Q:

What is the need of having a separate flag as Auxiliary carry in microprocessor 8085?

My question is "what is the need of using auxiliary carry?"  I mean 'AC' flag is set when D3 generates a carry. Then why not different flags for D2 or D4? 
Why suddenly we are treating D3 carry in a special way? 

A:

It is used for BCD arithmetic. Each decimal requires 4 bits. 

