Page 124 - DCAP507_SYSTEM_SOFTWARE

System Software

Notes 8.2 Other Loader Schemes

8.2.1 Binders
The implementation of any program requires four basic functionalities and those are allotment,
relocation, linking and loading. As we have also observed in direct linking loader for execution of
any program each time these four functionalities need to be performed. But performing all these
functionalities each time is time and space consuming task. Furthermore if the program contains
many subroutines or functions and the program requires to be executed repeatedly then this
activity turns out to be annoyingly complex. Each time for implementation of a program, the
allocation, relocation linking and loading requires to be done. Now performing these activities
each time augments the time and space complexity. In fact, there is no requirement to redo all
these four activities each time. Rather, if the results of some of these activities are accumulated in
a file then that file can be utilized by other activities. And doing allocation, relocation, linking and
loading can be avoided each time. The idea is to separate out these activities in separate groups.
Therefore dividing the essential four functions in groups decreases the overall time complexity of
loading process. The program which performs allocation, relocation and linking is called binder.
The binder performs relocation, generates linked executable text and accumulates this text in a file
in some systematic manner. Such type of module organized by the binder execution is called load
module. This load module can then be in fact loaded in the main memory by the loader. This
loader is also recognized as module loader. If the binder can generate the exact replica of executable
code in the load module then the module loader just loads this file into the main memory which
eventually decreases the overall time complexity. But in this process the binder should knew the
existing positions of the main memory. Although the binder knew the main memory locations
this is not the only thing which is sufficient. In multiprogramming environment, the area of main
memory obtainable for loading the program is determined by the host operating system. The
binder should also know which memory area is allocated to the loading program and it should
adjust the relocation information consequently. The binder which performs the linking function
and generates adequate information concerning allocation and relocation and writes this
information along with the program code in the file is known as linkage editor. The module
loader then accepts this rile as input, reads the information accumulated in and depending on this
information about allocation and relocation it performs the task of loading in the main memory.

Notes Although the program is repeatedly executed the linking is finished only once.
Furthermore, the flexibility of allocation and relocation aids efficient utilization of the
main memory.

Task Illustrate the function of binders.

8.2.2 Linkers

As we have discussed earlier, the execution of program can be performed with the assistance of
following steps:
1. Translation of the program (carried out by assembler or compiler).

2. Linking of the program with all other programs which are required for execution. This
also includes preparation of a program called load module.
3. Loading of the load module prepared by linker to some mentioned memory location.

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