MAX_PHYS_ADDRESS_BITS
#if (MAX_PHYS_ADDRESS_BITS > PGDIR_SHIFT + PGDIR_BITS)
#define MAX_PHYSMEM_BITS MAX_PHYS_ADDRESS_BITS
if (max_phys_bits > MAX_PHYS_ADDRESS_BITS) {
#define MAX_PHYS_ADDRESS (1UL << MAX_PHYS_ADDRESS_BITS)