I wrote:
>3100s and 5k/200s don't have this register.  I think other decstations
>do.

Perhaps I was confounding a  1 us-resolution counter with the ASIC
counter in the 3max+.  This register is NOT documented. To quote
Mills' xntp-kernel/README.kern

>2.3. Precision Clocks for DECstation 5000/240 and 3000 AXP Alpha
>
>The stock microtime() routine in the Ultrix kernel for Digital Equipment
>MIPS-based workstations returns system time to the precision of the
>timer interrupt interval, which is in the 1-4 ms range. However, in the
>DECstation 5000/240 and possibly other machines of that family, there is
>an undocumented IOASIC hardware register that counts system bus cycles
>at a rate of 25 MHz. The new microtime() routine for the Ultrix kernel
>uses this register to interpolate system time between timer interrupts.
>This results in a precision of 1 us for all time values obtained via the
>gettimeofday() and ntp_gettime() system calls.

--Jonathan