Protection in Cray

[Tzi-cker Chiueh]

I am currently investigating methods of minimizing virtual memory overheads
in high-performance computers. I think I have some way of solving 
logical-physical address translation. But I can't think of an efficient way 
of providing protection. I figure since Cray doesn't have virtual memory, maybe
it can teach me something about achieving protection inexpensively. 
Can anybody enlighten me about how Cray provides protection as normal virtual 
memory systems offer, or where can I find useful description in this regard ? 
Thank you. 

tzi-cker