BTW, if you want hard realtime on today's NetBSD, using an
interrupt handler for that task is probably easiest way.
Because interrupt priority level which is higher or equal to
IPL_CLOCK is already mostly realtime even on today's NetBSD,
if you exclude some devices which use that high priority (e.g.
3com etherlink III driver is one of such drivers, since it uses
splsched() internally).

You can see how to write such interrupt handler by looking at
IPL_SERIAL and splserial() handling in sys/dev/isa/com_isa.c and
sys/dev/isa/com.c .
--
soda