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Although the BCD correction circuit operates on nibbles, it is itself quite irregular. It needs to detect and to change the half-carry between the nibbles.

You'll also notice a lot of ALUs have different circuits for odd and even bits, and that's because the carry chain can be slightly faster (in some implementation styles) if it alternates between carry and not-carry with each bit.

So, a bit-pair might also be worth considering.

Good luck with the adventure!

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Don't forget that the z80 used a single 4-bit ALU twice, so two internal ticks; its ancestor the 8085 used an 8-bit ALU built from eight one-bit slices. But one other difference is that the 8085/z80 doesn't use the inverted borrow/carry that the 6502 does.

Neil

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One of the things about pass transistors - and I'm talking about NMOS on-chip circuit design, as elaborated in Mead & Conway - is that they don't have a simple propagation delay in the way that logic gates do. In fact, to a reasonable approximation, the propagation delay rises according to the square of the number of pass transistors in series.

So, for example, while the fastest adder circuit has logarithmic delay in the number of bits (5 units of delay for a 32 bit adder), and a normal carry chain with logic gates has linear delay in the number of bits (32 units of delay for a 32 bit adder), a carry chain built purely from pass transistors has quadratic delay (1024 units of delay for a 32 bit adder.

I don't know if these discrete devices behave similarly, but it might be worth an experiment.

Edit: of course an 8 bit machine is less affected by this than a 32 bit machine. Note that the 6502's 16 bit PC incrementer has a bit of lookahead to speed it up.

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Fascinating that e.g. HC family parts slow down as the supply voltage is reduced, yet the faster parts are lower voltage operating...

Watching this with interest...

Neil

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_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?