title: Non blocking system call and mode switch
body: <p>Suppose we invoke a system call for asynchronous IO. At the time of invoking system call, the mode changes from user mode to kernel mode . After invocation, the mode should immediately change back to user mode so that user application can proceed further(as it is non blocking).</p>
  
  <p>Now if the mode is changed to user mode then how will kernel proceed with IO as mode is changed from kernel to user mode ? Will kernel perform asynchronous IO in user mode ?</p>