Data interchange in two registers This picture is from Computer System Architecture 3rd Edition by Morris Mano. Is it possible to interchange the data of any two registers in a single clock pulse? I know that the data of DR (data register) and AC (accumulator register) can be interchanged in a single clock pulse. But other than this I don't understand how data replacement between two registers can be accomplished. 
Thank you.
Just count the busses.  Most of the registers in this design only have an input from the single common bus.  The only exception is the ACcumulator, which also has an input from the DR.