unit-iii-loader & linker - sri eshwar

System SoftwareLoaders & Linkers

Veningston .KDepartment of CSE

Government College of Technology, [email protected]
mailto:[email protected]

OutlineLoaders & Linkers

Basic loader functions Design of an Absolute Loader A Simple Bootstrap Loader Machine dependent loader features

Program Relocation Program Linking Algorithm and Data Structures for Linking Loader

Machine-independent loader features Automatic Library Search Loader Options

Loader design options Linkage Editors Dynamic Linking Bootstrap Loaders

24/August/2013 2System Software - Sri Eshwar College of

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Recap [1/3]Machine architectures

Simplified Instructional Computer (SIC)

SIC/XE [eXtra Equipment]

AssemblerTranslate mnemonic operation codes to their machine language equivalents

Assign machine addresses to symbolic labels used by the programmer

Write the object program


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Recap [2/3]Format of Object program


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Recap [3/3]

Object program contains translated instructions and data values from the source program, and specifies addresses in memory where these items are to be loaded


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Basic loader functionsLoading

Brings object program into memory

RelocationModifies the object program so that it can be loaded at an address different from the location originally specified

LinkingCombines two or more separate object programs and supplies information needed to allow references between them

may be a single system programLoader: loading and relocation

Linker: linking Linking Loader


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Absolute loader

No linking and relocation neededAll functions are accomplished in a single passRecords in object program perform

Header recordCheck the Header record for program name, starting address, and length (available memory)

Text recordBring the object program contained in the Text record to the indicated address

End recordTransfer control to the address specified in the End record to begin execution of the loaded program


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Loading of an absolute program [1/2]

Object program


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Loading of an absolute program [2/2]Program loaded in memory

No text recordthese locations remain unchanged


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Algorithm for an absolute loader


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Object Code Representation

Character formEach byte of assembled code is given using its hexadecimal representation in character form

Easy to read by human beings

Inefficient in terms of space and execution time

Binary formEach byte of object code is stored as a single byte in the object program

Most machine store object programs in a binary formLess space and loading time

Not good for reading


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A simple bootstrap loader

Bootstrap Loader (usually in ROM)When a computer is first tuned on or restarted, a special type of absolute loader, the bootstrap loader loads the first program (usually O.S.) to be run into memory

SIC bootstrap loaderThe bootstrap itself begins at address 0It loads the OS starting address 0x80No header record, end record or control informationThe object code is loaded into consecutive bytes of memory starting at address 80After loading the OS, the control is transferred to the instruction at address 80.Begins the execution of the program

24/August/2013System Software - Sri Eshwar College of

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SIC bootstrap loader


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Algorithm for SIC/XE bootstrap loader


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Machine-Dependent Loader Feature

Merits of an absolute loaderSimple and efficient

limitation of an absolute loaderProgrammer needs to specify the actual address at which it will be loaded into memory.It is difficult to run several programs concurrently, sharing memory between them.It is difficult to use subroutine libraries efficiently.

SolutionA more complex loader that provides

Program relocationProgram linking


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(Recap) Program relocation

Why?

It is desirable to load and run several programs at the same time

The system must be able to load programs into memory wherever there is room

The exact starting address of the program is not known until load time


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It is not practical to plan program execution (we do not know exactly when jobs will be submitted, exactly how long they will run)

(Recap) Example of program relocation


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Absolute program

Program with starting address specified at assembly time

The address may be invalid if the program is loaded into somewhere else.

Example


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The only parts of the program that requiremodification at load time are those that specify direct addressesThe rest of the instructions need not be modified

Not a memory address (immediate addressing)PC-relative, Base-relative

From the object program, it is not possible to distinguish the address and constant

The assembler must keep some information to tell the loaderThe object program that contains the modification record is called a relocatable program


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(Recap) The way to solve the relocation problem

For an address label, its address is assigned relative to the start of the program (START 0)

Produce a Modification record to store the starting location and the length of the address field to be modified.

The command for the loader must also be a part of the object program


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(Recap) Modification record

One modification record for each address to be modified

The length is stored in half-bytes (4 bits)

The starting location is the location of the byte containing the leftmost bits of the address field to be modified.

If the field contains an odd number of half-bytes, the starting location begins in the middle of the first byte.


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(Recap) Modification record


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(Recap) Relocatable Object Program


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Relocation

Loaders that allow for program relocation are called relocating loaders or relative loaders.

Efficient sharing of the machine requires that we write relocatable programs instead of absolute ones.

The way relocationis implemented in a loader is also dependent upon machine characteristics.

Linkingis not a machine-dependent function


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Relocation

Two methods for specifying relocation as part of the object programModification records

For a small number of relocations required when relative or immediate addressing modes are extensively used

Relocation bitsFor a large number of relocations required when only direct addressing mode can be used in a machine with fixed instruction format (e.g., the standard SIC machine)


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Object program with relocation by modification records


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Relocatable program for SIC


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Review on Relocatable program for SIC

The standard SIC machine does not use relative addressing (PC-relative, Base-relative)

All instructions except RSUB in this program need relocation [require 31 Modification records]

The modification record scheme is a convenient means for specifying program relocation; however, it is not well suited for use with all machine architectures


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Too many modification records

Relocation Bits

If there are many addresses needed to be modified, it is more efficient to use a relocation bit, instead of a Modification record, to specify every relocation.

When the instruction format is fixed

There is a relocation bit for each word of the object program

Relocation bits are put together into a bit mask

If the relocation bit corresponding to a word of object code is set to 1, the added to this word when the program is relocated


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Relocation Bits


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Relocation Bits

A bit value of 0 indicates that no modification is necessary [e.g. data constants and JSUB]

If a text record contains fewer than 12 words of object code, the bits corresponding to unused words are set to 0

A new Text record is created for proper alignment

Relocation bits are generated by the Assembler and used by the Loader.


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Program linking

Goal

Resolve the problems with EXTREF and EXTDEF from different control sections

A program is a logical entity that combines all of the related control sections.

Control sections could be assembled together, or they could be assembled independently of one another.

Control sections are to be linked, relocated, and loaded by loaders.


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(Recap)Expressionthe use of expressions as operand

The assembler evaluates the expressions and produces a single operand address or valueExpressions consist of

Operator+,-,*,/ (division is usually defined to produce an integer result)

Individual termsConstantsUser-defined symbolsSpecial terms, e.g. *, the current value of LOCCTR

ExamplesMAXLEN EQU BUFEND-BUFFERSTAB RESB (6+3+2)*MAXENTRIES


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(Recap)Relocation Problem in ExpressionsValues of terms can be

Absolute (independent of program location)Constants

Relative (to the beginning of the program)Address labels* (value of LOCCTR)

Expressions can beAbsolute

Only absolute termsMAXLEN EQU 1000

Relative terms in pairs with opposite signs for each Pair [independent on program starting address]

MAXLEN EQU BUFEND-BUFFER

RelativeAll the relative terms except one can be paired as described in

unpaired relative term must have a positive sign.

STAB EQU OPTAB + (BUFEND BUFFER)


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(Recap) Program blocks vs. Control sections

Program blocks (Assembler directive: USE)

Segments of code that are rearrangedwithin a single object program unit

Control sections(Assembler directive: CSECT)

Segments of code that are translated into independent object program units


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(Recap) Control Sections and Program Linking

Control sectionscan be loaded and relocated independently of the other control sections

are most often used for subroutines or other logical subdivisions of a program

the programmer can assemble, load, and manipulate each of these control sections separately

because of this, there should be some means for linking control sections together


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(Recap) External Definition and Reference

Instructions in one control section may need to refer to instructions or data located in another sectionExternal definition

EXTDEF name [, name]EXTDEF names symbols that are defined in this control section and may be used by other sectionsE.g. EXTDEF BUFFER, BUFEND, LENGTH

External referenceEXTREFname [,name]

EXTREF names symbols that are used in this control section and are defined elsewhereE.g. EXTREF RDREC, WRREC

To reference an external symbol, extended format instruction is needed


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(Recap) Records for Object Program

The assembler must include information in the object program that will cause the loader to insert proper values where they are required


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(Recap) Records for Object Program


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Program linking

Example

Program for Linking and Relocation

Use modification records for both relocation and linking

address constant

external reference


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Program for Linking and Relocation [1/3]


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About the assembly program

3 control sections: PROGA, PROGB, PROGC

List of items:LISTA, LISTB, LISTC

Set of references to external symbols

Instruction operands: REF1, REF2, REF3

Values of data words:REF4 through REF8

These programs are given to emphasize the relationship between the relocation and linking process.


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From the loader point of view

Programmer think of a program as a logical entity that combines all of the related control sections

However, from the loader point of view, there is no such thing as a program. There are only control sections that are to be linked, relocated, and loaded.

The loader do not need to knowwhich control sections were assembled at the same time


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Object programs [1/3]


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About the object program

Local reference is assembled using PC-relative instruction with no relocation or linking required

Modification record instructing the loader to add the value of the external symbol to this address field when the program is linked


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Note

The general approach taken is for the assembler to evaluate as much of the expression as it can

The remaining terms are passed on to the loader via modification records


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Example - 1

Consider REF4, the assembler for PROGAcan evaluate all of the expression in REF4except for the value of LISTC.

This results in an initial value of 000014and one Modification record


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Example - 2

Consider REF4, in PROGBcontains no terms that can be evaluated by the assembler

The object code therefore contains an initial value of 000000and three Modification record


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Example - 3

Consider REF4, for PROGCassembler can supply the value of LISTCrelative to the beginning or the program (but not the actual address which is not known until the program is loaded)

The initial value of this data word contains the relative address of LISTC000030

Modification records instruct the loader to add the beginning address of the program (i.e. the value of PROGC), to add the value of ENDA, and to subtract the value of LISTA


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Inference from the example 1, 2 & 3

The expression in REF4represents

a simple external reference for PROGA

A more complicated external reference for PROGB

A combination of relocation and external references for PROGC


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Exercise

To work through references REF5through REF8by yourself to be sure that you have understood how the object code and modification records are generated


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Programs after linking and loading


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Inference from the previous Figure

Note that each of REF4through REF8has resulted (after relocation and linking is performed) in the same value in each of the three programs, Since the same source expression appeared in each program


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Example

The value for reference REF4in PROGAis located at address 4054(the beginning address of PROGA+ 0054, the relative address of REF4within PROGA)

Next slide shows how this value is computed?


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Relocation and linking operationsperformed on REF4


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Note

For the reference that are instruction operands, the calculated values after loading do not always appear to be equal

This is because there is an additional address calculation step involved for PC-relativeor Base-relativeinstructions.


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Algorithm and data structurefor a linking loader

Programmer do not need to worry about how these calculations are actually performed by the loader

The algorithm and data structures does


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Algorithm and data structurefor a linking loader

Considerations by SIC/XE:

Most instructions uses relative addressing; no relocation is necessary

Modification records are used in this type of machine

A linking loader usually makes two passes over its input

Because some external symbols are processed before read


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Two passes linking loader

Two Passes Logic

Pass 1: assign addresses to all external symbols

Pass 2: perform the actual loading, relocation, and linking


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Linking loader - Pass 1 [1/3]

Assign address to all external symbolsOnly processes Header Record and Define RecordBuilds an external symbol table (ESTTAB)

its nameits addressin which control section the symbol is defined

Program Load Address (PROGADDR)The beginning address in memory where the linked program is to be loaded (supplied by OS).

Control Section Address (CSADDR)The starting address assigned to the control section currently being scanned by the loader.CSADDR is added to all relative addresses within the control section


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Add symbol to ESTAB

Control section name: (name, CSADDR) ESTAB

Get control section name from H record

If the first control sectionCSADDR = PROGADDR

When E record is encountered, read the next control section

CSADDR = CSADDR + CSLTH (known from H record)

EXTDEF: (name, CSADDR + value in the record) ESTAB

Get EXTDEF from D record


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At the end of Pass 1: ESTAB contains all external symbols defined in the set of control sections together with the address assigned to each

Print the load map if necessary (optional) useful in program debugging


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Linking loader - Pass 1 algorithm


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Linking loader - Pass 2Perform the actual loading, relocation, and linking

Only processes Text Record and Modification RecordGet address of external symbol from ESTABWhen read T record

Moving object code to the specified address

When read M record(+/-) EXTREF in M to be used for modification is looked up in ESTABThis value is added/subtracted from the indicated location in memory

Last step: transfer control to the address in EIf more than one control section specifies a transfer address: loader arbitrarily uses the last oneIf no control section contains a transfer address: transfer control to the first instruction (PROGADDR)


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Linking loader - Pass 2 algorithm


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Linking loader To improve efficiency

We can make the linking loader algorithm more efficient by

Assigning a reference number to each external symbol referred to in a control section

01: control section name02~: external reference symbols

This reference number is used (instead of the symbol name) in Modification recordsAvoids multiple searches of ESTABfor the same symbol during the loading of a control section.

Search of ESTAB for each external symbol can be performed once and the result is stored in a table indexed by the reference number.The values for code modification can then be obtained by simply indexing into the table.


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Examples of Using Reference Numbers [1/3]


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The reference numbers are underlined in the Refer and modification records



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unit-iii-loader & linker - sri eshwar

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