Programming Languages

Marjan Sirjani

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2. Language Design Issues

Design toRun efficiently : early languagesEasy to write correctly : new languages

Data typing features in MLClass of C++Package of Ada

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Major influences on designing a language

The underlying computer which programs execute upon;

The execution model, or virtual computer that supports the language on the actual hardware;

The computational model that the language implements.

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2.1. The Structure And Operation Of A ComputerA computer is an integrated set of algorithms and data structures capable of storing and executing programs.Hardware computer or virtual computer

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Major Components of a Computer (From a Programming

Language Designers View)

DataVarious kinds of elementary and structured

data.

Primitive operations

Sequence controlControlling the sequence of primitive

operations execution.

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Major Components of a Computer (From a Programming

Language Designers View)

Data access Controlling the data supplied to each execution of

an operation.

Storage management Controlling the allocation of storage for programs

and data.

Operating environment Providing mechanisms for communication with an

external environment containing programs and data.

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Data

Main memory

High-speed register

High-speed cache memory

External files

Data and Program

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operations

A set of build-in primitive operations Arithmetic operations on each built-in numeric

data (+,-,*,/) Testing various properties of data items (test for

zero, positive, and negative numbers) Accessing and modifying various parts of a data

item Controlling input-output devices Sequence control (jumps)

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Sequence Control

There is an interpreter :Fetch the instructionDecode instructionFetch designated operandsBranch to designated operationExecute primitive operations 1 to n

Using an address register

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Data Access

Access to operands of the operation

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Storage Management

Keeping all resources of the computer operating as much as possibleMemoryCentral processorExternal data devices

MultiprogrammingCache memory

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Operating Environment

The outside world of computer;

a set of peripherals and input-output devices

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Computer States

We saw the static organization of the computer,

We also must see the dynamic operation of it during program execution.

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Language Implementation

Translation (compilation) Accept programs in some source language as input and produce functionally equivalent programs

in another object language as output.

Assembler : assembly to machine language. Compiler : a high level language to assembly or

machine code. Loader,linker editor : machine code in relocatable form

to machine code really executable. Preprocessor : a high level language to a high level

language in a standard form .

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Language Implementation

Software simulation (interpretation)

the simulator executes the input program directly.

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Differences Between Software ImplementationsTranslation versus Interpretation

Physical input sequence –

Logical flow of control

Process each program statement exactly once –

might process some statements repeatedly

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Differences Between Software Implementations

Faster program execution – slow program execution

Loss of information about program – leaving statements in their original form

Object program much larger then the source program –

no space is needed for object program

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Firmware Computers

A common alternative to the strict hardware realization of a computer is the firmware computer,

simulated by a micro-program running on a special micro-programmable hardware computer.

(emulation)

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2.2. Virtual Computers

Hardware realizationPhysical devices

Firmware realizationmicroprogramming

Software simulationSome other programming language

Combination of these techniques

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Syntax and Semantics

Syntax: what the program looks like.

Semantics: the meaning given to the various syntactic constructs.

Example:

V: array [0..9] of integer;

int V[10];

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Hierarchies of Computers

Virtual computer developed by programmer

The language virtual computer

Operating system virtual computer

Firmware virtual computer

Actual hardware computer

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Binding and Binding Times

Binding of a program element to a particular characteristic or property : the choice of the property from a set of possible properties.

Binding time of the property for that element: the time during program formulation or processing when this choice is made.

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Examples Of Binding

Setting the value of an attributeA variable: name , type, storage areaA routine: name formal parameters of a

certain type, certain parameter-passing conventions,

A statement: associated actions.

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Classes of Binding Times

1. Execution time (run time)

(dynamic binding)

2. Translation time (compile time)

3. Language implementation time

4. Language definition time

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1. Execution time :On entry to subprogram or block.

Binding of formal to actual parameters in C.At arbitrary points during execution .

Binding of variables to values by assignments.

(can be modified repeatedly during execution)

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2. Translation time (compile time)Bindings chosen by the programmer

Variable names and typesBindings chosen by the translator

Relative locations of a data object in the storage allocated for a procedure

Bindings chosen by the loaderActual addresses

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3. Language implementation timeThe details associated with the

representation of numbers and of arithmetic operations.

(integer type memory representation)

4. Language definition timePossible statement forms, data structure

types, program structures. (integer type definition)

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Programming languages differ inThe number of entities with which they can

deal,The number of attributes to be bound to

attributes,The time at which such bindings occur

(binding time),The stability of the binding (fixed or

modifiable)

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Importance of Binding Times

Many of the most important differences among languages involve differences in binding times.

Early binding : efficiency.

(FORTRAN types, arithmetic operations)

Late binding : flexibility.

(ML types, string manipulations)

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Binding Time: An Example

X := X + 10

Set of possible types for variable X.

Type of variable X.

Set of possible values for variable X.

Value of the variable.

Representation of the constant 10.

Properties of the operator +.

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Attributes of a variable:NameScopeTypeL-valueR-value

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Name and Scope

Static scope binding: defines the scope in terms of the lexical structure of a program . (C, Pascal)

Dynamic scope binding: defines the scope of a variable’s name in term of program execution. (same dcl until new dcl)

(APL, LISP, SNOBOL)

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Name and Scope (dynamic binding)

{ /* block A*/ int x; … } { /* block B*/ int x; … } { /* block C*/ … x:= …; … }

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Type

Static typing: bind variables to their type at compile time, and the binding can not be changed during execution. Static type checking

Dynamic typing: run-time binding between variables and their types. (according to the value assigned)

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2.3. Language Paradigms

How does the program execute?

What sort of constructs does the language provide?

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Four basic computational models:

Imperative languages

Applicative language

Rule-based language

Object-oriented programming

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Imperative Languages

Imperative or procedural languages are command-driven or statement-oriented languages.Basic concept : machine state.The syntax has the form like: statement1; statement2; …

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Applicative Languages

Applicative or functional languages look at the function that program represents rather than just to the state changes as the program executes, statement by statement.

The syntax has the form like: functionn(…function2(function1 (data))…)

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Rule-based Languages

Rule-based or logical languages execute by checking for the presence of a certain condition and when it is satisfied, they execute an appropriate action.The syntax has the form like: enabling condition1 action1

enabling condition2 action2

…

enabling conditionn actionn

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Object-Oriented Languages

Object-oriented languages can be viewed as combining imperative and functional paradigms. Abstract data types inheritance

efficiency of imperative languages flexibility and reliability of functional languages.

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Generality of Computational Model

How one uses a programming language depends on the programmer.