If __HAVE_CPU_TIMESTAMP is defined, <machine/rnd.h> exists and defines
two functions:
1) u_int32_t cpu_timestamp(void);
returns the 32 low order bits of a reasonably high frequency counter.
2) int cpu_havetimestamp(void);
returns non-zero if cpu_timestamp() actually works.
The timestamp counter should run at a frequency greater than 1 MHz
(otherwise, microtime would be sufficient); other than that, the exact
frequency and origin of the counter are unspecified.
sommerfeld