Chapter Notes on Computing Part 4

8/6/2019 Chapter Notes on Computing Part 4

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Computing Revision Guide 2509

2008JimTang dOverbroecksCollege Page2of17

Making sure that the user can interact with the computer by providing aninterface

Ensuring that any errors in the computer are reported to the user, usuallywith messages on the VDU

Managing files on the hard drive and in memory and allowing the user tomanage files by providing a file management utility program

Providing utility programs such as FORMAT DISK, or DEFRAG

Scheduling. The OS provides programs that ensure the resources of thecomputer such as CPU time, are used as efficiently as possible by all thedifferent users and the different jobs.

Memory management (including virtual memory, paging, segmentation,threshing)Purpose of Memory Management

Allocate memory to allow separate processes to run at the same time Deals with allocation when paging Reallocates memory when necessary

Protects processes from each other

Protects the operating system Enables memory to be shared

Virtual memory

Use of backing store As if it were additional memory

Uses paging/fixed size units

Swap pages between memory & backing store to make space for pages needed

Holds part of program not in use

Allows programs to run that need more memory than is availablePaging and segmentation

Similaro Splitting a (large) program into smaller sectionso Only part of program needs to be in memoryo Some of program can stay on disko Allows program to run when there is insufficient memoryo Virtual memory

o Splitting memory into smaller sectionso Both need indexingo Sections of program need not be all together

Differenceso Segments may be variable size, pages are fixed sizeo Pages are smaller

Disk threshing

occurs when moving pages between memory & disk disk is relatively slow

high rate of disk access

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more time spent transferring pages than on processing

Scheduling to make efficient use of resources; job queues, priorities

Purpose efficient use of processor time efficient use of resources

maximise number of users no apparent delay for users

maximise throughput of CPU.Round robin

Each task allocated a time slice in turnSystem of priorities

Highest priority first

Interrupts a signal

that causes a break in the execution of the current routine/requestsprocessing time

the current routine can be resumed after the interruptPriorities for interrupts

more than one interrupt can occur at a time

deal with the more urgent interrupt first

because a more urgent interrupt has a higher priority

to avoid loss of data

Interrupt in I/O bound and processor bound

Program A is Processor bound has low priority Program B is I/O bound

has high priority Program B starts

Once data is being printed, program A has processor time until an interrupt requests more data for printing

Program B is given processor time

as it has higher priority

This is repeated until one program is complete

Otherwise if program A had priority it would stop B from runningInterrupt in FDE cycle

Interrupt flag is set Current instruction/cycle is completed

Interrupt is identified Interrupt priority is compared with current task

High priority

So current values are placed on a stack Start address of interrupt service routine is placed in PC

ISR (Interrupt Service Routine) is run



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To avoid loss of data/store data

Spooling

Output data To slow peripheral/disk drive E.g. for printing later/when appropriate

when processor is not busy/after interrupt To avoid delays Some peripherals (e.g.) printers) are relatively slow/speed mismatch References to jobs stored in a queue/buffer

each job called by its reference jobs can be prioritised.

Network operating systems and their main features; transparency,

directory services, security, network printingNetwork Operating System (NOS)

is an operating system that gives computers the ability to communicatewith other computers on a network.

Features

Controls communication between workstations themselves and betweenworkstations and servers

Allows and manages file sharing

Allows and manages printing and other resources Controls internet access amongst stations

Manages user accounts Manage security Manages automate system back-ups

Manage and reports on the use of the systems resources, such as eachusers hard disk allocation and how many prints they are allowed to do

Provides a number of different user interfaces.Transparency

Users are unaware of the hardware actions Users are unaware of the software actions

Users do not need to understand the NOS tasks Each user is unaware of other users

Privacy

restrict data access to certain people. Maintain

o passwords (accessing system; accessing files)o physical protection (some data at certain terminals only)o allow authorised accesso according to access rightso encryption.

Security Risks

Hardware failure

Fire



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Theft

Disgruntled employees

Hackers trying to break into the system Computer viruses

Methods for safety

Backups Physical security

Passwords Anti-virus

Network Printing

Packets from different users arrive at the print server and are stored in different files for different users Spooling (as above) When a file is complete it is added to the print queue and files are printed in turn

4.2 Translators (Compilers and interpreters)

Compilers and interpretersTranslator

(Convert) from high level language/source code/language used bypeople

to low level object code/machine code/binary/language used by

computerInterpreter

software

that translates & executes a program

one instruction at a time/line by line

each time the program is run / without generating an executableprogram

stops at first error found

Find erroro Used during program developmento

to debug (sections of) codeo Translates one lineo then allows it to be run before translation of next lineo Errors found more easily/quicklyo Starts to run program from any point

Compiler Used when program is completed

Executable program runs quickly

Executable program prevents purchaser from re-using code Translates the whole program as a unit



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and creates an executable programAdvantages of using an interpreter rather than a compiler

Errors are reported as they occur rather than together

easier to correct errors

Less memory is needed as only the object code from a single source code instruction in use is

held

interpreters are usually smaller programs than compilers Program can be restarted from any point

after correcting an error Can insert breakpoints (for debugging) to halt the program Can trace variable values which are displayed as the program is running Can set variable values while running the program

Disadvantages

the user needs an interpreter / the program runs more slowly

because the program has to be translated each time it is run

the stages : lexical analysis, syntax analysis; How are errors detected andreported?Lexical -> Syntax -> Code generation

Lexical analysis Reserved words/standard components are replaced by tokens

a token is a string of binary digits

of fixed length

Variable names are checked Variable names are entered in the symbol table

Constants are entered in the symbol table Data type (/space) required is entered in the symbol table

Error reporting occurs Unnecessary characters (white space, comments) are removed

Program is formatted ready for the next stage/syntax analysisSyntax analysis

The stream of tokens is split up (or parsed) into expressions

Check the database of the syntax rules Check the semantics

Syntax error

During syntax analysis

Statements are checked against the rules of the language Uses symbol table

(if statements are not valid) errors are reported To alert the user/programmer



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Code generation and optimisationCode generation

Produces machine code/executable code Several instructions for each high level language instruction

All variables are given addresses All constants are given addresses Relative addresses are calculated

Optimisation Makes code as efficient as possible

Speed of processing is improved Number of instruction is reduced

Source code High level language (version of program)

Written by user Easy for people to understand

Translated by (input to) compiler (accept translator)Object code

Low level/machine code/binary code

Used by computer Runs without further translation/executable

Produced by (output from) compiler (accept translator)Executable code

a complete program

..that the computer can run without further translation/has been compiled

machine code / low level language

Linkers and loadersLinker

combines modules/library routines

that are already compiled/in object code/in executable form

Loader

copies modules into memory

from backing store

ready for execution

completes address links to program.

4.3 Computer Architectures and the fetch-execute cycle (see also 4.5)

What is Von Neumann Architecture? Why are registers needed?Von Neumann

Instructions and data stored together.. in the same format

Instructions fetched and executed one after another

A single processor (control unit)Registers



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LocationsusedforspecificpurposesThe fetch-decode-execute-reset cycle and the use of registers

Registers using in a FDE cycle Program counter holds address of next instruction MAR points to the data/instruction to be used

MDR holds data/instruction when copying from/to memory CIR (Current Instruction Register) hold a copy of the instruction to be

executed

Copy contents of PC to MAR Increment PC

Load instruction pointed to by the MAR to MDR

Copy instruction from MDR to CIR Decode instruction (in CIR)

Execute instruction

Check for interrupt

Features of a processor that determine its performance (Clock) speed/pulse rate/in MHz more instructions executed at faster clock speed

Word size number of bits processed together

Bus size (data bus) number of bits processed/transferred together

Architecture e.g. single or multiple processor/pipelining

Cache size to allow faster access (to part of program and data)

Other machine architectures (eg parallel processor systems), pros andconsParallel processor system

Multiple processors used together/simultaneously

To perform a single job/different parts of the same program (at the sametime)

Program may be split into a number of tasks each of which may be processed by any available processor Pros

o Increased processing speed/multiple instructions processed at thesame time

o Different parts of program performed at the same timeo Performs complex tasks efficiently

Conso Not suitable for all programso Program may need to be rewritten

E.g. Weather forecasting/expert system



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Pipelining

processor is split into parts

each part handles one of fetch, decode, execute (reset)

Conso Change in instruction sequence/e.g. jump instruction: requires pipe

to be cleared

4.4 Data representation, data structures and manipulation

All topics from 1.4, plus

floating point binary representation using mantissa and exponent;normalisation; the trade-off between accuracy and range

data structures: (for each, how to add, change, delete)

list (linear, linked?)

Stack

LIFO Two pointers

Queue

FIFO One pointer

Add

Check for full queue Error if it is full

Move rear pointer to point to new item

insert data at rear of queueDelete

Check for empty queue

Error if it is empty Take data pointed to by front pointer

and move front pointer to next item

TreeIn-order

traverse left subtree

visit root

traverse right subtree

Delete

traverse tree until item is found..

and remove elements below item

store them

delete item (accept only if in correct place)

transfer stored items back onto tree



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(static and dynamic implementation of the above, pros and cons)Static

Pros Storage needed is known Easier to write program

Fewer errorsCons

Size restricted (to that declared when structure is created)

May waste memory

Searching binary, serialSerial Search

Start at the first item Look at each item in turn

Until it is foundBinary Search

Look at middle word Repeated halving

Until it is found Pros

o Faster for large set of data/fewer values need to be checked

Conso Data must be in order/more complex algorithm

Sorting insertion*, quick*

Insertion Sort Start with the second item If it is smaller the than number one, swap

Go to the third item, insert it into the correct position Repeat until all items in the list have been inserted into the correct

positionsQuick Sort

Select an item at random, the pivot

Create two new lists, one with all items less than pivot, other with itemsgreater than pivot

Repeat until lists only have one item

Merging*

* algorithms needed

** error at start of algorithmCheck empty(underflow) or full(overflow)

4.5 Programming paradigms

Pseudocode for writing algorithms

The nature and purpose of 3rd and 4th generation languages (?)



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Third Generation

Imperative

(The programmer must) give a step by step description of solution toproblemFourth Generation

Declarative Knowledge base/facts and rules

User can state what they want to achieve without giving detailed steps

Programming paradigms : low-level, object-oriented, declarative,procedural, functional. Explain with examples how they are typically used.Low level language

close to design of computer/lack of portability

machine-oriented language assembly language used for tasks connected with running the computer

uses mnemonics for instructions

uses variable names for addresses uses labels

1-1 correspondence between low-level language and machine codeObject-oriented

If must contain objects that have methods and data, with the data onlybeing accessible via the methods

The objects must be instantiate from a class, making use of polymorphismProcedural

3rd generation

step by step instructions to solve a problem to explain how to solve the problem

Declarative

4th generation state what to achieve

without giving detailed steps

uses facts and rulesFunctional

Breaking a maths problem down to the smallest possible units then writing a set of functions for those units Each function then receives parameters from outside the function,

manipulates the parameters mathematically and then outputs parametersto other functions

Characteristicso An instruction set geared towards handling maths problemso Being excellent at handling recursiono You can write mathematically-reliable programs

Structured programming techniques : use of functions and procedures;



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use of Jackson Structured Programming and Top-Down Design (Stepwiserefinement)

Procedure A subprogram that performs a task is given an identifier

..the identifier can be used as a program instructionParameter

An item of data/value (of a variable)

supplied to a function or procedureTop-down

Successively break task into simpler tasks

until tasks can be programmed Clear program structure

Easier to write program as a series of modules Identify common or standard modules

Tasks can be shared between many programmers according to their expertise

easier to de-bug.

How are parameters (value and variable) and variables (global and local)used?Global variable

May be used throughout program

including all proceduresLocal variable

Restricted to one procedure

Same variable name may be used for different values in different procedures

Problems with global variables

Difficult to trace errors not sure which module has changed a value or conflict between

variables

Different programmers may change them independently

Modules are not independent

How is the stack used? (return addresses, variables)Passing parameters between modules

Values

Addresses pushes on to stack

when module is called

Popped off stack

,by called module Need to be popped in reverse order (last in, first out)



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to allow successive calls

Recursion

Object-oriented languages : data encapsulation, classes and derivedclasses, inheritanceData encapsulation

Data can only be accessed via methods provided by objects.Class

Describes shared attributes and methods

A template for A set of objects

that have state and behaviourObject

An object is one instance of a class

..is an actual real-world entityInheritance

Class has its own attributes and methods

and those from its super classDerived class

A class has all the features of its superclass/class its come from

and has additional features of its own Inheritance

A class that uses another class

Declarative languages: backtracking, instantiation, predicate logic,

satisfying goalsDeclarative language

A programming language uses facts and rules Fact

o Predicateo which may have argumentso Something that is always (unconditionally) true

Ruleo May be used to define a relationship between factso Simple statement store in knowledge baseo A rule is true depending on a given condition

Goalo a query to be solved

E.g. expert system/medical diagnosis/equipment fault-finding/oilexploration/robot control

Reasons:o Further data can be added when availableo Large amount of informationo Order of storage irrelevanto Can explain how they have arrived at their solution

Backtracking



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Follow rules

towards goal

If goal fails go back to previous match and

start again from this point

towards an alternative goalInstantiation

If B is an instant of A, then A is instantiated to B

Functional languages: list processing, head and tail recursionSee Programming paradigmsTail recursion

The function call is at the end of the expression

Low-level languages: use of accumulator(s), registers, program counter,direct, indirect and indexed addressing of memoryAccumulator

temporary storage (in the ALU)

holds data being processed/result of calculation

deals with the input and output in the processorMemory address register (MAR)

Address of the data (instruction) to be fetched/of the next location to be accessed

Memory data register (MDR)

Temporary store for data/instruction being transferred to/from memory

Acts as a buffer

to compensate for different speedsProgram counter

Address of next instruction

increment to address of next instruction every time an instruction is fetched

change to address part of instruction/change to start of ISR when it is a jump instruction/when an interrupt occurs

Immediate addressing

Using constantDirect addressing

E.g. Store the contents of the accumulator at address 0101 Cons

o uses fixed memory location/not relocatable/limited address rangeIndirect addressing

go to vector address which holds location of the real address needed

Indexed addressing

modifies the address



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by adding the number

from the index register

Proso easy to alter the index registero easy to access a range of memory locations(eg for an array)

Backus-Naur form and syntax diagrams. How are they used?BNF:::= is defined by| means or< > means the meta variablee.g.::=0|1|2|3|4|5|6|7|8|9::==+|-

The terminal symbols are symbols which cannot be broken down any further4.6 Databases

Types of database : flat-file, network, hierarchical, relational; (all the rest ison relational database management systems - RDBMS)Flat-file

all the records are held in one table

each row in the table corresponds to one record each column in the table corresponds to a different field.

Tables are two-dimensional structuresRelational

Is kept as a number of related tablesHierarchical

Using a tree structure

Get more detailed information as you go to a lower levelNetwork

Records are cross-linked as well as linked above and below in a networkdatabase

Design a simple (?) relational database to 3rd normal form, defining tablesand views, and drawing Entity-relationship models and entity life historiesNormalisation

many-many relationships are not allowed

avoid duplicate dataThird normal form

Contains no functional dependencies between attributes other than the primary key (or a candidate key)

Limits duplication of data

Removes multi-valued attributes

Design forms for input, deletion, modification, querying of database

RDBMS compared to flat-fileFlat-file



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Duplicate data stored

so storage wasted/data redundancy

Inconsistent data more than one version of the data item

Program-data dependence so difficult to change data format

Change of data

may require rewriting file Lack of flexibility difficult to obtain new types of reports

RDBMS avoid data duplication

avoid inconsistent data

easier to change data easier to change data format

data can be added easily

easier to control access to data.

Primary, secondary and foreign keysEntity

A thing about which there is a need to record dataAttribute

A property of an entityPrimary Key

Unique value To indentify a record

Foreign Key

Primary key from one table

used as an attribute in another table Used to link tables

Access to the database for different users, at different timesAccess rights

Security

senior staff update data, others have read only access Privacy

financial data not seen by all staff Access data needed for their job

e.g. financial data only seen by those who need to Why it may change temporarily

o Data may become read onlyo while being used/updated by another usero to avoid inconsistent updates

to avoid attempts to sell the same car twice



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Structure of a RDBMS : data dictionary, data description language, datamanipulation language

Information stored in the data dictionary Names of tables/columns Characteristics of data (e.g. length, data type)

Restrictions on values in columns Meaning of data columns

Relationships between data Which programs can access data

Data Definition Language (DDL)

A language used to define the data structure (for a database)

Defines the schema Create attributes

Define data types Define primary/foreign keys

Define validation rules HLL/4GL

Data manipulation language (DML)

high level language/4GL part of DBMS

To access/query data To store data

To update data
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Chapter Notes on Computing Part 4