The ASYNCHRONOUS attribute  indicates  that  a  routine  may  be
  called  by  an asynchronous event (such as a condition handler).
  Since such an event can alter the values of variables within the
  routine  upredictably,  this  attribute  forces  the  routine to
  reference only local variables or variables  declared  with  the
  VOLATILE attribute.

  See the "HP Pascal Language Reference Manual" for  the  complete
  description of using the ASYNCHRONOUS attribute.