Locality of reference is the property that determines the effectiveness of cache memory. It refers to the tendency of a computer program to access the same set of memory locations repeatedly over a short period of time. In other words, programs tend to access a small subset of memory locations frequently, while ignoring the rest of the memory.

Explanation:

Cache memory is a small, high-speed memory that stores frequently used data and instructions. It is placed between the processor and the main memory, and its purpose is to reduce the average access time to data and instructions, thus improving the overall performance of the computer.

The effectiveness of the cache memory depends on how frequently the data and instructions are accessed. If the data and instructions are accessed frequently, then storing them in the cache memory can significantly reduce the access time. On the other hand, if the data and instructions are accessed infrequently, then storing them in the cache memory would not be very useful.

The property of locality of reference comes into play here. Programs tend to exhibit locality of reference, which means that they access a small subset of memory locations frequently, while ignoring the rest of the memory. This property is divided into two types:

1. Temporal locality: This refers to the tendency of a program to access the same memory locations repeatedly over a short period of time. For example, a loop that iterates over an array will access the same array elements repeatedly.

2. Spatial locality: This refers to the tendency of a program to access memory locations that are near each other. For example, an array of integers will be stored in contiguous memory locations, and a program that accesses one element of the array is likely to access nearby elements as well.

The cache memory takes advantage of the locality of reference property to improve performance. When the processor requests data or instructions, the cache memory checks if it already has the required data or instructions. If it does, then the cache memory returns the data or instructions to the processor, which saves time compared to accessing the main memory. If the cache memory does not have the required data or instructions, then it fetches them from the main memory and stores them in the cache memory for future use.

In conclusion, the effectiveness of the cache memory is based on the property of locality of reference, which refers to the tendency of a program to access a small subset of memory locations frequently. The cache memory takes advantage of this property to improve performance by storing frequently used data and instructions in a high-speed memory that is closer to the processor.