In the past, we have revised our books many-a-times. Most of the times, the reasons for revision were either the advancement of technologies or change in syllabus content. This time the reason behind the revision of book is the reorganization of subject computer Science (868) syllabus by ISC. This book is now completely based on the revised syllabus of computer Science subject for Class XL. While writing the content of this book, The intended aims of the subject have been kept in mind while keeping the explanations balanced and, lucid. This revised edition carries the same student-friendly presentation style like previous editions. This book also contains proper explanations of the concepts, definitions, important notes and tips, and numerous examples and solved problems to make the understanding easier.