They will probably design and contract fabbing for separate chips only when there is sufficient expected market interest.
The fact that a higher end iPad got its own AxxX line of chips implies that it gets high enough sales figures to warrant a separate chip.
I guess the question is what kind of sales figures to Apple auger for such new ARM based SKU's, and how many separate designs will there be.
For that matter will they be only fully integrated SoC designs with zero upgrade path - or will some models have PCIE slots or some proprietary Apple alternative for their homegrown GPU's (which it likely will be to prevent their use in PC's).
A 5nm mask set costs ~$15M.
Apple sells ~20M macs a year.
In other words it's actually not that big a deal (from the point of view of mask costs) to create a new mask set for [all macs as a whole].
Of course one has to add into that the probably larger costs of the design and verification of the alternative mac SoCs, but Apple so far seems to be doing a very good job of ensuring that IP can be reused fairly easily across designs (from watch to phone to iPad to ...) It helps when you aren't determined to cripple some of your SoCs to force some target customers to buy the more expensive SoC...
OK. So if all we do is have a separate design for Macs, we're talking less than a dollar in extra cost per SoC from masks. Throw in $10 extra for design, verification and it's still clearly a great deal.
Next question is how finely do we segment the Mac chips? THAT is the real question here, how finely Apple will segment the SoCs across the Mac range.
We don't know how many Macs are sold in different categories (MB+MBA, MBP+mini+iMac, iMac Pro+Mac Pro say). If Apple uses the iPad Pro chip for MB and MBP, that takes out a lot of the volume available for a dedicated Mac SoC. How much volume is left? 10% An extra $10 for mask sets and an extra $100 for verification is starting to become uncomfortable.
A third split, to an dedicated iMac Pro/Mac Pro chip makes things even worse, if those are, what, maybe a fifth of the MBP/iMac/mac mini numbers? A tenth?
OK, that's bad? How can we make it better?
(a) Mac SoCs only get updated every two years instead of every year. (Like iPad Pro has kinda sorta been for a while). That gets us a factor of two. The actual models could still possibly get mid-like kickers on alternate years, like camera upgrade, or faster flash, or whatever.
(b) Do we aggregate the power (number of cores, size of GPU) as we go up by more of smaller SoCs, rather than different SoCs? ie either chiplets, or simply putting two (or three or four) SoCs on the PCB? Both of these seem like reasonable choices.
Going forward Apple doesn't have to follow the path they were forced to follow by Intel's pricing. It's not clear that they believe it's great for customers to have iMacs at multiple levels of i3, i5, i7, i9 all also at different frequencies. I expect they will toss this sort of complication and offer a single iMac (8+8 cores or whatever, at a single frequency) and you'll choose like you choose your phone -- by screen size, by flash, maybe by amount of RAM.
Secondly chiplets are nicer than separate SoCs because smaller, lower power, faster communication. But separate SoCs are not a TERRIBLE choice, especially for rev 1. Intel And AMD seem to have reasons to want to avoid this (you kinda want to charge more for the 8 core than the 4 core, but in particular ways, so you land up making it difficult for anyone to want to put together a system from two 4-cores), but Apple will not have to engage in that nonsense.
Bottom line:
- the numbers can work if Macs all get iPad Pro SoCs, the high end ones (which for the next year I see as MBP, mac mini, iMac) getting saying 2 or even 3 or 4 SoCs on the PCB.
- the numbers can (apparently barely -- but that depends on design/verification costs that we don't know) work if the lowest end Macs get iPad Pro SoCs, the others get a Mac-specific SoC
Further trout in the milk: is Apple planning to put these things in its data centers? Who knows? I can see a way to do things without much disruption for this year (which I believe will be conventional SoCs, no chiplets, AND no iMac Pro, Mac Pro (so both higher core counts and extreme GPU demands). Next year with the A15 I can imagine multiple solutions, I have no feeling for which will happen.
Final data point. The people who do this sort of thing claim that there are *three* SoC part numbers in the macOS beta's corresponding to Apple Silicon SoCs. We have no idea what this means.
One is this year's developer silicon, one is A14X, one is A14Mac?
All three are A14Mac, but with some sort of modifications of the SoC like different amounts of RAM? (So far Apple has not used this sort of differentiator for say the SoCs that go into iPads with 4 vs 6GB, but that could change.)
Other modifications of the SoC, like the same basic design, but different mask sets (not THAT expensive) allowing for different transistor choices and thus higher frequency/higher power for the mini and iMac? (Think eg Qualcomm's one extra-fast A77 core on Snapdragon 865 and 865+)