The complex hardware and software architecture of the modern computer system is the root of all IT-related technologies. The book starts with a primitive state of zero cognition, firstly introduces the most basic digital circuits, and then shifts to computer operating systems and AI. This book uses plain language, well-placed questions and the entry points in line with the primitive cognitive thinking, so as to help readers gain insight into the entire bottom rung of the computer world. This book follows a writing principle of “firstly explain the reasons, think about them, then present the solution, and finally abstract it into a concept”. The book is clearly-organized, it is able to lead readers to retrace the cognitive path of the author. This book is a collection of popular science and professional knowledge which is explained with simple terms, diagrams and models. It is suitable for all IT practitioners.