This book is a comprehensive text on basic, undergraduate-level computer architecture. It starts from theoretical preliminaries and simple Boolean algebra. After a quick discussion on logic gates, it describes three classes of assembly languages: a custom RISC ISA called SimpleRisc, ARM, and x86. In the next part, a processor is designed for the SimpleRisc ISA from scratch. This includes the combinational units, ALUs, processor, basic 5-stage pipeline, and a microcode-based design. The last part of the book discusses caches, virtual memory, parallel programming, multiprocessors, storage devices and modern IO systems. The book's website has links to slides for each chapter and video lectures hosted on YouTube.