Re: Is it time to stop research in Computer Architecture ?

Tom Knight wrote:
"Andy \"Krazy\" Glew" <ag-news@xxxxxxxxxxxxxxx> writes:
As for the dynamic power, similar considerations: the total amount of
charge switched per cycle must stay below the square root curve. More
precisely, the total amount of charge*frequency*voltage must stay
below the square root curve.

Resonant power
recovery (at least of the clocks, and probably of much of the logic
with reversible techniques) becomes easy. We will do this, but
probably not in your phone.

Wow! Tom Knight, responding to my comp.arch post!

Tom, I almost said something about power recovery logic in my post. I am a huge fan. But not necessarily as far out on the path towards reversibility as your work has been. So much of your work seems (or at least seemed, in the early days) to be oriented towards the absolute lowest power.

Myself, I would be happy to save 30%, or 60%, via such techniques. And, to my limited understanding of this area, which is not my specialty, the sorts of things that I am thinking of *might* apply to my cell phone.

So, first some questions for you: what application domains do you have in mind in your work on adiabatic, charge recovery, resonant power recovery, logic? Space? Operating at cryogenic temperatures? Supercomputers? Or, motes on Earth?

Also, what keeps your techniques from being used in my cell phone?

Second, I will risk embarassment by talking about my own thoughts in this area. Disclaimer: I am not a circuits guy. But I never let lack of expertise prevent me thinking about a topic with common sense.

Back when I was at Illinois some 20 years ago I used to roll my eyes at the papers on reversible and adiabatic computing. They seemed to me to be an academic exercise, theoretical. But then I was TA'ing a VLSI design class at Bell Labs in Naperville, and one of the students, a physicist, asked me why we don't have chips with AC power supplies. *That* is what caused the blinders to be removed from my eyes. (Or, maybe, a new set of distorting lenses to be placed in front of my eyes.)

The basic idea of AC power supplies is that charge does not go all the way around. It moves in, it moves back. For the most part.

Now, most of our VLSI transistor technologies are assymmetric. You want P and N devices to be connected to different power rails. Doesn't have to be so, but that's mainly what we do - and the circuit guys look down on me for my wild ideas already, so I won't go down that path, or the asynch path, here. In any case, the circuits are assymetric: transistors are connected differently to hi and lo.

So: why not have an AC power supply. And define dual circuits. Use one circuit when power rail A is high and B is low, and the other circuit when power rail A is low and power rail B is high.

You need devices that can switch off the undesired circuit when the power rails are in the wrong configuration. And you need devices that can select which of the duals to be output. These devices must work in al power configurations.

If your "AC" power supply (actually, the signals on the at-least-2 power supply rails) was a square wave, this would be enough. But, it won't be. WLOG, let us imagine that the AC power supply is a sine wave. The devices will have to work within a range of changing voltages, say when "high" is between k=1/3 and k=1 Vmax=(Vmean+k*Vswing), and "low" is between k=1/3 and k=1 Vmin=(Vmean-k*Vswing). Can this be done? (I think so, but I am not an expert.)

You would also have to idle both of the dual circuits when the rails are between k=1/3 and k=-1/3. But, fortunately, that is at the fastest changing part of the cycle.

If you need continuous operation, you could use three power rails, i.e. three phases.

Distributing the control signals to switch the "dual" (or triple, or more...) circuits might be a pain.

But, we need not be thinking about the AC ticking away at gigahertz frequency. I.e. it does not need to be the clock. Why could we not run the AC power rails at a much lower frequency, and use the dual circuits for hundreds or thousands or more` clock cycles?

The biggest problem with this is that it requires dual circuits. But even that's not totally necessary. In many places we have similar, almost dual, circuits. If we can connect them, and switch them, we might be able to share these nets.

OK, OK, OK. This is not my area. But I would love to understand WHY something like this cannot work. Actually, I would love it to work even more.
.

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