List Of Definitions Paper 3

 List Of Definitions Paper 3

Term	Definition
Accumulator	-Stores results of calculations. -All input to and output from processor pass through the accumulator
Adaptive maintenance	-It changes to the system because external things have changed. -e.g. the sales tax rate changes
Backtracking	-If the result of one rule does not apply in a second rule, then go back to find another result of the first rule. -e.g. parent (john,dis) is found if we are searching for mother of dis. This fails the second part of the rule for mother because john is male, so backtracking is used to return to the next example satisfying the first part of the rule.
Bridges	-It links two LANs, which may or may not be similar. -It uses address information in packets. -It has the ability to learn the layouts of the networks.
Class	-A template for creating objects that share a common behaviour and common structure
Compiler	-Compiler translates entire program before run. -Compiler creates an object code. -Compiler must be present for translation.
Conceptual level	-Abstract representation of the whole database and the relationship between tables.
Corrective Maintenance	-It is needed to correct bugs found in the system once it is working. - e.g. There is no warning given that the till roll is about to end.
Current instruction register	-It holds the instruction while it is being executed. -Contains both operational code (op-code) and address(operand)
Data description language (DDL)	-It is used to define the data tables. - Specifies data types. - Specifies constraints on the data
Data encapsulation	- Data can only be accessed by the methods provided by the class. - e.g. Name can only be accessed from the class Person.
Data manipulation language (DML)	It allows the user to: - Insert,  Update, Delete, Modify/edit, Retrieve data
Declarative Language	-The programs are expressed as a number of rules and a set of facts. - The program specifies what must be done, not how to do it.
Direct implementation	The old system is removed and the new system replaces it completely and immediately.
Directory services	-Allow users to manage access to their own files while maintaining lack of access to others. -Maintain directory of software/services available to specified users.
Duplication of data on a network	Copy of entire database sent to and used by other machines
Dynamic data structure	Dynamic data structure can alter size dependent upon the number of items stored

Encryption	-Makes messages unintelligible -Provides security for data by making it impossible to understand -Key used to encrypt data and another to decrypt it. -Use of public and private keys
Exponent	-Is a two’s complement integer which -holds the power of 2 -by which the mantissa must be multiplied... -to give the original value.
External level	-External level gives the different views of the data seen by each of the users.
Flat file	-All the data is held in a single file -which is two dimensional -rows for records and columns for fields
Foreign key	-The primary key of another table that is used to link tables together.
Fragmentation of memory	-As jobs and files are loaded into memory they occupy space which when vacated leaves gaps in memory. -If a larger file is sent to that area, it has to be broken up to fit. -This splitting of available memory into discontiguous pieces is called fragmentation
Function.	-A procedure or small section of code which returns a specific value. -The value is returned whenever the function name appears.
Gantt chart	-A series of bars representing time to be taken on the different tasks which are needed to produce the system and relative timings of tasks. -Shows when different resources are going to be required. -Also shows reliance of one task on the completion of another. -Will show how long the whole system should take to complete.
Global variable	-the value of the variable exists throughout the code of the program - the variable used to hold the details searched for (needs to be used outside the procedure).
Goal	-The intention to find all instances that satisfy a rule/set of facts -e.g. If rule is male(X) then the goal is to find dis and david and john
Hypertext mark up language	-a computer language used to create multimedia pages -each page consists of the text to be displayed -tags providing special instructions about the display -provides links to pages
Index register	-contains a value which is added to the address (in the CIR) -in order to make the address of the data -incremented after use so that a set of data can be read one after the other without altering the raw address
Indexed addressing	- address in instruction is added to - a value held in a special register called the index register - allows a set of contiguous data (array) to be accessed without altering instruction

Indirect addressing	-address in instruction is the address of the address of the data - used to access areas of memory that are not accessible using the space available for the address in the instruction code
Inheritance	- Where one class is a subclass of another it can use its methods - Pupil can use getname() from Person
Instantiation	-A particular variable is assigned a value -e.g. if male(X) then X is instantiated to dis
Internal level	Internal level is the structure used for storage of the data
Interpreter	-Interpreter translates line of code and then runs it -Interpreter retains source code. -Interpreter must be present for run
Interrupt	Signal indicating that a device requires attention of CPU.
Interrupt (hardware)	-Interrupt generated by request of hardware  to close down safely in the event of power failure -e.g. printer out of paper
Interrupt (timer)	-Interrupt generated at fixed intervals to allow for display refresh -to control access to processor in multi access system
Intranet	Restricted to the organization -Limited number of users/pages -Access controlled by use of passwords -Allows for confidential/sensitive data
Linkers	-Used to combine already compiled procedures -with compiled program to create an executable file. -Deals with external references.
Loader	-Decides where to place programs and procedures - Loads program and procedures into memory - Adjusts memory addresses to match locations used
Local variable,	-The value of the variable only exists in the procedure - The counter used to control the loop so that it does not effect a repeat use of the variable.
Mantissa	-The fractional part of the representation -Holds the magnitude of the data.
Memory address register	Holds the address of the instruction/data that is next to be used
Memory data register	-Contents of any address that has been accessed are placed in here first before being used -May be an instruction or a piece of data
Message authentication	- Method of ensuring that message is from the person it claims to be from -Use of digital signature created using private key which can only be done by owner of key -Digital certificate from authority to authenticate author of message
Modems	-Necessary if communication link is analogue -Converts digital signals to analogue for transmission.
Object code	Code in machine code

Object-oriented	- data and the permitted operations on that data are defined together (class) - objects in classes can pass messages from one to another - classes can share some characteristics (inheritance)
Paging memory	-the division of memory into fixed size units - logical pages can then be assigned to any physical page in memory - records of the contents of each page in memory are kept in an index
Parallel implementation	-Old and new systems both used on all data - Example where it is essential that no errors arise in the new system, e.g. changing the software that produces student exam grades - Errors can be found in new system before it starts to operate
Parallel processing	-many processors are used  simultaneously, all doing some processing required by the application - Special non-linear programs must be produced
Parallel processor system	– Many (independent) processors working on the same program at the same time
Parameter	Information about a data item being given to a procedure when it is called
Parameter being passed by Reference	The address of the value is passed, allowing it to be altered if necessary
Parameter being passed by value	The value to be searched for is passed.
Partitioning data on a network	Part of the data in the database is held locally.
Perfective Maintenance	-changes which enhance the performance of the system - e.g. a flagging system at the tills to speed up the processing of bar codes
Phased implementation	- One part or module of old system is changed to new while remainder stays on old - Complex system with easily identified modules, e.g. control of a chemical plant may involve one reaction being switched to the new system first so that it can be monitored separately - Allows tight monitoring of new system with limited resources/training of personnel
Pilot implementation	-One complete system, representative of whole system is switched while remainder remains on old system. - Any system comprising a number of matching systems, e.g. a college record keeping system may alter one year group to ensure it works before changing the others - Allows system to be tested with a limited volume of data.
Primary key	Unique attribute or set of attributes used to identify the record or tuple.
Procedure	A self contained set of instructions.
Program counter	-Contains the address of the next instruction to be carried out -Controls the sequence of instructions
Protocol	It is a set of rules to allow communication between devices/computers/machines.
Rich text format	-These are a complex format used to store data from a word processor.  -They include information about fonts, sizes, colour and styles.
Routers	-Connects different types of network together -Software at a node on the network which directs messages down different routes according to their desired destination
Secondary key	A different attribute that allows the data to be accessed in a different order.
Security	-Different user types given different rights such as RO (Read Only) or RW (Read Write) or other protections on files/data - Passwords and ID to establish identity
Segmentation	-the division of software into logical parts which are of different sizes - individual segments can be present in memory without the need for the whole program to be there - an index is required to store the beginning, size and contents of each segment - leads to fractionalisation of the memory
Source code	Code written in a high level language
Spooling	temporary storage of input or output data on some form of backing storage
Static data structure	static data structure remains a fixed size throughout its use
Stepwise refinement	-Also known as top-down design -Split original problem into smaller parts -Continue splitting into smaller and smaller parts until each part can be considered to be a single process.
Subclass	A class that inherits the structure and methods of another class
Superclass	A class that passes down its attributes and methods
Switches	A device that allows many terminals all to use the same communications line at different times
Text only format	Contains only characters represented in ASCII on keyboard
Transparency	-Users are unaware of the hardware and software -they believe they are the sole users of a standalone -Users are unaware of communications
User documentation	-A series of manuals to explain the software in printed form and/or on screen. -Overview of package -Sample inputs/outputs. -Explanation of error messages. -Installation of software/hardware. -Quick reference guide. -How to carry out simple maintenance (like reloading a till roll).
Virtual memory	-used when there is not enough memory available - part of the storage is allocated to act as memory - this block is then imported to memory when it is needed - especially used when a piece of software is so big that it will not fit into memory
Von neumann architecture	-Program is stored in memory along with data. -programs and data are indistinguishable -Uses a single processor -Sequential carrying out of instructions

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