Tag: cache line

Cache Line | Cache Line Size | Cache Memory

Cache Memory-

 

Before you go through this article, make sure that you have gone through the previous article on Cache Memory.

 

We have discussed-

  • Cache memory is a random access memory.
  • It lies on the path between the processor and the main memory.
  • It bridges the speed mismatch between the fastest processor and the slower main memory.

 

Also Read-Cache Mapping Techniques

 

Cache Lines-

 

Cache memory is divided into equal size partitions called as cache lines.

 

  • While designing a computer’s cache system, the size of cache lines is an important parameter.
  • The size of cache line affects a lot of parameters in the caching system.

 

The following results discuss the effect of changing the cache block (or line) size in a caching system.

 

Result-01: Effect of Changing Block Size on Spatial Locality-

 

The larger the block size, better will be the spatial locality.

 

Explanation-

 

Keeping the cache size constant, we have-

 

Case-01: Decreasing the Block Size-

 

  • A smaller block size will contain a smaller number of near by addresses in it.
  • Thus, only smaller number of near by addresses will be brought into the cache.
  • This increases the chances of cache miss which reduces the exploitation of spatial locality.
  • Thus, smaller is the block size, inferior is the spatial locality.

 

Case-02: Increasing the Block Size-

 

  • A larger block size will contain a larger number of near by addresses in it.
  • Thus, larger number of near by addresses will be brought into the cache.
  • This increases the chances of cache hit which increases the exploitation of spatial locality.
  • Thus, larger is the block size, better is the spatial locality.

 

Result-02: Effect of Changing Block Size On Cache Tag in Direct Mapped Cache-

 

In direct mapped cache, block size does not affect the cache tag anyhow.

 

Explanation-

 

Keeping the cache size constant, we have-

 

Case-01: Decreasing the Block Size-

 

  • Decreasing the block size increases the number of lines in cache.
  • With the decrease in block size, the number of bits in block offset decreases.
  • However, with the increase in the number of cache lines, number of bits in line number increases.
  • So, number of bits in line number + number of bits in block offset = remains constant.
  • Thus, there is no effect on the cache tag.

 

Example-

 

 

Case-02: Increasing the Block Size-

 

  • Increasing the block size decreases the number of lines in cache.
  • With the increase in block size, the number of bits in block offset increases.
  • However, with the decrease in the number of cache lines, number of bits in line number decreases.
  • Thus, number of bits in line number + number of bits in block offset = remains constant.
  • Thus, there is no effect on the cache tag.

 

Example-

 

 

Result-03: Effect of Changing Block Size On Cache Tag in Fully Associative Cache-

 

In fully associative cache, on decreasing block size, cache tag is reduced and vice versa.

 

Explanation-

 

Keeping the cache size constant, we have-

 

Case-01: Decreasing the Block Size-

 

  • Decreasing the block size decreases the number of bits in block offset.
  • With the decrease in number of bits in block offset, number of bits in tag increases.

 

Case-02: Increasing the Block Size-

 

  • Increasing the block size increases the number of bits in block offset.
  • With the increase in number of bits in block offset, number of bits in tag decreases.

 

Result-04: Effect of Changing Block Size On Cache Tag in Set Associative Cache-

 

In set associative cache, block size does not affect cache tag anyhow.

 

Explanation-

 

Keeping the cache size constant, we have-

 

Case-01: Decreasing the Block Size-

 

  • Decreasing the block size increases the number of lines in cache.
  • With the decrease in block size, number of bits in block offset decreases.
  • With the increase in the number of cache lines, number of sets in cache increases.
  • With the increase in number of sets in cache, number of bits in set number increases.
  • So, number of bits in set number + number of bits in block offset = remains constant.
  • Thus, there is no effect on the cache tag.

 

Example-

 

 

Case-02: Increasing the Block Size-

 

  • Increasing the block size decreases the number of lines in cache.
  • With the increase in block size, number of bits in block offset increases.
  • With the decrease in the number of cache lines, number of sets in cache decreases.
  • With the decrease in number of sets in cache, number of bits in set number decreases.
  • So, number of bits in set number + number of bits in block offset = remains constant.
  • Thus, there is no effect on the cache tag.

 

Example-

 

 

Result-05: Effect of Changing Block Size On Cache Miss Penalty-

 

A smaller cache block incurs a lower cache miss penalty.


Explanation-

 

  • When a cache miss occurs, block containing the required word has to be brought from the main memory.
  • If the block size is small, then time taken to bring the block in the cache will be less.
  • Hence, less miss penalty will incur.
  • But if the block size is large, then time taken to bring the block in the cache will be more.
  • Hence, more miss penalty will incur.

 

Result-06: Effect of Cache Tag On Cache Hit Time-

 

A smaller cache tag ensures a lower cache hit time.


Explanation-

 

  • Cache hit time is the time required to find out whether the required block is in cache or not.
  • It involves comparing the tag of generated address with the tag of cache lines.
  • Smaller is the cache tag, lesser will be the time taken to perform the comparisons.
  • Hence, smaller cache tag ensures lower cache hit time.
  • On the other hand, larger is the cache tag, more will be time taken to perform the comparisons.
  • Thus, larger cache tag results in higher cache hit time.

 

PRACTICE PROBLEM BASED ON CACHE LINE-

 

Problem-

 

In designing a computer’s cache system, the cache block or cache line size is an important parameter. Which of the following statements is correct in this context?

  1. A smaller block size implies better spatial locality
  2. A smaller block size implies a smaller cache tag and hence lower cache tag overhead
  3. A smaller block size implies a larger cache tag and hence lower cache hit time
  4. A smaller bock size incurs a lower cache miss penalty

 

Solution-

 

Option (D) is correct. (Result-05)

 

Reasons-

 

Option (A) is incorrect because-

  • Smaller block does not imply better spatial locality.
  • Always, Larger the block size, better is the spatial locality.

 

Option (B) is incorrect because-

  • In direct mapped cache and set associative cache, there is no effect of changing block size on cache tag.
  • In fully associative mapped cache, on decreasing block size, cache tag becomes larger.
  • Thus, smaller block size does not imply smaller cache tag in any cache organization.

 

Option (C) is incorrect because-

  • “A smaller block size implies a larger cache tag” is true only for fully associative mapped cache.
  • Larger cache tag does not imply lower cache hit time rather cache hit time is increased.

 

Next Article- Magnetic Disk | Important Formulas

 

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