AQA Computing Revision By Rickesh Bhopal

37 Pages·2008·0.21 MB·English

AQA Computing
Revision
By Rickesh Bhopal
By Rickesh Bhopal
CPT1: Computer Systems, Programming and Network Concepts
By Rickesh Bhopal
Fundamentals of Computer Systems
Hardware and Software
Hardware –The physical components that make up a computer
Software –The computer programs that tell the computer what to do in response to a command
Classification of Software
There are two types of Software;
System Software–performs tasks needed to run the computer system
Operating System Software
acts as an interface between user and computer hardware

program which controls running of other programs

e.g. Windows, DOS, DFS (BBC operating system), MacOS, OS2, Unix

Utility Program
devised to perform operations on files or data

e.g. PK Zip (compression)

Library Program
part of library facilities of a computer

e.g. searches for lost files / restoring corrupted files

Compilers, assemblers, interpreters
compliers and interpreters are different types of program used to translate the source code in

programming languages such as Pascal, Visual Basic and C++ into code the computer can
execute.
An assembler performs similar function translating the statements of assembly code into

machine code.
Application Software– written to perform specific tasks for the user that is not any relevance of
running the computer. For example writing a letter.
General purpose applications software
includes all common application packages such as word processing, desktop publishing,

spreadsheet, database, CAD
software that can be made to do many different tasks

Special purpose applications software
software to perform a specific task

applications such as payroll, accounts or stock control systems

Bespoke Software
individual program written for a particular company

Bespoke or Off-The-Shelf?
By Rickesh Bhopal
Advantages of Bespoke:
tailor made for exactly what user wants

has no unwanted features

it can be written to run on specific hardware

can be integrated with existing software

may not be a suitable software package on the market

Advantages of Off-The-Shelf:
less expensive ascost of developing is shared across wider customer range

possible to speak to other users for evaluation before purchase

can be bought and installed straight away

more likely to have less bugs than newly written code

The Generation of Bit Patterns in a Computer
There are different interpretations that may be associated with a pattern of bits.
All digital computers use binary system for representing data of all types –numbers, characters,
sound and pictures. They consist of 2 symbols which can be anything such as + and –or 0 and 1.
Bits and Bytes
A binary digit (1 or 0) is known as a 'bit' which is short for BInary digiT.
Bits are grouped in 8-bit bytes. A byte can hold 256 different combinations of 0s and 1s, one byte
holds one character.
Word Size
The word size of a computer is the number of bits that the CPU (Central Processing Unit) can
manipulate simultaneously, they can have 8-, 16-, 32- or 64-bit word sizes (or even larger), this is
one factor that will determine the speed of the computer. The standard PC's have 32-bit processors.
However some have 64-bit processors such as the AMD Athlon 64.
By Rickesh Bhopal
Internal Components of a Computer
A computer system is composed of internal and external components.
The internal components are contained in the Central Processing Unit (CPU).
Processor
Main Memory or
Immediate Access
Store
VDU Output Visual
Keyboard Keyboard
Controller Display Unit
Input
Controller
Key:
(Disk) I/O Controller
Control Bus
Address Bus
Data Bus
Secondary Store or
Backing Store (eg
Magnetic Disk)
Processor
Contains the control unit and arithmetics/ logic unit (ALU) and registers

The control unit coordinates and controls all the operations carried out by the computer. It

operates by repeating three operations;
1. Fetch –cause the next instruction to be fetched from the main memory
2. Decode –produces signals which control the other parts of the computer such as the ALU
3. Execute –cause the instruction to be executed
The ALU can perform two types of operations, Arithmetic (subtraction, addition,

multiplication and division) and Logical (comparing data to determine whether one is smaller
than, equal to or greater than the other)
Registers are special memory cells that operate at very high speed

Main Memory
The computer can manipulate only data that is in main memory. Therefore, every program executed
By Rickesh Bhopal
and every file accessed must be copied from a storage device into main memory. The amount of
main memory on a computer is crucial because it determines how many programs can be executed
at one time and how much data can be readily available to a program. Also known as RAM.
Memory and the Stored Program Concept
Programs are stored in memory, each instruction after the last in consecutive memory

locations. Any deviation is dealt with in a special way.
Buses
Channels (set of parallel wires) within computer which carry data between independent

components
Shared transmission medium, therefore only one device can transmit along a bus at any one

time
Address Bus
Processor Keyboard Input VDU Output Disk I/ O Memory
Controller Controller Controller
Control Bus
Data Bus
Address Bus
When the processor wishes to read a word (8, 16 or 32 bits) of data from memory is puts the

address of desired word on the address bus
The width of the address bus determines the maximum possible memory capacity of the

system
Carries address of memory or I/O port

Unidirectional

Data Bus
Carries data

Bi-directional

By Rickesh Bhopal
If data bus is 8 bits wide, each instruction is 16 bits long, therefore processor must access

Main memory twice to fetch instructions
Increasing width increases system's speed and performance

Control Bus
Carries control signals

Bi-directional

Read address location from data bus, write from data bus to address location (in memory)

Ensures access to and use of data and address buses by different components of the system

Interrupt request, interrupt ACK, bus, reset

I/ O Controllers
These allow peripheral devices to be connected to the processor.

Can operate both input and output transfers of bits, e.g. Floppy disk controller

the controller is an electronic circuit board consisting of three parts;

1. interface allowing connection of controllers to the system of i/ O bus
2. set of data, command and status registers
3. interface that enables connection of controller to cable connecting device to computer
Secondary Storage
secondary storage, or external memory, is computer memory that is not directly accessible to

the central processing unit of a computer.
Secondary storage is used to store data that is not in active use.

Secondary storage is usually slower than primary storage, or internal memory, but also

almost always has higher storage capacity and is non-volatile, which makes it perfect for the
preservation of stored information in an event of power loss.
RAM
storing programs which are currently running

volatile (loses all information once machine is switched off)

ROM
non-volatile

contents permanently etched into memory chip at manufacturing stage

contains bootstrap loaded (instructs computer to load OS software as soon as computer is

switched on)
Cache
used to improve speed of computer

intermediate store between CPU and main memory

stores most recently/frequently used data

1Kb -512Kb

By Rickesh Bhopal
Fundamentals of Programming
Generations of Programming Language
First generation –Machine code
Executes directly without translation

Patterns of 0's and 1's

It is time consuming, error-prone and is rarely used by programmers.

Second Generation –Assembly language
Allows programmers to use mnemonics and denary numbers (0-9) instead of 0's and 1'

Also instructions to add or subtract 2 numbers are available

Examples; OS software, device drivers and encryption/decryption software

Must be translated to machine code before they can beexecuted using an assembler

They are low level language as they are close to machine code

Each type of computer will have its own assembly language

Third Generation – Imperative High Level Languages
The high level language instructions are executed in a programmer-defined sequence

Languages include BASIC, Pascal, COBOL and FORTRAN

One statement in a high level language is translated into several machine code instructions

before it can be executed. The term imperative high level language refers languages such as
Pascal and BASIC
Types of Program Translator
Assembler
Translates assembly code into machine code ready for the computer to execute it

As each computer has its own Assembly language it also has its own assembler, which can

be written in a high level language such as C
Compiler
Translates the whole high level language source code into object code which can then be

executed without he presence of a compiler
It scans through several times, each time performing a different check

Interpreter
Analyses the source code statement by statement as execution proceeds decoding each

statement and calling routines to carry out each instruction
No object code is produced, hence the program has to be interpreted each time it is to be run

Advantages of Compilers
Object code can be saved on disk and run whenever required without the need to compile

again, but if an error is found in the program it must be recompiled
The object code executes faster than interpreted code

The object code can be distributed or executed without the compiler present

Object code is more secure, as it cannot be read without a great deal of reverse engineering

By Rickesh Bhopal
Advantages of Interpreters
In program development, lengthy compilations are avoided each time an error is found

Easier to partially test and debug programs

Features of Imperative High Level Languages
Built-in Data Types
Integer VarNoOfSpaces : integer;
Real VarAverage : real;
String VarSurname : string;
Character VarFirstLetter : char;
Boolean VarFound : boolean;
Built-in User Defined
Records typeTStudent = record
FirstName : String [15];
Surname : String[25];
DateOfBirth : TDateTime;
end;
Symbolic types type TSeason = (String, Summer, Autumn, Winter);
Sub-range types typeTCapitalLetter = 'A' .. 'Z'
Programming Statements
Assignment Spaces := Spaces –1;
Iteration for Count := 1 to10
doWriteln(Counter);
Selection if Number = 0
then Number := Number + 1;
else Number := Number – 1;
Procedure calls Adjust (NoOfSpaces, NoOfStars);
Function calls FirstLetter := Initial (Surname);
Declarations
Type Definitions type TSeason = (String, Summer, Autumn, Winter);
Variable Declarations var NoOfStars : Integer;
Array Declarations var Total : array[1..150] of real;
var Sales : array[1..150, 1..5] of real;
Constant Definitions const Pi = 3.14;
Procedure Declarations procedureAdjust (varSpaces, Stars : Integer);
begin
Spaces := Spaces –1;
Stars := Stars + 2;
end;
Function Declarations function Initial (S: String) : String;
By Rickesh Bhopal
begin
S := LeftStr (S,1);
Initial := Uppercase (S);
end;
Function
A sub-program which returns a value

Name of function is used as a variable which stores the returned value

Procedure
Completely self-contained subprogram which may be called from the main program

Defined and given an identifier which may be used like any other programming instruction

Variable
Temporary storage space for data, referred to by its name

Data in a variable may vary or change within the program

Event Procedure
Procedure which is run when a certain event is triggered by the user

Scope
Refers to the range of statements for which a variable (or procedure is valid). A variable

doesn't exist outside its scope
Parameter
Information about a data item being supplied to a function or procedure when it is called

(aka argument)
Variables declared in the heading of a procedure/subroutine whose values can be passed to

and returned from it
Actual Parameter
Parameter passed to the procedure or function

Formal Parameter
Parameter which appears in the procedure or function

Passing by Value
A copy of the variable is used and is discarded when the subprogram exits

Passing by Reference
The memory location of the data is issued and the changes made to the subprogram affect

the variable permanently
By Rickesh Bhopal