src/Pure/System/options.scala

       def apply(name: String): String = get(name, Options.String, Some(_))
       def update(name: String, x: String): Options = put(name, Options.String, x)
     }
 
   def seconds(name: String): Time = Time.seconds(real(name))
+
+  def threads(default: => Int = Multithreading.num_processors()): Int =
+    Multithreading.max_threads(value = int("threads"), default = default)
 
 
   /* external updates */
 
   private def check_value(name: String): Options = {