Introduction. Linking, which combines two or more separate object programs and supplies the information needed to allow references between them.

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1 Index 1. Introduction 2. Functions of Loader 3. Translation & Linking of Program 4. Loading And Linking Scheme -Link-And-Go -Link-Load-And-Go 5. Difference Between Link-And-Go & Link-Load-And-Go 1

2 Introduction To execute an object program, we needs Relocation, which modifies the object program so that it can be loaded at an address different from the location originally specified. Linking, which combines two or more separate object programs and supplies the information needed to allow references between them. Loading and Allocation, which allocates memory location and brings the object program into memory for execution. 2

3 Introduction» Loader It is software processor which perform some low level processing of the program that form input to it in order to produce ready to execute program in the machine language.» Linking Loader performs some processing to put the various program forms together to make single program is known as linking.» Object Program It accept output of translator program & uses certain other similar program called object program. 3

4 Functions of Loader 1] Locate a program in appropriate area :- The Basic function of loading is to locate a program in appropriate area of the main memory of computer every area of the main memory of computer every time it is to be executed. In addition to loading loader performs 2 important functions:- 2] Loading of program to assigned area :- accept output of translator program form and uses certain other similar program called object program, it produce 1 ready to execute machine language program. 4

5 Functions of Loader 3] Linking of programs with each other :- e.g. Loader performs some processing to put the various programs forms together to make 1 single program. This is known as linking. Linking is necessary because program written by programmer, generally does not execute on its own without the presence of other object program in the computer storage. A program written in HLL may contain calls in certain standard functions like sin, cos, tan etc. which are not coded by programmer himself. Linking function makes address of program known to each other, so that such transfer can take place during execution. 5

6 Functions of Loader 4] Relocation of a program to execute properly :- Another Function commonly performed by a Loader is that of program relocation i.e. Simply moving a program from 1 storage area to another and modify the location sensitive code (address sensitive instructions). So that it can execute correctly in a new set of location; so that call to program by another program can execute efficiently. 6

7 Translation & Linking of Program. Source program Obj.Program Ready to run machine lang. program Translator Loader Computer System Result Other Obj.Program Data A program translator binds a program to translation time address. A loader modifies binding to bind program to load time address. 7

8 Loading And Linking Scheme Loading And Linking Scheme :- As loader perform loading of a program into assigned area, linkage editor perform linking of a program with each other, while relocation performs relocation of a program to execute property. There are two type of loading scheme :- 1] Link-And-Go. 2] Link-Load-And-Go. 8

9 1. Link-And-Go In the Link-And-Go scheme the linkage editor actually situated a program its load address perform all other linking function and passes control to the purpose of execution. Drawbacks of the scheme :- 1) Linkage editor co-exist with the program while perform the linkage task. 2) Linking has to be repeated for execution of program. 9 9

10 Link-And-Go Object Module Linkage Editor Ready To Execute Program 10

11 2.Link-Load-And Go:- The Link-Load-And-Go scheme require less storage because its construct a ready to execute form of a program and push it s the secondary storage. its necessary to load this program into the main storage without any relocation. such loader is called as absolute loader. This loader presents :- 1)Text of program which has been link for desired area. 2)The load address origin. 3)Length of the program

12 Link-Load- And-Go-Scheme Object Module Phase Library Linkage Editor Ready to Execute Program Absolute Loader 12 12

13 Difference Between Link-And-Go & Link-Load-And-Go Link-And-Go Link-Load-And-Go 1) In link and go scheme loader load the program into main storage perform necessary linking with other object module and transfer the control for cpu execution. 1) In link and go scheme loader load the program into main storage perform necessary linking with other object module and generate ready to run machine language program and store Secondary storage. 2) Linking is necessary every time when the program execute. 2) Linking is not necessary every time. 3) Memory requirement is more. 3) Memory requirement is less. 4) Linkage editor co-exits with user program every time. 4) Linkage editor is not needed co-exit user program every time. 5) Absolute loader is not present. 5) Absolute loader is present

Chapter 3 Loaders and Linkers -- Loader Design Options

Chapter 3 Loaders and Linkers -- Loader Design Options Loaders Linkage editor Linking before loading Dynamic linking Linking at the execution time Bootstrap loader Linkage editors Difference between a