TLB_REFTRAP
if ((pte & PTE_PROT(TLB_REFTRAP)) == 0)
pte = pa | PTE_PROT(TLB_WIRED | TLB_REFTRAP |
pte = PTE_PROT(TLB_REFTRAP);
(pte & PTE_PROT(TLB_UNCACHABLE|TLB_DIRTY|TLB_REFTRAP));
#define pmap_clear_reference(pg) pmap_changebit(pg, PTE_PROT(TLB_REFTRAP), 0)