was is your question????????
multiple things will effect the program design and implementaion
>compiler design
>standardization
>optimization
among the others..
sft devlopment is a very vast field, a whole branch sftware engg. is devoted to it. you must have heard abt SDLC
your will develop the software in teams and appointed one module only, if you are a starter, so dont worry abt modularity. if you are senior team leader, all the modularity maturity and security is strictly laid down by the company only. you will need to understand the rules.