QUESTION #391

How fast (ie: clockspeed in Hz) can a single processor in a computer be? The highest I've read is 4 GHz. What's the limit?

It is difficult to speak of an absolute frequency limit, since there are many different limitations on clock speed, some of which are technical, and some are purely physical.

One important assumption in circuit design is that all circuit elements are 'lumped'. This means that signal propagation time from one element to the other is negligible. 'Negligible' means that the time it takes for the signal produced at one point on the circuit to propagate to the rest of the circuit is (very) small compared to the times involved in circuit operation.

For all practical purposes, electrical signals travel at the speed of light. Let us take an example: Assume a processor which works at 1GHz. This means one billion clock cycles per second. This also means one clock cycle takes one billionth of a second, or a nanosecond. Light travels about 30cm (about a foot) in a nanosecond. So, the size of circuitry involved at such clock speeds better be much less than (at least 1/10 of) 30cm. So, your maximum circuit size is 3cm. Taking into account that the actual CPU core size is less than 1cm a side, we are still in safe waters.

Remember that this was for 1 GHz. If the clock speed is increased to 100GHz, a cycle will be 0.01 nanoseconds, and signals will only propagate 3mm in this time. So, your CPU core will ideally need to be about 0.3mm in size. It is quite hard to cram a CPU core into such a small space. So, we're still in safe waters, but somewhere between now and 100GHz, we're going to hit this physical barrier.

What happens when this size limitation is violated? What happens is that certain parts of your circuit are in the 'current' cycle, and other parts are in the 'previous' cycle. It gets really hard to handle such distributed systems. Then in design, you'll have to handle delays in signal propagation, circuit shape starts playing a strong role in design. In fact, it becomes more like separate processors than a single processor.

Then there are other technical problems, like overheating, circuit design, perhaps having stable clocks at such high frequencies and such. But I believe none are as serious as the physical limitation of clock speed vs. circuit size.
Answered by: Yasar Safkan, Ph.D., Sofware Engineer, Noktalar A.S., Istanbul, Turkey

Present day microprocessors (like the Pentiums) are built with the complementary-MOS (CMOS) technology. How fast a processor can be clocked finally depends on how small a switching time each MOS transistor can have. The switching time of a MOS Field Effect Transistor (MOSFET) goes down with its size. So, as long as you can make transistors smaller, you can get faster transistors and hence faster processors. This reduction in the size is termed as 'CMOS Scaling' and has been more or less following the celebrated Moore's Law.

The smallest feature size to be found in a processor today is some 150 nanometers (this is the so called 'channel length' in a MOSFET). You can go smaller than that, but with ever increasing difficulty. Conventional transistors may still work down to about 50 nanometers channel length, giving your processor clock speeds around 20 GHz or so, but beyond that, its anybody's guess. At such small lengths, a transistor isn't really well behaved, so one can't use it as one has been. Quantum effects reign supreme.

One can actually try to put some of these quantum effects to good use and make devices (like transistors) based on them. Single electron transistors (SETs) which have shown work at room temperatures hold promise in this regard. It may be possible one day to make integrated cicuits out of them. That would give processors speeds of as much as a terahertz. But one can be sure of not seeing any of these things in the market in the next 20 years at least. Till then, its old CMOS, inching towards higher speeds and smaller dimensions, saturating its potential in around 2020.