Linking In MS-DOSShubham Shah

Yashashwi Mahindrakar

Sagar Nadgauda

Linking In MS Dos System

First of All lets see what is linking…

Linking is the Process of collecting and combining various pieces of code and data into single fie.

Thus this file can be Loaded (Copied) into Main MEMORY and executed.

Linking can be performed at compile time, when the source code is translated into machine code, at load time, when the program is loaded into memory and executed by the loader, and even at run time, by application programs.

Linkers Intro.

On early computer systems, linking was performed manually.

On modern systems, linking is performed automatically by programs called linkers.

Linkers play a crucial role in software development because they enable separate compilation.

Instead of organizing a large application as one monolithic source file, we can decompose it into smaller, more manageable modules that can be modified and compiled separately.

When we change one of these modules, we simply recompile it and relink the application, without having to recompile the other files

Understanding Linkers will do???

Understanding linkers will help you build large programs. Programmers who build large programs often encounter linker errors caused by missing modules, missing libraries, or incompatible library versions. Unless you understand how a linker resolves references, what a library is, and how a linker uses a library to resolve references, these kinds of errors will be baffling and frustrating.

Understanding linkers will help you avoid dangerous programming errors. The decisions that Unix linkers make when they resolve symbol references can silently affect the correctness of your programs. Programs that incorrectly define multiple global variables pass through the linker without any warnings in the default case. The resulting programs can exhibit baffling run-time behavior and are extremely difficult to debug.

Understanding linking will help you understand other important systems concepts. The executable object files produced by linkers play key roles in important systems functions such as loading and running programs, virtual memory., paging, and memory mapping.

Understanding linking will enable you to exploit shared libraries. For many years, linking was considered to be fairly straightforward and uninteresting. However, with the increased importance of shared libraries and dynamic linking in modern operating systems, linking is a sophisticated process that provides the knowledgeable programmer with significant power. For example, many software products use shared libraries to upgrade shrink-wrapped binaries at run time. Also, most Web servers rely on dynamic linking of shared libraries to serve dynamic content.

===== LinKing ===== The Linker Diagram:-

More about Linking Static Linking

all code modules are copied into a single executable file

the same shared module may exist in many files

a location in this file implies the location in the memory image

target address for cross-module reference can be determined before run time

Dynamic Linking

needs help from OS ( that means the scheme varies depending on OS)

library modules are usually linked dynamically

inserts symbolic references in the executable file

these symbols are resolved at run time

Object Model Format The Object Module of MS DOS differs from the intel specification in

some respects .We also make some simplification for the purpose of this discussion. Details of the object module are as follows:

“An Object module is a sequence of object records.”

There are 14 types of object records, in which they contain 5 kinds of information :-

Binary Image , i.e the code and data generated by translator.

External references.

Public Definitions

Debugging information such as line number in the source program.

Miscellaneous information such as comments in the source program.

Object Records Of Intel 8088

Record Type Id(Hex) Description

THREADR 80 Translator header record

LNAMES 96 List of names record

SEGDEF 99 Segment definition record

EXTDEF 8C External names definition

PUBDEF 91 Public names definition record

LEDATA A1 Enumerated data(Binary Image)

LIDATA A3 Repeated data(binary Image)

FIXUPP 9D Fix up (i.e. relocation) record

MODEND 8B Module end record

Sample MS DOS assembly language program

Sr.no Statement Offset

0001 NAME FIRST

0002 COMPUTE SEGMENT

0003 EXTERN PHI:BYTE,PSI:WORD

0004 PUBLI APLHA, BETA

0007 ALPHA . . . . . . . 0015

. . .

. . .

. . .

0012 MOV AX, SEG PHI

0028

. . .

. . .

. . .

0029 BETA .. . . . . . . . . 0084

0036 COMPUTE ENDS

0037 END

Design of Linker

For linking in MS Dos system we will design a program named LINKER which performs both linking and relocation of absolute segments and of re locatable segments that cannot be combined with other re locatable segments.

Its output is a binary program which resembles a program with .COM extension in MS DOS.

This program is not relocated by the loader prior the execution .

(Note:- the difference between the LINKER and the LINK program of MS DOS :LINK produces a program with .EXE extension, which is relocated by the loader prior to execution)

Specification The LINKER invocation command has the following format:

LINKER <object module names> ,<executable file>,

<load origin>, <list of library files>

The linker performs relocation and linking of all named object modules to produce a binary program with the specified load origin.

The program is stored in the file with the name <executable file>.If the LINKER comes across an external symbol that is not defined in any of the object modules named in th LINKER command, it locates ian object module in one of the library files included in <list of library files>that contains a public definitions.

This method of resolving an external reference by automatically including an object module form a library file is called “AUTOLINKING”.

Datastructures.

Here the linker program has two pass organization .

In first pass object modules are processed to collect information concering segments and public definations into name table(NTAB)

The second pass performs relocation and linking to produce a binary program.

General Shortcut to remember of ms dos linking

pass 1:

allocates segments defined in SEGDEF

resolve external symbols

pass 2:

prepare memory image

if needed, disk space is also used

expand LIDATA

relocations within segment

write .EXE file

General Difference between linux and ms dos linker

Linux linker GOT(Global Offset table ) & lazy modeling.

Where as the ms dos linker does pass 1 & pass2 linking wise

Thank you

Any questions