Chapter 5Chapter 5

Abstraction I: Abstraction I:

EncapsulationEncapsulation

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In the construction of large programs, In the construction of large programs, we need to design and implement new we need to design and implement new data types.data types.• Designing the specification of the abstract Designing the specification of the abstract

data type (attributes and operations).data type (attributes and operations).• Implementing it.Implementing it.

Like aLike a section section in a university registration in a university registration system.system.

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Basic mechanisms providing the Basic mechanisms providing the programmer ability of creating new programmer ability of creating new data types and operations on that type:data types and operations on that type:• SubprogramsSubprograms

– correct use of the new type is the programmer correct use of the new type is the programmer responsibility.responsibility.

• Type declarationsType declarations– the programming language partly support the the programming language partly support the

correct use of the new type.correct use of the new type.

• InheritanceInheritance

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5.1. 5.1. Abstract Data TypesAbstract Data Types

Early languages (FORTRAN,COBOL)Early languages (FORTRAN,COBOL)• subprogram definitionsubprogram definition

Ada and C++ Ada and C++ • package and classpackage and class

data typedata type• a set of data objectsa set of data objects• valid operations on them (early 1970s)valid operations on them (early 1970s)

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Data AbstractionData Abstraction

A set of A set of data objects,data objects, ordinarily using ordinarily using one or more type definitions,one or more type definitions,

A set of A set of abstract operations abstract operations on those on those data objects, anddata objects, and

EncapsulationEncapsulation of the whole in such a of the whole in such a way that the user of the new type way that the user of the new type cannot manipulate data objects of the cannot manipulate data objects of the type except by the operations defined.type except by the operations defined.

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Information HidingInformation Hiding

Each program component should Each program component should hide as much information as hide as much information as possible from the users of the possible from the users of the component.component.

Program designProgram design

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EncapsulationEncapsulation

The user of the abstractionThe user of the abstraction• Does not need to know the hidden Does not need to know the hidden

information in order to use the information in order to use the abstraction, andabstraction, and

• Is not permitted to directly use or Is not permitted to directly use or manipulate the hidden information manipulate the hidden information even if desiring to do so.even if desiring to do so.

Language designLanguage design

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5.2. Encapsulation By 5.2. Encapsulation By SubprogramsSubprograms

An abstract operation defined by An abstract operation defined by the programmer.the programmer.

A subprogram definition:A subprogram definition:• specificationspecification• implementationimplementation

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Specification of a Specification of a SubprogramSubprogram

The The name name of the subprogram.of the subprogram. The The signaturesignature (prototype) of the (prototype) of the

subprogram (number, order and subprogram (number, order and data type of arguments and data type of arguments and results).results).

The The action action performed by the performed by the subprogram, (a description of the subprogram, (a description of the function it computes).function it computes).

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In C :In C :• void Sub(float X, int Y, float *Z, int *W);void Sub(float X, int Y, float *Z, int *W);

In Ada :In Ada :• procedure Sub(X: in REAL; Y: in integer; Z: procedure Sub(X: in REAL; Y: in integer; Z:

in out REAL; W: out BOOLEAN)in out REAL; W: out BOOLEAN)

• Sub : real1* integer * real2 --> real3 * Sub : real1* integer * real2 --> real3 * BooleanBoolean

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Implementation of a Implementation of a SubprogramSubprogram

Subprogram body:Subprogram body:• local data declarationslocal data declarations• statementsstatements

float FN(float X, int Y)float FN(float X, int Y) signaturesignature

{float M(10); int N;{float M(10); int N; dcldcl

… … statementsstatements

}} subprograms are like primitive operations, subprograms are like primitive operations,

but the programmer must explicitly declare but the programmer must explicitly declare the arguments and results types.the arguments and results types.

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Subprogram Definition and Subprogram Definition and InvocationInvocation

Definitions and activationsDefinitions and activations• Programmer writes a subprogram Programmer writes a subprogram

definition as a static property of a definition as a static property of a program.program.

The definition serves as a template for The definition serves as a template for creating activations during execution.creating activations during execution.

• During execution, if the subprogram During execution, if the subprogram is called (or invoked), an activation of is called (or invoked), an activation of it is created. it is created.

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SubprogramSubprogram definition definition is like a is like a type definitiontype definition

and subprogram and subprogram activationactivation is like is like a data object of that type.a data object of that type.

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Subprogram ActivationSubprogram Activation

Is a type of data object,Is a type of data object, as a block of storage that contains as a block of storage that contains

certain component data items,certain component data items, storage allocated when it is created, storage allocated when it is created,

freed when it is destroyed,freed when it is destroyed, has a has a lifetimelifetime (between (between callcall and and returnreturn),), it is executed, reference and modify it is executed, reference and modify

other data objects during execution. other data objects during execution. (these are the differences with other data objects)(these are the differences with other data objects)

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Implementation of Implementation of Subprogram Definition and Subprogram Definition and invocationinvocation

Float FN(float X, int Y) Float FN(float X, int Y)

{const initval=2;{const initval=2;

#define finalval 10#define finalval 10

float M(10);int N;float M(10);int N;

……

N = initval;N = initval;

if (n<finalval){…}if (n<finalval){…}

return (20*X+M(N));}return (20*X+M(N));}

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Components needed for Components needed for an activation of the an activation of the subprogramsubprogram

FN’s signature: storage for FN’s signature: storage for parametersparameters and and resultresult

DCLs: storage for DCLs: storage for local variableslocal variables storage for storage for literalsliterals and and defined defined

constantsconstants statements: storage for statements: storage for executable codeexecutable code

fig. 5.1. fig. 5.1. Page 205Page 205

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Structure of a subprogram Structure of a subprogram activationactivation

Split the template into two parts:Split the template into two parts:• A static part, A static part, code segment. code segment.

– Constants and executable code. Constants and executable code. – Invariant during execution.Invariant during execution.– Shared by all activations.Shared by all activations.

• A dynamic part, A dynamic part, activation record.activation record.– Parameters,function results, local data , Parameters,function results, local data ,

housekeeping data (temporary storage areas, housekeeping data (temporary storage areas, return points, linkages for referencing non-local return points, linkages for referencing non-local variables).variables).

– A copy for each activation. A copy for each activation. (fig. 5.2. P.206)(fig. 5.2. P.206)

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Storage ManagementStorage Management

Access to the components: using Access to the components: using the base-address-plus-offset the base-address-plus-offset calculation.calculation.

Allocation of a block of Allocation of a block of appropriate size on a subprogram appropriate size on a subprogram call, freeing the block on return.call, freeing the block on return.

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Generic SubprogramsGeneric Subprograms

A generic subprogram name is A generic subprogram name is overloaded .overloaded .

p.207, p.208p.207, p.208

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Type DefinitionsType Definitions

Type definition: definition of a Type definition: definition of a class of data objects.class of data objects.• A type nameA type name• declaration of the structure of a class declaration of the structure of a class

of data objects.of data objects.

p. 209,210p. 209,210

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Advantages of type Advantages of type definitionsdefinitions

Simplifying program structure,Simplifying program structure, Simplifying the program Simplifying the program

modification,modification, as an argument to a subprogram.as an argument to a subprogram.

A kind of encapsulation and A kind of encapsulation and information hiding.information hiding.

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implementationimplementation

A type definition is used only during A type definition is used only during translation.translation.• Enters the information from a type Enters the information from a type

definition into a table during definition into a table during translation,translation,

• using it during execution.using it during execution.

(storage representation: storage (storage representation: storage management, type checking)management, type checking)

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Type EquivalenceType Equivalence

What does it mean to say that two What does it mean to say that two types are “the same”?types are “the same”? (a data type issue)(a data type issue)

What does it mean to say that two What does it mean to say that two data objects of the same type are data objects of the same type are “equal”?“equal”? (a semantic issue, r-value of a data (a semantic issue, r-value of a data

object)object)

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Type EqualityType Equality

ProgramProgram main(input, output); main(input, output);

typetype Vect1: Vect1: arrayarray[1..10] [1..10] ofof real; real;

Vect2: Vect2: arrayarray[1..10] [1..10] ofof real; real;

var var X,Z: Vect1; Y:Vect2;X,Z: Vect1; Y:Vect2;

procedure procedure Sub(A: Vect1); Sub(A: Vect1);

… …

end;end;

begin -begin -main programmain program

X:= Y;X:= Y;

Sub(y)Sub(y)

end.end.

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Type EqualityType Equality

Name equivalenceName equivalence• only if they have the same only if they have the same namename..• in Ada, C++, subprogram parameters in Pascalin Ada, C++, subprogram parameters in Pascal

Structural equivalenceStructural equivalence• if they define data objects that have the same if they define data objects that have the same

internal components. internal components. ( the same storage ( the same storage representation so the same accessing formulas representation so the same accessing formulas and the same run-time implementation)and the same run-time implementation)

• older languages (FORTRAN, COBOL, PL/1), C, older languages (FORTRAN, COBOL, PL/1), C, PascalPascal

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Name Equivalence Name Equivalence DisadvantagesDisadvantages

There can be no There can be no anonymous types.anonymous types.• If we have If we have

var W: array[1..10] of real;var W: array[1..10] of real;

W can’t be used as an argument, its type has W can’t be used as an argument, its type has no name.no name.

A single global type definition must be A single global type definition must be used. A single type definition must be used. A single type definition must be used in all or large parts of a program used in all or large parts of a program (transmitting as an argument through a (transmitting as an argument through a chain of subprograms).chain of subprograms).

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Structural Equivalence Structural Equivalence DisadvantagesDisadvantages

Several questions arise when two types are Several questions arise when two types are structurally equivalent.structurally equivalent.• For records, must the component names be identical?For records, must the component names be identical?• Or does it suffice to have the same number and type of Or does it suffice to have the same number and type of

components in the same order?components in the same order?• If record component names must be identical, then If record component names must be identical, then

must the components be in the same order?must the components be in the same order?• Must array subscript ranges be identical?Must array subscript ranges be identical?• Or is it sufficient to have the same number of Or is it sufficient to have the same number of

components?components?• Must the literals in two enumeration types be the same Must the literals in two enumeration types be the same

and in the same order? and in the same order?

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Structural Equivalence Structural Equivalence DisadvantagesDisadvantages(cont.)(cont.)

Two variables may be structurally Two variables may be structurally equivalent, even though the equivalent, even though the programmer declares them as programmer declares them as separate types.separate types.• For example: type Meters = integer;For example: type Meters = integer;

liters = integer;liters = integer;

var Len: Meters;var Len: Meters;

Vol: Liters;Vol: Liters;

many type errors may go undetected.many type errors may go undetected.

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Structural Equivalence Structural Equivalence DisadvantagesDisadvantages(cont.)(cont.)

Determining whether two complex Determining whether two complex type definition are structurally type definition are structurally equivalent, if done frequently, may equivalent, if done frequently, may be a costly part of translation.be a costly part of translation.

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Data Object EqualityData Object Equality

Once the compiler determines that Once the compiler determines that two objects are the same type, are the two objects are the same type, are the two objects equal?two objects equal?

• Stack equality.Stack equality.• Set equality.Set equality.

Programmer-defined data, separate Programmer-defined data, separate operation for equaloperation for equal

P. 214P. 214

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Type Definitions with Type Definitions with ParametersParameters

Parameterizing the type definition Parameterizing the type definition to be used repeatedly with to be used repeatedly with different substitutions for the different substitutions for the parameters.parameters.

Parameters likeParameters like• class size in Adaclass size in Ada• a type in MLa type in ML• no parameterized types in Pascalno parameterized types in Pascal

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5.4. Storage Management5.4. Storage Management

Different features in a language causes Different features in a language causes different storage management techniques different storage management techniques to be used.to be used.• FORTRAN: no recursive calls, no dynamic FORTRAN: no recursive calls, no dynamic

storage management.storage management.• Pascal: stack-based storage management.Pascal: stack-based storage management.• LISP: garbage collection. LISP: garbage collection.

Language implementers decide about the Language implementers decide about the details. details.

Programmers don’t know about it.Programmers don’t know about it.

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Major Run-Time Elements Major Run-Time Elements Requiring StorageRequiring Storage

DataData operationsoperations

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Data and program Data and program Requiring StorageRequiring Storage

Code segments for translated user Code segments for translated user programs.programs.

System run-time programs.System run-time programs.• Supporting user programs.Supporting user programs.• Like library routines, software interpreters or Like library routines, software interpreters or

translator, storage management routines.translator, storage management routines. User-defined data structures and User-defined data structures and

constants.constants. Subprogram return points.Subprogram return points.

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Referencing environments.Referencing environments.• Identifier associations (LISP A-list)Identifier associations (LISP A-list)

Temporaries in expression evaluation.Temporaries in expression evaluation.• Recursive function calls make it a lot.Recursive function calls make it a lot.

Temporaries in parameter Temporaries in parameter transmission.transmission.• Resulting values for evaluation of actual Resulting values for evaluation of actual

parameters are stored in temporaries until parameters are stored in temporaries until the total evaluation is completed.the total evaluation is completed.

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Input-output buffers.Input-output buffers.• Temporary storage areas used between Temporary storage areas used between

the time of the actual physical transfer the time of the actual physical transfer of the data to or from external storage of the data to or from external storage and the program-initiated input / output and the program-initiated input / output operation.operation.

Miscellaneous system data.Miscellaneous system data.• System data like tables, status System data like tables, status

information for input-output, ...information for input-output, ...

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Major operations requiring Major operations requiring storagestorage

Subprogram call and return operations.Subprogram call and return operations.• Activation record,Activation record,• local referencing environment, …local referencing environment, …

Data structure creation and destruction Data structure creation and destruction operations.operations.• new - dispose in Pascal.new - dispose in Pascal.• malloc - free in Cmalloc - free in C

Component insertion and deletion operations.Component insertion and deletion operations.• ML and Lisp list operations, inserting a ML and Lisp list operations, inserting a

component into a list.component into a list.

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Programmer- and System- Programmer- and System- Controlled Storage Controlled Storage ManagementManagement

Programmer control of storage Programmer control of storage managementmanagement• place a large and often undesirable place a large and often undesirable

burden on the programmer,burden on the programmer,• may interfere with the necessary may interfere with the necessary

system-controlled storage system-controlled storage management.management.

Programmer can cause dangling Programmer can cause dangling references and garbage.references and garbage.

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What language?What language?• Protection for the programmer by using a Protection for the programmer by using a

language with strong typing and language with strong typing and effective storage management features,effective storage management features,

• decrease in performancedecrease in performance

OROR• performanceperformance• more risk in having errors and fail during more risk in having errors and fail during

execution.execution.

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Storage Management Storage Management PhasesPhases

Initial allocationInitial allocation RecoveryRecovery Compaction and reuseCompaction and reuse

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Static Storage Static Storage ManagementManagement

SimplestSimplest static allocationstatic allocation no run-time storage managementno run-time storage management no concern for recovery and reuseno concern for recovery and reuse efficientefficient in COBOL and FORTRANin COBOL and FORTRAN

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In FORTRANIn FORTRAN• each subprogram is compiled each subprogram is compiled

separately,separately,• the code segment includes an the code segment includes an

activation recordactivation record– compiled program,compiled program,– its data areas,its data areas,– return point location,return point location,– miscellaneous items of system data.miscellaneous items of system data.

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Stack-Based Storage Stack-Based Storage ManagementManagement

Simplest run-time storage management Simplest run-time storage management technique.technique.

Based on the nested last in first out Based on the nested last in first out structure in subprograms calls and structure in subprograms calls and returns.returns.

Automatic compaction.Automatic compaction. In Pascal : a single central stack of activation records, In Pascal : a single central stack of activation records,

and a statically allocated area for subprogram code and a statically allocated area for subprogram code segments and system programs.segments and system programs.

P.222, fig. 5.5P.222, fig. 5.5

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Heap Storage Heap Storage Management: Fixed-Size Management: Fixed-Size ElementsElements

A heap is a block of storage within A heap is a block of storage within which pieces are allocated and freed in which pieces are allocated and freed in some relatively unstructured manner.some relatively unstructured manner.

Need for heap , when a language Need for heap , when a language permits storage to be allocated and permits storage to be allocated and freed at execution time.freed at execution time.

Fixed size elements allocated => no Fixed size elements allocated => no need for compaction.need for compaction.

Page 224, fig. 5.7.Page 224, fig. 5.7.

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RecoveryRecovery

The problem: identification of reusable The problem: identification of reusable element, solutions:element, solutions:

Explicit return by programmer or system.Explicit return by programmer or system.• Natural, but cause garbage and dangling Natural, but cause garbage and dangling

reference. P.226reference. P.226 Reference counts.Reference counts.

• Cost of maintaining. P.227Cost of maintaining. P.227• popular with parallel processing systems.popular with parallel processing systems.

Garbage collection.Garbage collection.

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Garbage CollectionGarbage Collection

Dangling references more dangorousDangling references more dangorous

Two stagesTwo stages• MarkMark

– garbage collection bit, set off if it is active.garbage collection bit, set off if it is active.

• SweepSweep– links the “on” elements to the free list.links the “on” elements to the free list.

When is a heap element active?When is a heap element active?– There is a pointer to it from There is a pointer to it from

– outside the heapoutside the heap– another active heap elementanother active heap element

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Three critical assumptionsThree critical assumptions• any active element must be reachable by a any active element must be reachable by a

chain of pointers beginning outside the heap.chain of pointers beginning outside the heap.• It must be possible to identify every pointer It must be possible to identify every pointer

outside the heap that points to an element outside the heap that points to an element inside the heap.inside the heap.

• It must be possible to identify within any It must be possible to identify within any active heap element the fields that contain active heap element the fields that contain pointers to other heap elements.pointers to other heap elements.

P. 231P. 231

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Heap Storage Heap Storage Management:Management:Variable-Size ElementsVariable-Size Elements

More difficultMore difficult if space for programmer defined if space for programmer defined

data structures is sequential, like data structures is sequential, like arrays or activation records.arrays or activation records.

Major difficulty : reuse of Major difficulty : reuse of recovered space.recovered space.

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Initial allocation and reuse.Initial allocation and reuse. reuse directly from a free-space list.reuse directly from a free-space list.

• First-fit methodFirst-fit method• best-fit methodbest-fit method

keeping free-space list in size order.keeping free-space list in size order. Recovery with variable-size blocks.Recovery with variable-size blocks.

• In first word of each block: a length indicator.In first word of each block: a length indicator. Compaction and memory fragmentation Compaction and memory fragmentation

problem.problem.

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Compaction approaches:Compaction approaches:• Partial compactionPartial compaction

– only adjacent free blocksonly adjacent free blocks

• Full compactionFull compaction– active blocks may be shiftedactive blocks may be shifted

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End of chapter 5End of chapter 5