OS X El Capitan Kernel The OS X kernel is based on FreeBSD and Mach 3.0 and features an extensible architecture based on well-defined kernel programming interfaces (KPIs). OS X was the first operating system to ship as a single install that could boot into either a 32-bit or 64-bit kernel, either of which could run 32-bit and 64-bit applications. The purpose of this book, then, is to serve as the manual that should have accompanied OS X—version 10.11 in particular. OS X El Capitan: The Missing Manual is designed to accommodate readers at every technical level. The primary discussions are written for advanced-beginner or intermediate Mac fans.