You can look at the OSX kernel code, just grab Apple's XNU repository. I've built bootable kernels from it. The code's not terrible, but the weirdest part to me

You can look at the OSX kernel code, just grab Apple's XNU repository. I've built bootable kernels from it. The code's not terrible, but the weirdest part to me is that there's the OSFMK microkernel portion, and then there's the BSD subsystem. They use different types, and they have their own syscall tables, and it was never clear to me where a particular bit of code would reside. I only dabbled in it, of course.