+/*
+ * Get value of an interval timer. The process virtual and
+ * profiling virtual time timers are kept in the u. area, since
+ * they can be swapped out. These are kept internally in the
+ * way they are specified externally: in time until they expire.
+ *
+ * The real time interval timer is kept in the process table slot
+ * for the process, and its value (it_value) is kept as an
+ * absolute time rather than as a delta, so that it is easy to keep
+ * periodic real-time signals from drifting.
+ *
+ * Virtual time timers are processed in the hardclock() routine of
+ * kern_clock.c. The real time timer is processed by a timeout
+ * routine, called from the softclock() routine. Since a callout
+ * may be delayed in real time due to interrupt processing in the system,
+ * it is possible for the real time timeout routine (realitexpire, given below),
+ * to be delayed in real time past when it is supposed to occur. It
+ * does not suffice, therefore, to reload the real timer .it_value from the
+ * real time timers .it_interval. Rather, we compute the next time in
+ * absolute time the timer should go off.
+ */