Q: What is the difference between top down and bottom up design?
A: Top down design proceeds from the abstract (entity) to get to the concrete (design). Bottom up design proceeds from the concrete (design) to get to the abstract (entity).
Top down design is most often used in designing brand new systems, while bottom up design is sometimes used when one is reverse engineering a design; i.e. when one is trying to figure out what somebody else designed in an existing system.
Bottom up design begins the design with the lowest level modules or subsystems, and progresses upward to the main program, module, or subsystem.
With bottom up design, a structure chart is necessary to determine the order of execution, and the development of drivers is necessary to complete the bottom up approach.
Top down design, on the other hand, begins the design with the main or top-level module, and progresses downward to the lowest level modules or subsystems.
Real life sometimes is a combination of top down design and bottom up design.
For instance, data modeling sessions tend to be iterative, bouncing back and forth between top down and bottom up modes, as the need arises.
[Continued on next page...]
______________________________________________________
