sub program control

8/6/2019 Sub Program Control

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Chapter 9: Subprogram Control

Subprogram Sequence Control

Attributes of Data Control Parameter passing

Explicit Common Environments

Subprogram Control :interaction among subprogramshow subprograms pass data among themselves


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Subprogram Sequence Control

Simple subprogram call returnCopy rule view of subprograms:

the effect of a call statement is the

same as if the subprogram werecopied and inserted into the mainprogram.


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Simple flow of execution

CALL

RETURN


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Simple call-return subprograms

Execution of subprograms

Subprogram definition.

Subprogram activation.


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Subprogram definition

The definition is translated into a template,

used to create an activation each time asubprogram is called.


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Subprogram activationa code segment(the invariant part) -

executable code and constants,

anactivationrecord (the dynamicpart) -local data, parameters.

created anew each timethe subprogramis called,

destroyed whenthe subprogramreturns.


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System-definedpointers

y Current-instructionpointer

CIPaddressofthe nextstatementtobe

executed

y Current-environmentpointer CEPpointertothe activationrecord.


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Oncall

instruction

Anactivationrecord iscreated

CurrentCIP and CEP are saved inthe createdactivationrecord asreturnpoint

CEP isassigned the addressofthe activationrecord.

CIP getsthe addressofthe firstinstructioninthecode segment

The executioncontinuesfrom the addressinCIP


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Onretu

rn

The old values ofCIP and CEP are retrieved.

The executioncontinues from the address inCIP

Restrictions of the model:

at most one activationof any subprogram


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The simplestimplementation

Allocate storage for asingle activation record

statically asanextensionofthe code segment.Used inFORTRANand COBOL.

The activationrecord isnotdestroyed - only

reinitialized for eachsubprogramexecution.

Hardware support- CIP isthe programcounter,CEP isnotused, simple jumpexecuted onreturn.


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Stack-basedimplementationThe simplestrun-time storage managementtechnique

callstatements: push CIP andCEPreturnstatements: popCIP andCEP offofthe stack.

UsedinmostCimplementations

LISP: usesthe stackasanenvironment.


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Recursive Subprograms

SpecificationSyntactically - no difference

Semantically - multiple activations of thesame subprogram exist simultaneously atsome point in the execution.

E.G. the first recursive call creates a secondactivation within the lifetime of the firstactivation.


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Implementation

Stack-based -

CIP and CEP are stored in stack, forminga dynamic chain of links.

A new activation record is created foreach call and destroyed on return.

The lifetimes of the activation recordscannot overlap - they are nested.


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Attributes of Data ControlData control features determine theaccessibility of data at different points during

program execution.

Central problem:

the meaning of variable names, i.e. thecorrespondence between names and memorylocations.


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Names and Referencing

Environments

Two ways to make a data object available as an

operand for an operation

Direct transmission

Referencing through a named data object


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Directtransmission

A dataobjectcomputed atone pointastheresultofanoperation may be directly

transmitted toanotheroperationasanoperand

Example: x = y + 2*z;

The resultofmultiplicationistransmitteddirectly asanoperand ofthe additionoperation


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Referencingthrough

anameddataobject

Adataobjectmay be givenaname

whenitis created,

the name may thenbe usedtodesignate it

as anoperandofanoperation.


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Programelementsthatmaybe

namedTobe discussednextVariablesFormalparameters

Subprograms

resolved at translation time:Defined types

DefinedconstantsLabelsException namesPrimitive operations

Literalconstants


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Associations and Referencing

Environments

Association: binding identifiers to particular dataobjects and subprograms

Referencing environment: the set of identifierassociations for a given subprogram.

Referencing operations during programexecution: determine the particular data object orsubprogram associated with an identifier


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Localreferencingenvironment

The setofassociations created onentrytoa subprogram

formal parameters,

local variables, and

subprograms defined only withinthatsubprogram


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Non-localreferencingenvironment

The setofassociations foridentifiers used withina subprogram

not created onentry toit

Global referencingenvironment:associations created atthe startofexecutionofthe

mainprogram, available tobe used ina subprogram

Predefined referencingenvironments:predefined associations inthe language definition


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A

ssociationsVisibility of associations

Associationsare visible ifthey arepartofthe referencing environment.Otherwise associationsare hidden

Dynamic Scope of associations

The setofsubprogram activationswithinwhich the associationisvisible


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AliasesforDataObjects

Multiple namesofadataobject- separate environments- noproblem- in asingle referencing environment- called

aliases.

Problemswith aliasing Can make code difficulttounderstand

Implementation difficultiesatthe optimizationstep - difficulttospotinterdependentstatements-nottoreorderthem


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Example of aliasingProgram main;

var I: integer;

procedure Sub1 ( var J: integer);

begin

(* I and J refer to same dataobject *)

end;

begin

. Sub1(I);

.

end.


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Static and Dynamic Scope

The dynamic scope of an association for anidentifier:

the set of subprogram activations in which the

association is visible during execution. tied to the dynamic chain of subprogram

activations.

The static scope of a declaration

the part ofthe program textwhere thedeclared identifier is used.


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Dynamicscope rules

Staticscope rules

Dynamic scope rules :Relate references with associations fornamesduring program execution

Static scope rules :

relate references with declarations ofnamesinthe programtext.


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B

lock structure

Block-structured languages :

y Each program or subprogram is organized asa set of nested blocks.

y

Each block introduces a new localreferencing environment.


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Subprogram A

Declaration of X

Declaration of Y

Hidden to A

SubprogramB

Declaration of Y

Declaration of Z

Use of Y

Use of X

Use of Z

Static scoperules forblock-structured

programs


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Local Data and Local Referencing

Environments

Local environment of a subprogram:various identifiers declared in the subprogram :

variables, parameters, subprogram names.

Static scope rules: implemented by means of atable of the local declarations

Dynamic scope rules:Retention - Associations and the bound values

are retained after executionDeletion - Associations are deleted


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Implementationofdynamicscope

rules

Bymeansofalocalenvironmenttable toassociate

names, typesandvalues.

Retention: the table iskeptaspartofthe codesegment

Deletion: the table iskeptaspartoftheactivationrecord, destroyedaftereach execution.

sub program control

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