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Translation Look-aside Buffers

  • using multi-level page tables can save space.
  • For example, how many bits do we need for the frame number for a 32-bit virtual address, 1 GB physical memory, and four kB pages? 1GB = 30 physical address bits. 4KB is 12 offset bits. Thus we need 30-12 = 18 bits for the frame number.
  • but this multi-level can also be very expensive because we need to look up that many times
    • potential solution: cache this! called translation look-aside buffers.
  • Similar to Cache Average Access Time,
TTranslation=TTLB Lookup+(1PTLB Hit)×TPage Table WalkT_\text{Translation} = T_\text{TLB Lookup} + (1 - P_\text{TLB Hit}) \times T_\text{Page Table Walk}