data characters and bits are transmitted at a fixed rate with the transmitter and receiver synchronized. This eliminates the need for start-stop elements and provides greater transmission efficiency.

Star topology - All devices are connected to a central hub. This is the topology used with twisted pair (10baseT and 100baseT) network cabling.

System Board - This is the main board in the computer that has the processor, the RAM, the BIOS ROM, the timing and control, the interrupt and direct memory access hardware and the keyboard interface on it. Modern DOS computers also have the Hard and Floppy Disk Drive interfaces and the Parallel and Serial ports built into the System Board. Also referred to as a Mother Board.

System Management Bus (SMBUS) - A two wire interface through which simple devices can communicate with the rest of a computer system.

TSR - Terminate and Stay Resident (or Memory Resident) programs stay resident in memory after loading and remain active even when other programs are running. These programs usually provide a utility function, such as mouse control, telecommunication, or menu shells.

Thin Client - A Thin Client is a minimal computer system that has only the barest of Operating Systems. It loads everything else including Applications from a Network Server. An alternative name is Network Computer, not to be confused with Networked Computer. The opposite to a Thin Client is a Fat Client.

Topology - The way devices are interconnected on a Local-Area Network or Interface. There are three principal topologies, Bus topology (also called Daisy Chain topology), Ring topology and Star topology.

UART - Universal Asynchronous Receiver/Transmitter. A chip that provides all the funtions needed to implement a serial (communications) port in a computer. Provides the parallel to serial and serial to parallel conversions and the flow control required for one bidirectional communications channel.

Universal Serial Bus (USB) - A high speed Serial Interface Bus that allows mutiple devices (modem, mouse, keyboad, printer, optical scanner, camera etc) to be connected to the same Bus Connector via a simple four wire cable. Data rates as high as 12Mb/s and up to 127 devices connected to the same bus. An alternative high speed Serial Bus may be FireWire.

Video Memory or Video RAM - The 128K byte address region above the 640 KB of the conventional memory area used by Video Interface cards. The assigned addresses are from A0000 to BFFFF hex. The actual physical memory devices are mounted on the Video Interface card and are addressed in this memory space.

Windows CE - Is a Windows platform for a broad range of communications, entertainment and mobile-computing devices. It is an entirely new Operating System built from the ground up to make possible new categories of business and consumer non-PC based devices that can communicate with each other, share information with Windows-based PCs, and connect to the Internet.

XMS Memory - Microsoft's eXtended Memory Specification (XMS) for programs to request memory above the conventional memory area.

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Copyright © Phil. Storr, last updated 26th December 1998