This book presents material on computer organization and architecture including the design of CPUs, microprocessors, and computer systems. It uses simple CPU designs like the Very Simple CPU and Relatively Simple CPU to illustrate concepts without complexity. Each chapter connects the concepts to the Relatively Simple CPU design and shows how the ideas are implemented in real-world machines. The book comes with simulation software for the Relatively Simple CPU that allows students to write programs in its assembly language and simulate execution to see internal organization and data flow within the CPU.