From "Design Principles of the I/O Kit": https://developer.apple.com/library/content/documentation/DeviceDrivers/Conceptual/IOKitFundamentals/Features/Features.

From "Design Principles of the I/O Kit": https://developer.apple.com/library/content/documentation/DeviceDrivers/Conceptual/IOKitFundamentals/Features/Features.html#//apple_ref/doc/uid/TP0000012-TPXREF101 https://developer.apple.com/library/content/documentation/De... > "OS X is largely the product of two strains of operating-system technology: Mac OS 9 (and its predecessors) and BSD. Given this pedigree, one might have expected Apple to adopt the device-driver model of Mac OS 9 or FreeBSD. Instead, Apple chose to redesign the model. Several reasons motivated this decision. > "First, neither the Mac OS 9 driver model nor the FreeBSD driver model offers a set of features rich enough to meet the needs of OS X. The OS X kernel is significantly more advanced than its Mac OS precursors; it handles memory protection, preemptive multitasking, multiprocessing, and other features not present in previous versions of Mac OS. Although FreeBSD is capable of handling these features, the BSD model does not offer other features expected in a modern operating system, including automatic configuration, driver stacking, power management, and dynamic loading of devices." It sounds like switching to a different kernel, and extending it to support everything that the Mac needs, would be a lot more work than finding and fixing the issues they have now. 3 nasty security bugs in 6 months is bad, but how many would they have if they changed kernels entirely? Mac OS X 10.0 sure wasn't bug-free. (For fun times: try creating a new user on 10.0 with login "root".) I suspect it's very similar to why they didn't switch the display layer to X11: https://developers.slashdot.org/comments.pl?sid=75257&cid=6734612 https://developers.slashdot.org/comments.pl?sid=75257&cid=67...