Why this statement renders TWO multipliers in XST?

As part of its logic simplifications, the synthesiser is expanding tmp1 in the expression being assigned to dout as follows:

assign dout = sign ? ~(en ? din_a_abs * din_b_abs : 16'b0) + 1'b1 : (en ? din_a_abs * din_b_abs : 16'b0);

and there are your two multipliers.

You need to tell the synthesiser not to expand the expression in this way. This can be done with a keep attribute on tmp1.

// synthesis attribute keep of tmp1 is true;

Regards, Allan.

Well probably you have a timing constaint that forces tmp1 to be duplicated to meet it, 1 inst has its output made -ve.the other as is. You may have a switch to stop this replication.

You could try putting the cond sgn negation in series with one of the

2 inputs or put it on the next pipeline or relax the timing, either way should give 1 mul.

regards

johnjakson_usa_com