> [...]
> For signed, convert (if negative) values to positive, and convert back
> if either were negative (a[31] xor b[31]) and also detect overflow
> with |result[63:32]?
> [...]

Converting negative integers back and forth requires quite a bit of logic, as 
the conversion cannot be done quickly, the logic has to walk all the 32 bits 
through. I've implemented both signed and unsigned multiplication with a single 
66-bit signed multiplier which takes advantage of the FPGA's built-in hardware 
multipliers.

Cheers,
  rdiez
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