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On average, how many transistors is a transistor connected to in a CPU?

What is the average fan-out of a gate in a CPU?

What is the average fan-in of a gate in a CPU?

(I'm guessing these numbers are much less than the average of 7,000 connections between one average neuron and other neurons in the human brain mentioned in the Contemporary transistors question).

In the Apollo Guidance Computer and the Cray 1, every gate has the same maximum fan-in: max fan-in of 3 in the AGC, max fan-in of 5 in the Cray 1. What is the maximum fan-in of any gate in today's CPUs?

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    \$\begingroup\$ Is there any electronic design question in there or are you just looking for some numbers for the heck of it? \$\endgroup\$
    – Kellenjb
    Commented Jul 26, 2011 at 13:40
  • \$\begingroup\$ Mostly because I am curious. But I've been told that there is a very good reason that there is a surprisingly small maximum fan-in and fan-out for the gates in a full-custom VLSI chip -- in particular, they always use a tree of gates, each with limited fan-in, to produce the "result is all zero" signal, rather than a functionally equivalent single 32-input NOR gate. \$\endgroup\$
    – davidcary
    Commented Aug 20, 2011 at 3:17

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Three to Five sounds about right. The problem is that computers today run so darned fast that the gate capacitance really starts to add up and blurs the edges of those would-be digital signals... that being said I don't have any references to defensible statistics on actual processors for you, sorry.

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In most processors which have 45nm or 32nm transistors, the fan out is usually 4 to 5. As the transistor size reduces, the technology and design constraints reduce the fan out number.

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