An Introduction to the Software
Top Down Design

Top down design is the name of a design process, where one could break down the project or objective or task, into smaller modules, level after level, until it reaches a level where it is easy, to be implemented. It is also know as Stepwise design.

 

The modules are separate entities, with their own functionalities. The modules are the breakdowns of the Task.  In the first level, we have the Task. In the second level, we have the Task as broken down into modules, so as to simplify the task. In the third level, we have each of the modules,  broken down into sub modules. And the process continues, level after level, until we reach a level consisting of sub modules, that can be implemented with ease.

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After all that has been done. We then we go up, level by level. The module above implements its logic in which it includes the logic of the sub modules below.  The module, above that module, implements its logic, which includes the logic of all the modules below it. We keep going this way until we reach the top, where the Task is implemented by a logic, which includes the logic of all the modules below it. This greatly simplifies, the design and execution of the Task.

 

One  could think of the modules as black boxes, where you send in the query, and get back a result. In the software context, the modules are functions, which implement the purpose of the module.

 

This design was widely used, in Software programming,  before the introduction of Object Oriented Design, popularly known as OOD. The advent of the language, C++, in the early 1990s, popularized OOD.  C++ is the Object Oriented version of C.

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The programming language, which extensively used Top Down design, was the C programming language. In the programming of this language, we would first do a design. A Top Down design. We would take the project, and divide it into functions, and each function would be broken down to more functions. This process would go on, until the complexity of the function would go down, and it could be coded with ease.

 

We will first do the design by drawing a chart known as a flow chart. In this chart, we put the project in level 1. We then break down the project and create many functions, in the second level. Those functions would be further broken down, resulting in many functions, in the third level. This process would continue, until we reach a level, where the complexity of the function is minimal. We find such a level for each of the streams from the project, which is at the top. This is how top down design is accomplished.

We then do a pseudo code for each of the functions, under the project, with a distinct input and output. We treat each of the functions as black boxes, and define what the inputs should be, and what the function would output.

 

After all this have been done, we start with the coding of the functions, and the whole project.

 

After the functions have been coded, we call the main functions, in the 'main()' program.

The main functions then call the functions under them, and those functions call the ones under them as per the Top Down design that we did. Thus coding is very simplified, as we reduced the complexity of the project by breaking it down, with Top Down design. 

 

The main program and functions are then compiled using the appropriate C compiler.

The compiler gives out the executable of the program, which is the end result for the software project.

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