Write the structure and advantages of TLB.

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A Translation look aside buffer can be defined as a memory cache which can be used to reduce the time taken to access the page table again and again. It is a memory cache which is closer to the CPU and the time taken by CPU to access TLB is lesser then that taken to access main memory.TLB follows the concept of locality of reference which means that it contains only the entries of those many pages that are frequently accessed by the CPU.

OS Translation Look aside buffer

In translation look aside buffers, there are tags and keys with the help of which, the mapping is done. TLB hit is a condition where the desired entry is found in translation look aside buffer. If this happens then the CPU simply access the actual location in the main memory. However, if the entry is not found in TLB (TLB miss) then CPU has to access page table in the main memory and then access the actual frame in the main memory. Therefore, in the case of TLB hit, the effective access time will be lesser as compare to the case of TLB miss. If the probability of TLB hit is P% (TLB hit rate) then the probability of TLB miss (TLB miss rate) will be (1-P) %. Therefore, the effective access time can be defined as;

    EAT = P (t + m) + (1 – p) (t + k.m + m)

Where, p → TLB hit rate, t → time taken to access TLB, m → time taken to access main memory k = 1, if the single level paging has been implemented.

By the formula, we come to know that

  1. Effective access time will be decreased if the TLB hit rate is increased.
  2. Effective access time will be increased in the case of multilevel paging.

Advantages-

The advantages of using TLB are-

  • TLB reduces the effective access time.
  • Only one memory access is required when TLB hit occurs.
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