The prom interface code appears to be somewhat bogus.

As Allen pointed out, prom_open() in prom.c returns 32 bits
of the 64 that the SRM callback returns.  Prom_init() in if_prom.c
then pretends that prom_open() returned the entire 64 bits.

Also, it appears that the code thinks that the 64 bit value returned
from the SRM always has the same format. (the u struct in prom_return_t
in prom.h)  Different callbacks return different formats, according to
the Alpha AXP Architecture Reference Manual.

-Dave