How can I set a process to have a non-degrading absolute priority, like the
npri command in IRIX?

I have to say I don't really understand the NetBSD priority system at all.
It seems very different to the one used in IRIX.
I understand about 'nice', but this only appears to apply an offset to
whatever priority the system decides to give a process. When I run 'top',
I see a column headed PRI, and another marked NICE. NICE is fairly obvious.
But the numbers in the PRI column seem to keep changing. Presumably low
numbers have greater priority. What determines these numbers, and how can
I fix them for a particular process? Is there a setting for 'real-time'?
(I'd like to increase the priority of the X server, because sometimes when
disk access is going on, the pointer moves rather jerkily.)

Thanks for any attempts to de-confuse me!

Ian