code: def SXTH(self, params):
          
          Ra, Rb = self.get_two_parameters(r'\s*([^\s,]*),\s*([^\s,]*)(,\s*[^\s,]*)*\s*', params)
  
          self.check_arguments(low_registers=(Ra, Rb))
  
          def SXTH_func():
              if self.register[Rb] & (1 << 15):
                  self.register[Ra] = 0xFFFF0000 + (self.register[Rb] & 0xFFFF)
              else:
                  self.register[Ra] = (self.register[Rb] & 0xFFFF)
  
          return SXTH_func
docs: STXH Ra, Rb
  
          Sign extend the half word in Rb and store the result in Ra