如何把LWIP协议初始申请空间放到外存里？

void LwIP_Init(void)
{
  /* Initializes the dynamic memory heap defined by MEM_SIZE.*/
  mem_init();
  /* Initializes the memory pools defined by MEMP_NUM_x.*/
  memp_init();
}
void
mem_init(void)
{
  struct mem *mem;
  LWIP_ASSERT("Sanity check alignment",
    (SIZEOF_STRUCT_MEM & (MEM_ALIGNMENT-1)) == 0);
  /* align the heap */
  ram = LWIP_MEM_ALIGN(ram_heap);
  /* initialize the start of the heap */
  mem = (struct mem *)ram;
  mem->next = MEM_SIZE_ALIGNED;
  mem->prev = 0;
  mem->used = 0;
  /* initialize the end of the heap */
  ram_end = (struct mem *)&ram[MEM_SIZE_ALIGNED];
  ram_end->used = 1;
  ram_end->next = MEM_SIZE_ALIGNED;
  ram_end->prev = MEM_SIZE_ALIGNED;
  mem_sem = sys_sem_new(1);
  /* initialize the lowest-free pointer to the start of the heap */
  lfree = (struct mem *)ram;
  MEM_STATS_AVAIL(avail, MEM_SIZE_ALIGNED);
}
void
memp_init(void)
{
  struct memp *memp;
  u16_t i, j;
  for (i = 0; i < MEMP_MAX; ++i) {
    MEMP_STATS_AVAIL(used, i, 0);
    MEMP_STATS_AVAIL(max, i, 0);
    MEMP_STATS_AVAIL(err, i, 0);
    MEMP_STATS_AVAIL(avail, i, memp_num);
  }
  memp = LWIP_MEM_ALIGN(memp_memory);
  /* for every pool: */
  for (i = 0; i < MEMP_MAX; ++i) {
    memp_tab = NULL;
    /* create a linked list of memp elements */
    for (j = 0; j < memp_num; ++j) {
      memp->next = memp_tab;
      memp_tab = memp;
      memp = (struct memp *)((u8_t *)memp + MEMP_SIZE + memp_sizes
#if MEMP_OVERFLOW_CHECK
        + MEMP_SANITY_REGION_AFTER_ALIGNED
#endif
      );
    }
  }