VALID_START_PFN_5
psize_t npages2 = (VALID_END_PFN_5 - VALID_START_PFN_5);
for(pf = VALID_START_PFN_5; pf < VALID_END_PFN_5; pf += PF_STEP) {
pa += ctob(VALID_START_PFN_5);