title: How to interact between Nios and FPGA?
body: <p><strong>Example:</strong>
  Let's assume there is a Nios running on a FPGA that sends randomly (or every second) a string to an attached display over the SPI interface. On the other hand there is the FPGA code that monitors a pushbutton. Every press on this button should send a string to the same attached display.</p>
  
  <p><strong>Question:</strong>
  How works the interaction (or communication) between the FPGA and Nios in general or in such described case? How is it possible to 'inform' Nios that the pushbutton is pressed when this code is running under the FPGA code? Maybe there is a documentation about this topic to get an idea how it works...</p>
  
  <p>Thanks in advance</p>