> I find it unlikely that Apple were doing most of the testing of macOS-on-ARM (which probably occurred for years prior to the M1 announcement, and prior to the A14X being created) directly using the iOS device architecture.

Of course they would be doing it like that, since the macOS kernel was already ported to ARM for iOS. They even handed out developer kits that literally used an iPad chip.

https://en.wikipedia.org/wiki/Developer_Transition_Kit_(2020...

macOS on ARM is very clearly a descendant of the way iOS works. It's just macOS userspace on top of an ARM XNU kernel, which was already a thing. The way the boot process works, etc. is clearly iOS plus their new fancy Boot Policy stuff for supporting multiple OSes and custom kernels.

> Doing that wouldn’t have allowed them to develop AS support for regular PCIe devices attached through Thunderbolt, for example, since there’s nothing like a PCIe lane in that architecture.

iPhones and iPads use PCIe! How do you think WiFi/storage are attached? PCIe isn't only a desktop thing, it's in most phones these days, the Nintendo Switch, Raspberry Pi 4, etc. Most modern ARM embedded systems use PCIe for something. You'd be hard pressed to find a high-end ARM SoC without PCIe lanes.

They didn't have Thunderbolt, but since Apple rolled their own bespoke controller too, there is absolutely nothing to be gained by bringing that up on Intel first. Instead they probably did the same thing everyone does: bring it up on FPGAs. Possibly as PCIe add-ons for existing iPad chips, possibly hosting a whole (reduced) SoC; both approaches would probably be mixed at different design stages.

I'm sure they also had at least one unreleased silicon spin/design before the M1, during this project. You never quite get these things right the first time. No idea if that would've made it close to tape-out or not, but I'm sure there was something quite a bit different from the M1 in design at some point.

> Use a regular Intel machine as a host

> Put whatever recent-generation ARM CPU they made, onto a PCIe card

You can buy one of those, it's called a T2 Mac. Indeed some Apple Silicon technologies came from there (e.g. how SEP external storage is managed), but not in the way you describe; most of the T2<->Intel interface was thrown away for Apple Silicon, and now the OS sees things the same way as BridgeOS did natively on the T2 on those Macs.