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Comment by nhaehnle | Hacker News Reader

No, he's right, although I can imagine that the impact wasn't too bad for early ARM and Z-80.

No matter how it is implemented concretely, decoding or "using" an address of N-bits needs N levels of logical circuitry. Each level of circuitry takes time for signals to pass through. Of course, the time taken within each level is less than the clock cycle time of your CPU, but it adds up. Too many levels, and you can't do the addressing in your allocated cycle time any more.

So there's a trade-off. That trade-off may well have been worth it for the Z-80 and early ARM, but you'll note that apparently ARM switched away from that model as well.

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That's not really true. You can decode N-bits in one step, for example, via 2^N N-bit compares. For small N, that's totally doable.

You still have to then AND the output of that decoder with the data lines which go to the registers right? OP's friend just mentioned 1 extra gate in the data path, and I presume this would be it.

Yep, but a 7-input AND gate will have basically the same delay time as an 8-input AND gate. They don't scale linearly as it would if you stacked 6 or 7 gates in series (although that's typically how you do it in math or programming)

I like examples: 74AHC00 2-input gate has typ 4.5ns propagation delay at 25˚C/15pF/3-3.6v http://www.nxp.com/documents/data_sheet/74AHC_AHCT00.pdf

74AHC30 8-input gate has typ 5.0ns propagation delay at 25˚C/15pF/3-3.6v http://www.nxp.com/documents/data_sheet/74AHC_AHCT30.pdf

Oh of course yeah. I was picturing it as a separate AND when it could be bunged in with the ANDs for individual register selects and write/load enables and so on also. Cool, thanks.

Fanout isn't instant. If you want to power 2^N gates, you need N time to power them all from a source signal.

Yep, the long bitlines are a large factor in timing for register files. The timing is something like O(n) for n storage lines because it grows linearly with capacitance.

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