http://damaga37e3smvdiz473qnzyfnul275erstpaihy2apvwq3bspjxyvnad.onion/books?page=10
This book explains in detail the various components of XNU - BSD, Mach, platform expert, Kernel Extensions and the IOKit environment. It goes further into memory management (vm_map , pmap and the kernel zone allocator), processes, threads, Mach IPC internals, the Virtual Filesystem Switch, and more.