Various sources state that they rebased to OSF Mach Kernel 7.3, which was based on Mach 3 and parts of Mach 4. The OSF MK ancestry of macOS XNU can still be seen in paths:
https://github.com/apple-oss-distributions/xnu/tree/main/osf...
The Darwin 0.1 and 0.3 releases contain the old kernel, derived directly from NeXTSTEP, and that's the direct derivative of Mach 2.5. The later XNU appears to be a reconstruction of that kernel with unencumbered code and that's also when IOKit replaced DriverKit.
EDIT: Apple calls out Mach 3.0.
https://developer.apple.com/library/archive/documentation/Da...
https://developer.apple.com/library/archive/documentation/Da...
Apple took Mach 3.0 and collapsed it into a single address space for performance purposes. Certainly reasonable.
True microkernels are, alas, more of an ideology than a practical reality, as the long-suffering GNU/HURD team discovered; Tanenbaum has been clear that the MINIX/NetBSD experiment was more about principles than performance. That said, certainly many hypervisors have attained success with configurations that coincidentally happen to be the same footprint as a microkernel.
[0] https://vita.militaryembedded.com/5159-open-virtualization-a...
HURD is Mach-based; it is stuck in a pre-L4 world and thus irrelevant to this conversation.
Very 40 years ago technology. Still better than ~60yr ago UNIX, although they're trying to ultimately be UNIX-like.
It is sad Mach and HURD still keep coming up in microkernel discussion.
The story goes that the early days were dominated by some ridiculously bad project management that makes Cathedral and the Bazaar era "Cathedral" a nice and well maintained project.
Most importantly, GNU Hurd lacked the "ecological niche" to drive work towards it because it couldn't deliver enough, just like lack of information about 386BSD led to Linux (Linus explicitly said that if he had known about 386BSD, he would have worked on it and maybe forked it)
NT and XNU never had microkernel origins - Cutler explicitly refuted this at a conference in the early 1990s
The alternative is a microkernel-based system, in which most of the OS runs as separate processes, mostly outside the kernel. They communicate by message passing. The kernel's job is to handle the message passing, interrupt handling, low-level process management, and possibly the I/O. Examples of this design are the RC4000, Amoeba, Chorus, Mach, and the not-yet-released Windows/NT.
That is why I thought Windows NT had originally been intended to be a microkernel.
Wikipedia has a decent depiction of the hybrid structure, but quoting from the NT Design Workbook:
> This specification describes the kernel layer of the NT OS/2 operating system. The kernel is responsible for thread dispatching, multiprocessor synchronization, hardware exception handling, and the implementation of low-level machine dependent functions.
> The kernel is used by the executive layer of the system to synchronize its activities and to implement the higher levels of abstraction that are exported in user-level API's.