Explain the performance of cache in computer architecture?

The main reason for containing cache memory in a computer is to increase system performance by decreasing the time required to access memory. The component of cache performance are cache hits and cache misses.

Each time the CPU accesses memory, it determines the cache. If the requested data is in the cache, the CPU accesses the data in the cache, instead of physical memory, this is a cache hit. If the data is not in the cache, the CPU accesses the data from the main memory. This is a cache miss.

The average memory access time, T_M is the weighted average of the cache access time, T_C plus the access time for physical memory T_P. The weighting factor is the hit ratio h. T_M can be expressed as

                            T_M=h T_C+(1−h)T_P

Since T_C is much less than T_P, increasing the hit ratio reduces the average memory access time. The table shows for T_C=10 ns,T_P=60 ns, and various values of h.

Hit ratios and average memory access times

Consider that a computer system has an associative cache, a direct-mapped cache, or a two-way set-associative cache of 8 bytes. The CPU accesses the following locations in the order shown. The subscript for each value is the low-order 3 bits of its address in physical memory.

A₀ B₀ C₂ A₀ D₁ B₀ E₄ F₅ A₀ C₂ D₁ B₀ G₃ C₂ H₇ I₆ A₀ B₀

It can determine the hit ratio and average memory access times for this access pattern for each of the three cache configurations, assuming again that

T_C=10 ns and

T_P=60ns.

First, consider the associative cache. It is initially empty uses a FIFO replacement policy. The table shows the contents of the cache as each value is accessed. Seven out of the 18 accesses are hits, yielding a hit ratio of h = 0.389 and an average memory access time of

T_M=40.56 ns.

Cache activity using associative ache

Ginni

Updated on: 27-Jul-2021

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