Cache Memory Design

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Cache memory is a very high-speed semiconductor memory which can speed up the CPU. It acts as a buffer between the CPU and main memory. It is used to hold those parts of data and program which are most frequently used by CPU. The parts of data and programs are transferred from the disk to the cache memory by the operating system, from where the CPU can access them.

In this chapter, we will explain in detail about cache memory along with its advantages and disadvantages.

What is Cache Memory?

In digital systems like computers, the cache memory is a high-speed volatile semiconductor memory used to store data and instructions frequently accessed by the central processing unit of the system.

The cache memory acts as a buffer between the processing element and main/primary memory, more specifically RAM (Random Access Memory). It is mainly used to provide a faster access to the recent and most frequently used data and programs.

The cache memory is employed for improving the performance and efficiency of the digital systems, as it reduces the time required for accessing the data.

Cache Memory Design

In this section of the article, we will discuss different concepts involved in the design of cache memory −

Purpose of the Cache Memory

The main purpose of the cache memory is to store frequently used data and instructions. This helps in reducing the access time.

Size of the Cache Memory

It is found that small size cache memory results in better performance improvement of the system.

Cache Memory Hierarchy

Cache memory is generally organized in multiple hierarchy levels, where each level is called a cache level or cache layer. A computer system typically has multiple cache levels, most common of them are L1 (Level 1 Cache), L2 (Level 2 Cache), and L3 (Level 3 Cache). Here, the cache memory L1 is the smallest, fastest and closest to the CPU of the system, while the L2 and L3 cache memories are larger and slower than L1 cache.

Structure of Cache Memory

Cache memory is typically divided into blocks of a fixed size. Each block has a specific data storage capacity. The structure of the cache memory is formed by grouping all these blocks together into cache sets.

Mapping Techniques for Cache Memory

The mapping techniques are used to determine how the memory blocks are mapped to cache blocks. The following three types of cache mapping techniques are commonly used −

• Direct Mapping − Direct mapping is a simple cache mapping technique in which each memory block is mapped into a certain cache block. Although, this technique can lead to a high rate of conflicts.
• Fully Associative Mapping − In this mapping technique, each memory block can be placed in any cache block, hence this technique has high flexibility. However, it requires addition hardware.
• Set Associative Mapping − This mapping technique is a combination of direct and fully associative mappings. In this technique, the cache memory is divided into cache sets, and each memory block can be placed in any cache block within its corresponding cache set.

Cache Replacement Algorithms

When a memory block is required to be accessed into a cache block that is already occupied, then a cache replacement algorithm is needed to determine which memory block should be replaced to free up space in the cache memory for the new memory block.

The following three are the common cache replacement algorithms −

• First-In First-Out (FIFO) Algorithm − This algorithm replaces the memory block that exists in the cache memory the longest.
• Least Recently Used (LRU) Algorithm − This algorithm replaces the memory block that has been fetched least recently.
• Random Replacement (RR) Algorithm − This algorithm replaces any memory block randomly.

Performance of Cache Memory

The performance of the cache memory is generally measured in terms of its hit rate. The hit rate specifies the percentage of memory accesses that result in cache memory hits. A high hit rate indicates that a significant portion of the memory accesses is satisfied from the cache memory. This provides enhanced system performance.

All these are the fundamental concepts of cache memory design. Now, let’s have look into the advantages and disadvantages of cache memory design.

Types of Cache Memory

Cache memory is classified on the basis of "levels". Where, each level describes its accessibility and closeness to the processing element of the digital system.

The classification of cache memory is done in the following three levels −

L1 (Level 1) Cache Memory

It is also known as primary cache memory. The L1 cache memory is the fastest one. But it is very small in size and mainly used in the processor chip in the form of CPU cache.

L2 (Level 2) Cache Memory

It is also called secondary cache memory. It has more capacity as compared to the L1 cache memory. It can be used in the processor chip as CPU cache or it can be a separate chip.

L3 (Level 3) Cache Memory

This one is a specialized cache memory designed to enhance the performance of L1 and L2 cache memories. However, the L3 cache memory is significantly slower than L1 or L2 cache memories.

Features of Cache Memory

The key features of cache memory are listed below −

• Cache memory is faster than main memory.
• It consumes less access time as compared to main memory.
• It stores the program that can be executed within a short period of time.
• It stores data for temporary use.

Advantages of Cache Memory

In digital systems, the cache memory provides several advantages, as it improves the overall performance and efficiency of the system. Some of the key benefits of the cache memory are highlighted below −

• Cache memory provides a faster data access speed and reduces the total access time. This characteristic of the cache memory helps to speed up the execution of tasks.
• Cache memory helps to reduce the memory latency by storing recent and most frequently used data and instructions. Also, it minimizes the dependency on slower primary memory or RAM. This feature also results in improved system performance and efficiency.
• Cache memory operates at the same speed as the CPU. Hence, it can provide a steady stream of input data and instructions that reduces the idle time of the CPU. Therefore, it also improves the CPU utilization.
• Cache memory bridges the gap between the high-speed, expensive cache memory and the slow-speed, cheap main memory. It provides a balance between speed, capacity, and cost.

Disadvantages of Cache Memory

However, the cache memory offers several advantages. But it also has some disadvantages which are listed below −

• Cache memory has a very smaller storage capacity. Thus, it cannot be used to hold all the data and instructions required by the processing unit.
• Cache memory is expensive to the design and manufacture. It also increases the overall complexity of architecture of the digital system.
• Sometimes, the cache pollution may occur when irrelevant data stored in the cache memory and there is no enough space for useful data. This significantly degrades system performance.

Conclusion

In conclusion, the cache memory is a high-speed semiconductor memory primarily used in digital systems to improve their performance and efficiency. The use of cache memory reduces the data access time and speeds up the task execution. However, being a quite expensive memory, it can increase the overall cost of the system.

In this chapter, we covered all the important concepts related to cache memory such as its purpose, features, advantages, and disadvantages.