What are ports, really?

By: Travis Downs (travis.downs.delete@this.gmail.com), January 24, 2020 1:16 pm
Room: Moderated Discussions
Some modern CPUs define their instruction dispatch behavior in terms of ports. E.g., Intel Skylake has 8 ports, 0 through 7. p015 handles AVX, p0156 scalar ALU stuff, p23 loads and store address generation. I won't go on more about it here because it has already been well covered.

Sometimes we loosely equal ports with ALUs. We might say "Haswell got another scalar ALU", but really "Haswell got another port, capable of executing scalar instructions (yes, no doubt there is an extra ALU too).". There are many more execution units than there are ports. For example multiplier and divider units, vector and integer units, perhaps a pdep/pext/popcnt/etc unit, all hiding beyond a few ports.

So what are ports? Why do they exist? I don't really have an intuition. What is the hardware design reason for them?

Is it mostly a consequence of the design of the bypass network? E.g., a port represents all the things that share a bus on the bypass network: only one thing can (start) executing on a port, per cycle, because there can be only one thing on the bypass/register busses every cycle?
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What are ports, really?Travis Downs2020/01/24 01:16 PM
  What are ports, really?Seni2020/01/24 02:22 PM
    What are ports, really?Travis Downs2020/01/24 02:29 PM
      What are ports, really?Seni2020/01/24 02:31 PM
    What are ports, really?Travis Downs2020/01/24 02:34 PM
      What are ports, really?Wilco2020/01/24 02:59 PM
        Thank you (NT)Travis Downs2020/01/25 09:48 AM
      What are ports, really?Seni2020/01/24 04:50 PM
        What are ports, really?Travis Downs2020/01/25 09:56 AM
          What are ports, really?Seni2020/01/25 10:38 AM
            Thanks Seni! (NT)Travis Downs2020/01/25 11:39 AM
  What are ports, really?Adrian2020/01/24 02:54 PM
  What are ports, really?Ricardo B2020/01/24 02:59 PM
  What are ports, really?anon.12020/01/24 08:03 PM
    What are ports, really?anon.12020/01/24 08:31 PM
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