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Wednesday, May 22, 2013

What are Address Binding, Dynamic Loading and Dynamic Linking?

In this article we shall discuss about three interrelated concepts namely address binding, dynamic loading and dynamic linking.

1. Address Binding: 
- There are two types of addresses for the computer memory. 
- These are called the physical address and the logical address. 
- A physical memory location is allocated to a logical pointer by address binding process.
- This is actually nothing but associating the physical address and the logical address with each other. 
- Sometimes logical address is also referred to as the virtual address. 
- This concept is an important part of the memory management. 
- Operating system is responsible for carrying out address binding on behalf of the applications and programs that need an access to the memory. 
- A program cannot be executed without bringing it to the main memory. 
- The instructions of the program have to be bound to right address spaces in the physical memory. 
- Address binding is simply a scheme for performing this job. 
- It can be thought of as something similar to address mapping. 
- Address binding can be carried out at any of the following times:
Ø  Compile time
Ø  Loading time
Ø  Execution time

- In execution time binding, whenever the program requires access to memory, it has to go through a register called the relocation register and is similar to the base register. 
- Then the offset is added. 
- But in binding during the loading time, same thing is done but every time this register need not be evaluated. 
- The addresses are mapped at the time of loading the program in to the memory. 
- If there is a change in the base address, the whole program has to be reloaded.

2. Dynamic Loading: 
- This mechanism is very useful for a program as it helps it do the following things:
Ø  Loading library in to the main memory.
Ø  Retrieving the address of the variables and routines that are contained in the library.
Ø  Accessing those variables and executing those routines.
Ø  Unloading the library.
- Dynamic loading is very much different from the load time linking and static linking. 
- Dynamic loading allows a system to start up even of the libraries are absent. - It also helps in discovering the absent libraries and then gaining the additional functionality. 
- Dynamic loading is a very transparent process since it is the operating system that handles it. 
- Main advantages are firstly, it helps in fixing the patches at once without having the need for re-linking them and secondly, it provides protection to the libraries against modification that is not authorized. 
Dynamic loading find its major use in the implementation of the software plugins.
- It is also used in the implementation of the computer programs where requisite functionality is supplied by the different libraries and user has the freedom to select the libraries he/ she wishes to provide.

3. Dynamic Linking: 
- This is an important part of the binding process. 
- The purpose of the dynamic linking is resolving the references or symbols and links to the library modules. 
- This process is carried out by a linker program. 
- This programs searches for a set of library modules in some given sequence. 
This process takes place during the creation of the executable file. 
- The resolved references may be addresses of the jump calls and the routines. - These may in different modules or in the main program.
- Dynamic linking resolves them in to relocatable address or fixed address through allocation of the memory to each of the memory segment of the referenced module. 

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