Is anyone else having problems booting current NetBSD/sparc kernels on
Ultra systems ?

With GENERIC_SUN4U and custom kernels I get:

kernel trap 10: illegal instruction
Stopped in pid 0,1 (system) at netbsd:atomic_cas_ptr:  UNDEFINED
db> bt
proc0_init(...)
main(...)
cpu_initialize(...)

AFAICT NetBSD/sparc SUN4U kernels are using the sparc atomic ops not
the sparc64 ones.

Robert Swindells