Mac OS X Internals: A Systems Approach

The line between RISC and CISC microprocessors has been growing fuzzier over the years, particularly as the focus of microprocessor companies has shifted to microarchitecture. It is common to see companies attempting to optimize superscalar, out-of-order execution. In other words, modern RISC processors have become more CISC-like and vice versa. An equally interesting evolutionary circle can be observed in kernel design: "technically monolithic" kernels have evolved to contain enough modularity and flexibility to offer many of the benefits provided by microkernels.