Re: counter doubt
I reckon nothing, yes I dont think there is any difference. It depends upon how u are using a count. For example, a simple 4 bit counter can be treated as a -7to+7 twos compliment(signed) counter or a 0to15 unsigned counter.
Or you can simply have a dedicated sign bit as suggested by Mr Srinivasan. But then an up counter would actually be a down counter when the sign bit says its a -ive count.
Let me know where do you want to use it, and i would be able to offer some more help. As Mr Srinivasan has also given a way to do it, you may want to choose either.