Here are all three volumes of the original 1985 edition of Inside Macintosh as a searchable PDF:
Everyone and their grandmother builds Linux kernels. Many people build BSD, and some brave men even compile the OS X kernel every now and then. Why not compile your own Solaris kernel for a change?
Imagine you’re an i386 user mode application on a modern operating system, and you want to make a syscall, for example to request some memory or create a new thread. But syscalls can be made in various ways on the i386 family of CPUs (int, call gates, sysenter, syscall), and CPUs tend to support only a subset of them. But hardcoding “int” into the kernel is a waste of resources on modern CPUs, because sysenter is a lot faster.