为什么我在C中的以下代码中遇到了段错误？

Void tlb_insert(VPAGE_NUMBER new_vpage, PAGEFRAME_NUMBER new_pframe, BOOL new_mbit, BOOL new_rbit)
{
  // Starting at the clock_hand\'th entry, find first entry to
  // evict with either valid bit  = 0 or the R bit = 0. If there
  // is no such entry, then just evict the entry pointed to by
  // the clock hand.

  int m;
  int evct = clock_hand;
  for (m = clock_hand; m < (num_tlb_entries); m++){
    if (tlb[m].vbit_and_vpage & VBIT_MASK == 0 || tlb[m].mr_pframe & RBIT_MASK == 0){
      evct = m;

      break;
    }
  }

  // Then, if the entry to evict has a valid bit = 1,
  // write the M and R bits of the of entry back to the M and R
  // bitmaps, respectively, in the MMU (see mmu_modify_rbit_bitmap, etc.
  // in mmu.h)

  if (tlb[evct].vbit_and_vpage & VBIT_MASK == 1){
    PAGEFRAME_NUMBER pfr = tlb[evct].mr_pframe & PFRAME_MASK;
    int val1 = tlb[evct].mr_pframe & RBIT_MASK;
    int val2 = tlb[evct].mr_pframe & MBIT_MASK;
    mmu_modify_rbit_bitmap (pfr, val1);
    mmu_modify_mbit_bitmap(pfr, val2);
  }

  // Then, insert the new vpage, pageframe, M bit, and R bit into the
  // TLB entry that was just found (and possibly evicted).
   tlb[evct].vbit_and_vpage = VBIT_MASK | new_vpage;
   tlb[evct].mr_pframe = new_mbit | (new_rbit | new_pframe); 
  // Finally, set clock_hand to point to the next entry after the
  // entry found above.
  clock_hand = evct + 1;
}

//Writes the M  & R bits in the each valid TLB
//entry back to the M & R MMU bitmaps.
 void tlb_write_back()
{
   int n;
  for (n = 0; n < num_tlb_entries; n++){
    if (tlb[n].vbit_and_vpage & VBIT_MASK == 1){
       PAGEFRAME_NUMBER pfr = tlb[n].mr_pframe & PFRAME_MASK;
       int val1 = tlb[n].mr_pframe & RBIT_MASK;
       int val2 = tlb[n].mr_pframe & MBIT_MASK;
       mmu_modify_rbit_bitmap (pfr, val1);
       mmu_modify_mbit_bitmap(pfr, val2); 
    }   
    } 
  } 

tlb[evct].vbit_and_vpage = VBIT_MASK | new_vpage;
tlb[evct].mr_pframe = new_mbit | (new_rbit | new_pframe);