> > >Again, I don't think you are reading what I am posting. In the XC3S400
> > >there are almost 7200 FFs and LUTs. So there are nearly 3600 LUTs
> > >capable of being RAMs and SRs. How many do you really need??? That is
> > >56,000 bits of distributed RAM, almost a quarter as much as the block
> > >rams! Don't you need some LUTs to use as logic???
> >
> > The wider address rams will require external muxing/control to
> > implement with only 4 LUTs/CLB usable as ram rather than 8.
>
> That is my poit of view. I don´t care (not much) of having just half
> of the LUTs confurable as memory (I´ve read what you wrote, Rick), but
> I didn´t like loosing those dual ported bigger blocks (you didn´t read
> carefully what I wrote). To have the same function I'll need a lot of
> additional logic and/or a clock two times faster. So, my DSP designs
> need a lot of more CLBs in Spartan3 than in Virtex2, and I'm not Ray.
>
> Luiz Carlos Oenning Martins
> KHOMP Solutions
You can feel how you wish about your designs, but even the loss of the
64 bit dual ports and the 128 bit single port rams is not signficant. To make a 64 bit dual port RAM requires 8 LUTs for ram (same as in VII) and one LUT for the read mux and possibly two more LUTs for the WEs. But if this is part of a larger ram block you are making half of the WEs would have been required anyway. So it is not a "large" amount of logic, just a bit more.If you are making really large blocks where the longer runs on the address and data can slow it down significantly, then you likely are better off with the block rams.
Considering the much lower price of the XC3S parts, all this sounds to me like a benefit, not a liability. Think of it as paying for the LUTs that have RAM and getting the other LUTs for free :)