25836196 linkers amp loaders

7/28/2019 25836196 Linkers Amp Loaders

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Chapter 5 (System Programming by John J. Donovan)


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Chapter Content:


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Users source program decks areconverted to object program decks byassembler & compilers.

The Loaderis a program whichaccepts the object program forexecution by computer & initiates the

execution.


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Allocate space in memory for the program(Allocation).

Resolve symbolic references between objectdecks (Linking).

Adjust all address dependent locations suchas address constants, to corresponds to theallocated space (Relocation).

Physically place the machine instruction &data into the memory (Loading)


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In this scheme the assembler runs in one part of thememory.

The assembler places the assembled machineinstructions and data as they are assembled directly

into their assigned memory locations. When the assembly is complete the assembler

causes a transfer to the starting instruction of theprogram.

This scheme was used by WATFOR FORTAN Compiler. In this scheme the assembler simply places the code

into the core & the loader consists of one instructionthat transfers to the starting address of newlyassembled program.


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Disadvantages:

The portion of memory is wantedbecause the core occupied by assembleris unavailable to the object Program.

It is necessary to retranslate the user

program decks, every time it is run. It is very difficult to handle multiple

segments, specially if the source

program are in different languages. It is difficult to produce orderly modularprogram.


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SourceProgram

deck

Compile-and-go translator

Programloaded inmemory

Assembler


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In this scheme the assembled program isloaded into the core, which eventually freesup the space allocated to assembler in theprevious scheme.

This scheme requires a LOADER, whichaccepts the assemble machine instructions,data & other information present in theobject format and place machine instruction& data in core in an executable format.

No reassembly requires to run the programat later date.

Possible to write different subroutines indifferent languages.


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Figure 5.3

Loader


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In this scheme the assembler outputsthe machine language translation ofthe source program in same form as

that of assemble-and-go scheme,except that the data is punched oncards.

The loader simply accepts the machinelanguage text & places it into core at alocation prescribed by the assembler.


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Figure 5.4


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Disadvantages:

The programmer must specify theassembler the address in the core wherethe program is to be loaded.

In case of multiple subroutines, the

programmer must remember the addressof each and use that absolute addressexplicitly in other subroutines to performsubroutine linkages.


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When we call a subroutine with a symbolreference rather than its absolute address, aseparate mechanism is needed to understandthe symbol reference. Ex: Branching stmt.

The mechanism is typically implemented witha relocating or direct linking loader.

EXTERN: Symbols are defined in one programbut are used in present program.

ENTRY: Symbol defined in one program andreferenced in other.


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The assembler assembles each proceduresegment independently and pass on to theloader text & information as to relocation& intersegment reference.

There are two methods for specifyingrelocation as part of the object program.

a) Modification Record : In this, for each

source program the assembler outputs thetext prefix by a transfer vector thatconsists of address containing names ofthe subroutines referenced by the source

program.


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The length of entire program and length oftransfer vector portion is provided by the

assembler to the loader.

Call tosubroutine

Transfervector

location tosubroutine

Actual corelocation ofSub routine

B) Relocation Bit: The assemblerassociates a bit with each instruction or

address field. If this bit field equals 1then corresponding address field mustbe relocated, otherwise the field is notrelocated.


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Disadvantages:

The Transfer vector linkage is onlyuseful for transfers, and is not suitedfor loading or storing external data.

The transfer vector increases the sizeof object program in memory.


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The assembler provides1. The length of segment

2. A list of all entries and their relative locationwithin the segment

3. A list of all external symbols

4. Information as to where address constants areloaded in the segment and a description ofhow to revise their values.

5. The machine code translation of the sourceprogram and the relative addresses assigned


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Fig 5.7


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External Symbol Dictionary (ESD) record:Entries and Externals

(TXT) records control the actual object

code translated version of the sourceprogram.

The Relocation and Linkage Directory

(RLD) records relocation informationThe END record specifies the starting

address for execution


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Di d


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Disadvantages:

It is necessary to allocate, relocate,

link, and load all of the subroutineseach time in order to execute aprogram

loading process can be extremely timeconsuming.

Though smaller than the assembler,the loader absorbs a considerableamount of spaceDividing the loading process into two

separate programs a binder and a

module loader can solve these


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Binder :

A binder is a program that performs the samefunctions as the direct linking loader

allocation, relocation, and linkingOutputs the text in a file rather than memorycalled a load module.

The module loader merely has to physically

load the module into memory.


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Core image builder: Produces a load module that looks verymuch like a "snapshot" or "image" of asection of core,

Called Core image module.

Link editor, can keep track of therelocation InformationThe load module can be further

relocated

The module loader must perform

allocation and relocation as well as

Di d t


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Disadvantages:

If a subroutine is referenced butnever executed if the programmer had placed a call

statement in the program but was never

executed because of a condition thatbranched around it

the loader would still incur the

overhead or linking the subroutine.All of these schemes require the

programmer to explicitly name allprocedures that might be called.


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If the total amount of memory required by allsubroutines exceeds the amount available

The module loader loads the only the proceduresas they are needed.

Allocating an overlay structureThe Flipper or overlay supervisor is the portion of

the loader that actually intercepts the "calls" andloads the necessary procedure.


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Example:

Suppose a program consisting of fivesubprograms (A{20k},B{20k},C{30k}, D{10k}, and E{20k}) thatrequire 100K bytes of core.Subprogram A only calls B, D and E;subprogram B only calls C and E;

subprogram D only calls E

subprogram C and E do not call anyother routines

Note that procedures B and D are

never in used the same time; neither


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A20K

B20K

C30K

D10K

E20K


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The loading and linking of external referencesare postponed until execution time.

The loader loads only the main program

If the main program shouldexecute a branch to an external address,

reference an external variable

The loader is called

Only then has the segment containing the externalreference loaded.

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