VALID_END_PFN_4
psize_t npages4 = (VALID_END_PFN_4 - VALID_START_PFN_4);
psize_t npages2 = (VALID_END_PFN_4 - VALID_START_PFN_4);
for(pf = VALID_START_PFN_4; pf < VALID_END_PFN_4; pf += PF_STEP) {
pa = (paddr_t) random() % (paddr_t) ctob(VALID_END_PFN_4);