A free program for Android, by Engineering Hub.

OS Concepts is a free Computer Science and Engineering lecture-based mobile application for learning concepts of operating system in an easy and intuitive way. This app is designed to help students understand important topics in operating systems and to serve as a quick reference guide for those topics. This app is intended to be used by students, faculty, and anyone interested in learning more about operating systems. It contains a lot of material that can be used to help you in your studies.

OS Concepts is a lecture-based app that focuses on Computer Science and Engineering topics. It contains a lot of material that is easy to understand and learn.