1. Search for the place you want to insert the "a".. or just decide the place.
2. Move every character after that point forward by one place.
3. write "a" to the desired position.
You are going to get incomplete answers with incomplete questions. I could write some code if you are willing to tell me (us) what exactly you want. It is very difficult to write code for the general problem.
EDIT: The info I (we) need is how to determine the position where to insert the characters. And is the length of inserted characters known or do you need some code to find that out..
I still don't know the language!!! Are we to assume asm?
If so the data ( string ) will be in an array...
When writing the array you stop at the desired position...
Write the new array...
Then carry on with the first array...
Oops... My bad... I thought winavr was an IDE... Sorry.... Well then much easier just read through the string library documentation.... C is designed for string manipulation..