Mike Treseler wrote:
> Andy Botterill wrote:
>
>
>>WARNING:Xst:646 - Signal <borrow> is assigned but never used. This
>>unconnected signal will be trimmed during the optimization process.
>>
>>What I would like to do is to find out which lines are caing this
>>warning and see if I have missed something or the synthesis tool has
>>made a mistake.
>
>
> It is most likely a logical error in the code.
I agree with that I'm trying to figure out how to track it down.
>
>
>>All sixteen op-codes assign something to borrow. Sometimes true,
>>sometimes inverse and sometimes 0.
>
>
> OK, but what xst is claiming is that
> no register is being assigned *to* by
> the "borrow" register.

(* KEEP = "TRUE" *){borrow, Rd_contents}=Rn_contents - shifter_operand;
There are at least three occurences similar to above.
Some are assigned to zero which may give the synthesis tool the idea
that it could be optimised.

>
> In other words, "borrow" does not appear
> on the right side of any active assignment
> that affects a top port output..

borrow does not go to a top level port but it is used in the carry logic
which goes to a primary output.
>
> -- Mike Treseler