geo said:
Where did you get that, please? I'm aware of exactly one credible source for that, and frankly that one struck me as an ATI rep having a "bad speculation day" post NV40 transistor count head-scratching.
Just a question from a noob on this forum (though not a noob in chip design): I've been following this thread for a long time now and every now and then this discussion about the amount of transistors comes up.
Why does it matter?
Because, honestly, in a 4 semiconductor company I've worked for, this has never been even mentionned... The only thing that counts is the size of the die and the percentage of memory of that die. Number of transistors is only used by the evil side (that's the marketing department for us, engineers) who use it to impress potential customers.
There are a number of ways in which you can exagerate the amount of transitors:
- lie about it knowingly. Hey, who's going to prove you wrong?
- lie about it unknowingly: no engineer is actually going to spend time counting them. Really. So all the numbers you read are based on extrapolations. "Let's say, x mm2 at a typical average of so many transitors per mm2, hmmm, that should be this amount of transistors." The fact that transistor densities for RAMs are way higher than those of random logic make it much easier to fudge numbers. Who's going to count the exact amount of mm2 of RAMs of the total die? Or the IO pads of a chip take a huge amount of area, even though they usually only contain only a few (huge) transistors. etc etc.
- optimistic counting. The last one is very easy to do: in current designs, there's a lot of redunancy, especially when it comes to RAMs. Say you put a number ram of 64 KB rams on your chip to implement your transmogrifier. In a 90nm process, there's no way you're *not* going to add redundant row and/or columns to increase the yields (dramatically.) If this adds an overhead of 10% for the RAM, but increases the yield by more than that, nobody is going to stop you from counting those extra 10% as real transistors, even if they're never going to do anything useful in addition.
In other words: nobody on the engineering side ever talks about the amount of transitors. So if Nvidia and ATI are using different numbers of same sizes of dies: it's all marketing. Just ignore it.
The parameters that *do* count are:
- die area
- % of area used for RAM. (On one side more is better, since it allows more redundancy. On the other side less is better, since densitity is higher which makes it hard to produce error free when there's no redudancy, which is the case when you're dealing with a lot of small RAMs.)
- Amount of logic redundany (typically, this is rather small.)
- cell density. This is mostly determined by the amount of wiring that interconnects that cells. An elegant design uses less wiring to to the same kind of stuff.
