BIOS
Acronym for basic input/ output system. It is a set of instructions stored in Read Only
Memory (ROM) that lets your computer's hardware and operating system communicate with
application programs and peripheral devices such as hard disks, printers and video adaptors.
BIOS extensions
In the PC, extensions to the main BIOS that enable the computer to work with add-on
devices, such as hard disk controllers and EGA or VGA adaptors. The ROM chips containing
these extensions do not have to be located on the motherboard; they can also be on expansion
boards plugged into the expansion bus. Any BIOS extensions needed to run these expansion
boards are loaded automatically when you boot your computer.
BIOS parameter block
It is a part of the boot record contained on every formatted disk that contains information
about the disk's physical characteristics. This information includes the version number of the
operating system used to format the disk, the number of bytes per sector, the number of sectors per cluster, per track and per disk, and is provided for use device drivers.
ABIOS
Acronym for Advanced Basic Input/ Output System. A set of firmware service routines built
into the IBM PS/2 series of computers that use the Micro Channel Architecture (MCA) to
support multitasking operating systems such as OS/2.
Address
1. The precise location in memory or on disk where a piece of information is stored. Every byte in memory and every sector on a disk have their own unique addresses.
2. To reference or manage a storage location.
Address bus
The electronic channel, usually from 20 to 32 separate lines wide, used to transmit the
signals that specify locations in memory. The number of lines in the address bus determines the number of memory locations that the processor can access, as each line carries one bit of the address.
Absolute addressing
The explicit specification of memory location by using its address rather than by using an
expression from which the address can be calculated. Also known as direct addressing.
Relative addressing
The specification of a memory location by using an expression to calculate the address,
rather than explicitly specifying the location by using its address.
Adaptor
A printed circuit board that plugs into a computer's expansion bus to provide added
capabilities. Common adaptors for the PC include display adaptors, memory expansion adaptors, input/ output adaptors that provide serial, parallel, and games ports, and other devices such as internal modems, CD-ROMs, or network interface cards. One adaptor can often support several different devices; for example, an input/ output adaptor may support one parallel port, a games or joystick port, and several serial ports.
Accelerator board
An add-on printed circuit board that replaces that main processor with a higherperformance
processor, so you can upgrade your system without replacing monitor, case, keyboard, and so on.
Using an accelerator board can reduce upgrade costs substantially.
However, there are other factors to consider, such as disk access time, in determining the overall performance of your system.
Graphics accelerator board
Aspecialised expansion board containing a graphics coprocessor as well as all the other
circuitry found on a video adaptor. By offloading most of the graphics processing tasks from the main processor onto the graphics accelerator board, you can improve the performance of your system considerably, particularly if you are a Microsoft Windows user.
Access
To retrieve data from a storage device such as a hard disk, or to login to a computer system
or network.
Access denied
In DOS, an error message indicating that your last command attempted to act on a file or
directory in a way that DOS does not allow.
You will see this error message if you use a DOS command to try to change or delete a read -
only file (A file that had the read-only attribute set), or if you try to copy more files or directories onto a disk than there are directory entries for in the root directory of the disk.
The root directory can only hold a fixed number of file entries (depending on the type of disk).
Even though there may be plenty of free space, if there are no more empty entries in the root
directory, DOS cannot add your new file and responds with the error message. By the way, this
size limitation applies only to the root directory, not to ordinary subdirectories.
Access mechanism
In a floppy or hard disk drive, the component that positions the read/ write head over the
surface of the disk, so that data can be read from or written to the disk.
Access Server
In networking, a computer that provides access for remote users dialling in, and allows
those users to access network resources as though they were working at a computer directly
attached to the network. Access servers can be computers designed for this purpose and sold as
part of the network, or can be made of multiport CPU cards installed in a PC on the network.
Access time
The period of time that elapses between a request for information from disk or memory,
and the information arriving at the requesting device.
Memory access time refers to the time it takes to transfer a character from memory to or from the processor, while disk access time refers to the time it takes to place the read/ write heads over the requested data. RAM may have an access time of 80 nanoseconds or less, while hard disk access time could be 18 milliseconds or less.
Account
On local area networks or multi user operating systems, an account is set up for each user.
Accounts are usually kept for administrative or security reasons, although in communications and online services, accounts are used as a method of identifying a subscriber for billing purposes.
ACK
Abbreviation for acknowledgement. In communications, a control code, ASCII06, sent by
the receiving computer to indicate that the data has been received without error, and that the
next part of the transmission may be sent.
Active-matrix screen
An LCD display mechanism that uses an individual transistor to control every pixel on the
screen.
Active-matrix screens are characterized by high contrast, a wide viewing angle, vivid colours, and fast screen refresh rates, and they do not show the streaking or shadowing that is common with cheaper LCD technology.
Active partition
That part of the hard disk containing the operating system to be loaded when you start or
restart the computer.
You can install two different operating systems (perhaps DOS and OS/2) on your hard disk,
but each must be in its own separate area, or partition. Only one portion can be active at any
given time, and to change from the DOS to the non-DOS partition, you may have to use the DOS FDISK command.
Active window
In an operating system or application program capable of displaying multiple windows on the
screen at the same time, the active window is the window that contains the cursor. If a window is active, its title bar changes colour to differentiate it from all the inactive windows. Only one
window can be active at a time.