Basic CS-Operating System [TCS Placement]: Sample Questions 27 - 27 of 35

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Question 27

Operating System

Describe in Detail


What is the Translation Lookaside Buffer (TLB) ?


  • In a cached system, the base addresses of the last few referenced pages is maintained in registers called the TLB that aids in faster lookup.
  • TLB contains those page-table entries that have been most recently used.
  • Normally, each virtual memory reference causes physical memory accesses- one to fetch appropriate page-table entry, and one to fetch the desired data.
  • Using TLB in-between, this is reduced to just one physical memory access in cases of TLB-hit.
  • A translation Lookaside buffer (TLB) is a memory cache that stores recent translations of virtual memory to physical addresses for faster retrieval.
  • When a virtual memory address is referenced by a program, the search starts in the CPU.
  • First, instruction caches are checked. If the required memory is not in these very fast caches, the system has to look up the memory՚s physical address.
  • At this point, TLB is checked for a quick reference to the location in physical memory.
  • When an address is searched in the TLB and not found, the physical memory must be searched with a memory page crawl operation.
  • As virtual memory addresses are translated, values referenced are added to TLB.
  • When a value can be retrieved from TLB, speed is enhanced because the memory address is stored in the TLB on processor.
  • Most processors include TLBs to increase the speed of virtual memory operations through the inherent latency-reducing proximity as well as the high-running frequencies of current CPU՚s.
  • TLBs also add the support required for multi-user computers to keep memory separate, by having a user and a supervisor mode as well as using permissions on read and write bits to enable sharing.
  • TLBs can suffer performance issues from multitasking and code errors. This performance degradation is called a cache thrash.
  • Cache thrash is caused by an ongoing computer activity that fails to progress due to excessive use of resources or conflicts in the caching system.
Given the Image is Defining the Translation Lookaside Buffer (TLB)

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