It would be more accurate to say that it's most of a fairly random subset of the OS that was made open source back in the day - there's very little of newer pro

It would be more accurate to say that it's most of a fairly random subset of the OS that was made open source back in the day - there's very little of newer projects (regardless of what 'layer' of the OS they're at), including ones that replaced older ones, some old stuff is gone, and I'm not sure it ever really counted as complete. Some examples: - The main kernel is open source (although not 100% identical to the version in OS X), but many kernel extensions are closed source, including quite basic stuff like disk image mounting. - In one open source kernel extension I'm interested in, IOUSBFamily, functionality has been randomly disappearing in favor of empty .cpp files. Like several of the opensource releases, it doesn't actually compile... - A version of CoreFoundation (C level API) is available, but not the full thing. The Objective-C runtime is available, but the core Objective-C libraries have never been. - libxpc, which is rather low-level functionality added a few releases ago, was never open sourced; launchd is gone in this release, but before that it didn't compile, thanks to missing xpc headers. - Bootloaders: BootX from PowerPC was open source, but the x86 EFI stuff was always closed source. - libm is gone this release. cctools may be gone. Some libSystem libraries such as malloc and pthread were separated out in a previous release, but not open sourced; they're back now, though.