entered into RCS

From-SVN: r11984

gcc/objc/THREADS.MACH

+This readme refers to the file thr-mach.c.
+
+Under mach, thread priorities are kinda strange-- any given thread has
+a MAXIMUM priority and a BASE priority.  The BASE priority is the
+current priority of the thread and the MAXIMUM is the maximum possible
+priority the thread can assume.  The developer can lower, but never
+raise the maximum priority.
+
+The gcc concept of thread priorities is that they run at one of three
+levels;  interactive, background, and low.
+
+Under mach, this is translated to:
+
+interactive -- set priority to maximum
+background  -- set priority to 2/3 of maximum
+low         -- set priority to 1/3 of maximum
+
+This means that it is possible for a thread with the priority of
+interactive to actually run at a lower priority than another thread
+with a background, or even low, priority if the developer has modified
+the maximum priority.
+
+