This blog post is first one of a series of posts which will deal with various topics related to xHCI (usb 3.0). I'm Rama Teja one of the participants in GSOC 2017. My project is to develop xHCI driver for ReactOS with the help of my mentor Thomas Faber. In the xHCI documentation given by Intel there are three major topics i.e., Register Interface, Data Structures and Operational model. The hardware controller's register interface is used to send commands to it or get the status etc., Data structures are components in the memory space allocated to the Driver. Operational model defines how the controller works i.e., how to initialize the controller, how to connect a device etc.