On 2009-10-29, Timothy Normand Miller wrote: > On Thu, Oct 29, 2009 at 7:24 AM, André Pouliot <[email protected]> > wrote: > > Petter Urkedal wrote: > >> On 2009-10-27, Andre Pouliot wrote: > >>> 2009/10/26 Petter Urkedal <[email protected]> > >>>> 2. Do we need to support extraction of the upper 32 bits of a > >>>> multiplication? If not, then mult and umult are also equivalent except > >>>> possibly for the flags. I think we can reuse the carry flag for > >>>> overflow of an unsigned multiplication in analogy to the additive > >>>> instructions. > >>>> > >>> The cost for supporting the selection of the upper or lower part of the > >>> multiplication is small since we can will probably use a signed 33 bits > >>> multiplier. Yes it does carry a cost to doing it that way but the > >>> multiplier > >>> will be reuse to do the shift operation in both direction. It should also > >>> support rotate instruction. > >> > >> We have 18 × 18 -> 36 multipliers at our disposal. A 32 × 32 -> 64 > >> multiplier takes 4 of these, whereas 32 × 32 -> 32 takes 3. Also the > >> final adder has the width of result. So, I think it costs enough that > >> we should carefully consider whether we need it. > >> > > The multiplier serve as a barrel shift for the data alignment of the > > adder. It also serve for doing right and left shift. If we were only > > doing the 32 x32 multiplication doing a 32 bit result you would be right. > > Oh, yeah. I didn't even think about that. You're right. Way to > minimize the logic here!
We're talking about the FP adder here, right? I haven't started thinking about sharing between int and FP yet, but in any case, isn't a barrel shifter a lot simpler that the extra logic required by 32 × 32 → 64 multiply? Or do the hard-coded units in the FPGA make that irrelevant? I would be nice to have 64 bit result just to make the ALU more complete, if we have a sufficiently strong excuse ;-) _______________________________________________ Open-graphics mailing list [email protected] http://lists.duskglow.com/mailman/listinfo/open-graphics List service provided by Duskglow Consulting, LLC (www.duskglow.com)
