This is a book about computer organization and architecture. It focuses on the function and design of the various components necessary to process informa-tion digitally. We present computing systems as a series of layers, starting with low-level hardware and progressing to higher-level software, including assem-blers and operating systems. These levels constitute a hierarchy of virtual machines. The study of computer organization focuses on this hierarchy and the issues involved with how we partition the levels and how each level is imple-mented. The study of computer architecture focuses on the interface between hardware and software, and emphasizes the structure and behavior of the system. The majority of information contained in this textbook is devoted to computer hardware, and computer organization and architecture, and their relationship to software performance.