CS Introduction to Computers 2

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Computer SystemsMay 16,2011


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Module: Computer System

Learning Unit: Computer System Components

The contents here are for Aricent internal training purposes only and do not carry any commercial value Restricted Aricent Group 2011 2


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Objective

Computation & ComputersComputer HardwareInstruction Set

Assembly & High Level Language

The contents here are for Aricent Group internal training purposes only and do not carry any commercial value Restricted Aricent Group 2011 3


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Computation & Computers

A Program is a set of instructions however unless the instructions from thisset can be executed it is not useful.

The set of instructions to achieve a task.

Consider the following problem: A person wants to buy pearls to make necklaces. He has to make five necklaces

and each necklace requires 10 pearls. How many pearls does he need to buy?

Consider Me as a machine who can provide him the result to this problem.


Let us say the Machine does not know how to find the answer to this question.However it can be instructed to do so through a program using Multiplication Operation.

How would the program for this machine look like?

No of Pearls = Multiply 5 by 10

The Machine (I) knows how to do multiplication so it can provide the result.

This is a program written in a set of instructions which can be executed by theMachine called Me.


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Let us change to a machine which does not understand Multiplication.Change it to a child who knows counting, addition, subtraction but notmultiplication.

What will be the program that needs to be written?No_of_pearls = 0;

no_of_necklace = 10;

While (no_of_necklace > 0){

no of earls = no of earls + 5


_ _ _ _

no_of_necklace = no_of_necklace - 1}

The program changes, because the programming language changesbecause the machine changes

You can't have multiplication instruction in a program written for the child

Conclusion:

With every machine you have an instruction set which can be used to instruct themachine


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What is a machine?

Everything that we see is a machine

or is capable of behaving like a machine

Mostly you can change a program written in a language to an equivalent

program in another language as it happened in our previous example and


o w g ve you e es re resu

How will you ask the machine to execute multiple instructions in a program

There has to be an interface through which you instruct the machine

Car has parts like clutch, gear, accelerator, brake and you can operate these devices to do

the required thing


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How does a car move ?

Put the Car in the Neutral state

Start it

Press the Clutch

Take the gear to state One

Press Accelerator


The Car starts moving

A Simple Model of a Car ?

States how many ?

Inputs how many ? State Transition how many transitions ?

Finite State Machine


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Computation

~Information Processing

Information : Data

e.g., Temperature measured in Celsius

Processing

Take the value measured in Celsius

Convert from Celsius to Farenheit


Show the value in FarenheitComputer

Equipment which can do Information Processing by an algorithmic process

Algorithm

Is a finite list of well-defined instructions for accomplishing some task that, given an initial state, willterminate in a defined end-state


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Calculator vs Computer?

Are they same?

No, why not

The defining feature of modern computers:

They can be programmed

A list of instructions (algorithm or the program) can be stored in the computer and can


be carried out any time in the future

Distinguishes them from all other machines


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How will you Design a computing machine? which components are

required?

Machines has to provide interface to enter the Instructions and data

Machine has to have an Instruction Set which would be used to process

data

Mostly computer instructions are simple, e.g.:


move some data from one location to another

Machines has to provide interface to provide/display the result to the user

Provide capability to store data and instructions, the defining feature


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Structure - Top Level

MainMemory

Peripherals

CentralProcessingUnit

Computer


Computer

InputOutput

SystemsInterconnection

Communicationlines


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Computer Hardware

Arithmetic and Logic Unit

Program Control Unit

CPU

Instruction RegisterProgram Counter

Stack Pointer

Register2

Register1

Registern


Memory-I/O BUS

Main MemoryDevice 1(DISK)

Device 2 Device n


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Components

The Control Unit and the Arithmetic and Logic Unit constitute the

Central Processing Unit

Data and instructions need to get into the system and results out

Input/output

Temporary storage of code and results is needed

Main memory



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What is a program?

A sequence of steps

For each step, an arithmetic or logical operation is done

For each operation, a different set of control signals is needed



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Function of Control Unit

For each operation a unique code is provided

e.g. ADD, MOVE

A hardware segment accepts the code and issues the control signals

We have a computer!



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Instruction Cycle

Two steps:

Fetch

Execute



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Fetch Cycle

Program Counter (PC) holds address of next instruction to fetch

Processor fetches instruction from memory location pointed to by PC

Increment PC

Unless told otherwise

Instruction loaded into Instruction Re ister IR


Processor interprets instruction and performs required actions


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Execute Cycle

Processor-memory

data transfer between CPU and main memory

Processor I/O

Data transfer between CPU and I/O module

Data processing


Some arithmetic or logical operation on data

Control

Alteration of sequence of operations

e.g. jumpCombination of above


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Interrupt Cycle

Added to instruction cycle

Processor checks for interrupt

Indicated by an interrupt signal

If no interrupt, fetch next instruction

If interrupt pending:


Save context

Set PC to start address of interrupt handler routine

Process interrupt

Restore context and continue interrupted program


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Multiple Interrupts

Disable interrupts

Processor will ignore further interrupts while processing one interrupt

Interrupts remain pending and are checked after first interrupt has been processed

Interrupts handled in sequence as they occur

Define priorities


Low priority interrupts can be interrupted by higher priority interrupts When higher priority interrupt has been processed, processor returns to previous

interrupt


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Connecting

All the units must be connected

Different type of connection for different type of unit

Memory

Input/Output

CPU



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Memory Connection

Receives and sends data

Receives addresses (of locations)

Receives control signals

Read

Write


Timing


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Input/Output Connection

Similar to memory from computers viewpoint

Output

Receive data from computer

Send data to peripheral

Input


Receive data from peripheral

Send data to computer


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CPU Connection

Reads instruction and data

Writes out data (after processing)

Sends control signals to other unitsReceives (& acts on) interrupts



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Buses

There are a number of possible interconnection systems

Single and multiple BUS structures are most common

e.g. Control/Address/Data bus (PC)e.g. Unibus (DEC-PDP)


Ask what is a bus error ?


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What is a Bus?

A communication pathway connecting two or more devices

Usually broadcast

Often grouped

A number of channels in one bus

e.g. 32 bit data bus is 32 separate single bit channels



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Data Bus

Carries data

Remember that there is no difference between data and instruction at

this level

Width is a key determinant of performance

8, 16, 32, 64 bit

different set of control signals is needed



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Address bus

Identify the source or destination of data

e.g. CPU needs to read an instruction (data) from a given location in

memory

Bus width determines maximum memory capacity of system

e.g. 8080 has 16 bit address bus giving 64k address space



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Control Bus

Control and timing information

Memory read/write signal

Interrupt request

Clock signals



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Instruction Set

Instruction Set of a Machine

Operands & Operators

Operators are those operations that are supported on the machine, e.g., add,

subtract

What are the operands of Compute hardware

C example


int a,b,c; a = b + c; These are variables

Can Operands be Variables for Machine Instructions?

No, these are symbolic names and a general purpose hardware can not work with

symbolic names

Operands have to be memory locations

Either Registers or Memory Addresses


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Instruction Set


Types of Instructions

Instructions to perform fundamental Arithmetic/Logical Operations

Add/subtract operation

Example a = b + c //sum of b and c is placed in a

Converted into machine language of a particular machine


00000010001100100100000000100000

Basically means: add $R1, $R2, $R3, i.e., sum of values kept in CPU registers R2 andR3 is to be placed in register R1

Similarly sub $R1, $R2, $R3

Value kept in register $R3 subtracted from value in $R2 is placed in register $R1


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Instruction Set


Data Movement Instructions

Load and Store Instructions

10001101001010000000010010110000

Load $R1, ($A1) //Load data from memory location M1 into Register R1

10101101001010000000010010110000

Store R1 A1 //Store data from Re ister R1 into memor location M1



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Instruction Set


Logical Operations

AND

Instruction looks as follows: AND $R1, $R2, $R3 // $R1 = $R2 & $R3

Bit-by-bit OR

NOT


Shift right


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Instruction Set


What Distinguishes a Computer from a Calculator?

Instructions for making Decisions

Conditional Branch Instructions, an example

Branch on equal

beq $R1, $R2, L1 //go to the statement labelled L1 if value in R1 == R2


Jump L2 // jump to the statement labelled L2Other such instructions

Branch if not equal

Set on less than


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Instruction Set

An Issue

CPU Instruction Set is in the binary form as the CPU understands only 0s

and 1s

very difficult for human beings to read/write such instructions

Programming is very difficult

Solution


Assembly Language

Symbolic Representation of CPU Instruction Set

Symbolic name for operators and operands

add $R1, $R2, $R3

sub $R1, $R2, $R3

One statement in Assembly produces exactly one machine instructionThere is one-to-one correspondence between machine instructions and statements

in the program


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Instruction Set

Assembly Language

Symbolic Representation of CPU Instruction Set

How do you write a program in this language

Start an Editor

Type characters corresponding to instructions

These characters are saved in ascii format


e.g., Assembly language statement: add $R1, $R2, $R3

Stored form:

a d d $ R

01100001011001000110010000100000001001000101 00100011 0001

Can you execute this statement?


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Instruction Set

Assembly Language

program written in this language can not run as it is in not in binary format

You need a converter from Assembly language to machine language

Assembler



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Programming in High Level Languages

Assembly Language

Is difficult to write programs in Assembly Language also

Not Portable can run on only one type of machine

Very Difficult to Debug and very hard to maintain

Performance can be much better if you write in Assembly performance

critical applications especially which are small in size can be written in this


anguage

Some programs require detailed access to hardware, like device controllers

in some embedded applications so those programs can be written in

Assembly only

Many times in time critical systems a mixed program is a necessity (80-20

principle)


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Programming in High Level Languages

High level Language

Better Maintainability

Closer to language like English

Easier to understand

Easier to write, test & debug programs

Portable across platforms


You need a tool called compiler to convert into native machine language


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Revision History

Version

No.

Date of

Release

Author Reviewer Approver Changes

3.0 16-May-11 Soma Tiwari BS Aligned as per new template



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CS Introduction to Computers 2