acao chapter - 01

7/30/2019 ACAO Chapter - 01

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Chapter No. 1


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What Is Computational

Model?

The common basis of programminglanguage and computer architecture is

known as computational model.Provides higher level of abstraction than

the programming language and the

architecture.Computational model is the combination

of the above two.


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Model? contn

E.g Von Neuman Architecture andimperative languages, reduction

architecture and functional languages.


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Model? contn


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What is a computer program?

It is an executable representation of some algorithmdesigned to solve some real world problem.

There are thus two elements to a computer program:

Logic - what we what the program to achieve.

Control - how we are going to achieve the end goal.

ALGORITHM = LOGIC + CONTROL


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Imperative Languages

Model of computation based on a step by stepsequences of commands.

Program states exactly how the result is to beobtained.

Destructive assignment of variables.


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Imperative Languages

cont

Order of execution is crucial, commands canonly be understood in context of previouscomputation due to side effects.

Control is the responsibility of the programmer.

E.g ALGOL, Pascal, Ada and C


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Functional Languages

A program in a functional language consists of a

set of (possibly recursive) function definitions

and expression whose value is output as theprogram's result.

Functional languages are one kind of declarative

language.

Declarative languages allow the programmerto concentrate on the logic of an algorithm(declarative languages are goal driven,control is

not the concern of the programmer)


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Declarative Langauge

Model of computation based on a system whererelationships are specified directly in terms of theconstituents of the input data.

Made up of sets of definitions or equations describingrelations which specify what is to be computed, not howit is to be computed.

Non-destructive assignment of variables.

Order of execution does not matter (no side effects).

Expressions/definitions can be used as values.

Programmer no longer responsible for control.


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Interpretation of the Concept

of a Computational Model

The computational model comprises of three sets of

abstraction:Computational Model

Basic Items of

ComputationProblem Description

Model

Execution

Model


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Basic Items of Computation

This is the specification of the items thecomputation refers to the kind computations(operations) that can be performed on them.

E.g of items of computations are:

data, objects or messages, arguments andfunctions, elements of sets and predicate

declared on them.


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Problem Description Model

Refers to both style and method of problem description.

Problem Description Model

Style Method


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Problem Description Style

It specifies how the problems in a particularcomputational model are described.

Style

Procedural Declarative


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Procedural Style

In a procedural style the algorithm for solvingthe problem is stated. A particular solution isthen declared. (Imperative languages usesprocedural style)

int nfac (int n) {

int fac = 1;

if (n > 0)for ( int i = 2; i


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Declarative Style

Facts and relationships related to the problemhave to be stated.

Declarative Style

Using Functions

(applicative

computational

model)

Using predicts (predict

logic based

computational model)


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Declarative Style

Functional style

relationships areexpressed using

functions. E.g. (square (n)

(* n n))

This is a functionsquare,that express therelationship between theinput n and the outputvalue n*n.

Logic style

relationships aredeclared using

expressions known asclauses.

E.g. square(N, M):-

M is N*N

Clauses can be used toexpress both facts andrules.


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Problem Description Method

Procedural style

the problemdescription model

states how a solutionof the given problemhas to described.

Declarative style

the problemdescription model

states how theproblem itself has tobe described.


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Execution Model

Execution model consists of three components.

Interpretation of

the computation Execution Semantics

Execution Model

Control of the

execution

sequence


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Interpretation of the Execution

How to perform the computation?

It relates to problem description method

Problem description method and the interpretation ofthe computation mutually determines and presumeseach other.

In Von Neumann computational model, problemdescription is the sequence of instructions which specify

data and sequence of control instructions and theexecution of the given sequence of instructions is theinterpretation of the computation.


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Execution Semantics

A rule that prescribes how a single execution step is tobe performed.

The rule is associated with the chosen problem

description method and how the execution of thecomputation is interpreted.


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Execution Semantics

State transition

semantics

SLD-

resolution

Execution Model

Dataflow

semanticsReduction

semantics


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Control of the Execution

Sequence

Control of the execution

sequence

Control

Driven

Data

Driven

Demand

Driven


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Control Driven

In control driven execution it is assumed thereexist a program consisting of sequence ofinstructions.

The execution sequence implicitly given by theorder of the instructions

Explicit control instructions can also be used to

specify a departure from the implicit executionsequence.


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Data Driven

It is characterized by the fact that an operationis activated as soon as the data is available.

Also, known as eager evaluation.


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Demand Driven

The operations will be activated only when theirexecution is needed to achieve the final result.

Also known as lazy evaluation because thedelayed until needed philosophy is applied.


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Relationships Between the Concepts of

Computational Model, Programming

Language and the Architecture

Computational

model

Computer

architecture

Programming

language

Specification toolImplementationtool


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Basic Computational Models

Turing

von Neumann

dataflow

applicative

object based

predicate logic based


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Von Neumann Computational

Model

Basic items of computation

data is the basic item of computation

data items are identified by names in order todistinguish between different data items used in thesame computation.

The named entities are known as variables in aprogramming language and in architectures

Multiple data assignments are allowed.


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Von Neumann Computational

Model

Problem description model

The computational task is specified as a sequence ofinstructions (Procedural Model).

Execution model

the computation is performed according to the givensequence of instructions.

Instruction execution follows a state transitionsemantics and the model behaves just like finite statemachine.

Each instruction transfers the state of the machine tothe present state to next one, in a definite way asspecified by the semantics of the instruction.


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Corresponding

programming

languages

Corresponding

architectures

Computational Model


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Key Concepts Related to

Computational Model

Granularity

From computational models point of view granularityis interpreted as the complexity of the items

computation.

From parallel architectures point of view granularityis interpreted as size of parallel computations thatcan be executed without any synchronization or

communication..Granularity can be classified as fine grained and

coarse grained.


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Granularity

Low

High

Language Class

Conventional

assembly

language

ConventionalHigh

language

Granularity Example

Fig 1.21 The interpretation of granularity for programming languages


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Key Concepts Related to

Computational Model

Typing

the concept of typing is used at a higher level inconnection with programming languages but from

computational models point of view typing oflanguages and architecture is closely related.

In typed languages there exist a concept of data typeand the compiler or interpreter checks the

consistency of the types used in function invocation,expressions etc.

the language may be strongly typed or weakly typed.

Strongly typed languages are Pascal, Miranda, hope,

C


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Typing

Weekly type languages are LISP, FP. They are alsoknow as untyped languages

Typed architectures are commonly known as tagged.

They provide a mechanism for typing the data beingstored or processed, by extending the data word bytag.

The tag contain the type identification and usually 3-

5 bits long.Tagging bridge the gap between untyped

architecture and weakly typed language

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