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Is the only purpose of transistors in a cpu to act as the switches that define its instruction set? And if so then why does increasing the number of transistors increase its speed?

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This question is answered in more detail at this Electrical Engineering Stack Exchange question:… (It is not clear if that kind of detail is appropriate here.) – Paul A. Clayton Dec 17 '12 at 2:40

Off the top of my head:

  • More cache, which is faster than RAM

  • More SIMD instructions, which process faster than single-data instructions

  • More cores, so you can do two or more things at once

  • More functional units, like built-in FPUs, and multiple ALUs

  • Pipelines, so each core can do more things at once

  • Better processing logic, like more sophisticated branch prediction logic

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If you're curious about this sort of thing I highly recommend the book The Elements of Computing by Noam Nisan and Shimon Schocken (at least the first half). After going through it you'd be able to answer your own question in great detail with a full appreciation of the parts involved.

The companion website actually has some sample chapters and notes. It's a very approachable book. I went through it on my own with no problem and then took a brand new class at my university that used it as its primary text.

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The simple answer is that more transistors doesn't make the rest of them go faster, but instead of doing one thing per time period, we can now do two (with some limitations).

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Increasing the number of transistors is usually done by making them smaller, which makes them closer together, which reduces signal propagation time.

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