A cache bus connects a processor core directly to its cache memory, operating at a higher speed than the system bus. It is used to reduce the time it takes to read or change frequently accessed data. Modern processors use a large amount of internal cache, and each core of a multi-core processor can have its own cache or share a large common cache. A special type of cache bus, called an inter-core bus, is used to solve consistency issues.
A cache bus is a dedicated high-speed bus that a computer’s processor uses to communicate with its cache memory. Also known as the backside bus, it operates at a much higher speed than the system bus. A cache bus connects a processor core directly to its cache; it works independently of the processor bus, transferring data along a wider and less restricted path. A bus cache is used in most modern processors to reduce the time it takes to read or change frequently accessed data.
In the 1980s, cache memory was usually located on the motherboard, not on the processor chip itself. The cache was accessed via the processor bus, just like normal system memory. The amount of cache memory was often quite small and was only offered as an optional system performance enhancement.
As processor speed and efficiency increased in the early 1990s, the processor bus became a bottleneck; fast cache memory needed a way to interact with the processor without waiting for system memory and input/output operations to finish. In the mid-1990s, most new processors adopted a dual-bus architecture to solve this problem. A high-speed cache bus has been created to access the cache directly. This bus isn’t used for anything else – all other data transfers use the slower processor bus, also known as the front-side bus. The processor can use both buses simultaneously, resulting in substantially better performance.
Early dual-bus designs frequently used cache memory located on the motherboard; large amounts of on-chip cache were not yet cost-effective due to production throughput issues. Later designs often incorporated a mix of internal and external caching as rendering improved. Modern processors usually use a large amount of internal cache; many include 8 megabytes (MB) or more, compared to older models which often only had 8 kilobytes (KB). In modern designs where the entire cache is on chip, the cache bus can be quite short with a very large data path, 512 bits in some processors. The bus typically runs at the same speed as the processor itself. The end result is that the contents of the cache can be read or changed very quickly.
Each core of a multi-core processor can have its own cache or share a large common cache. In both cases, a cache bus connects each core to the appropriate cache memory. When each processor core has its own separate cache, consistency issues can arise. For example, when a core updates data in its cache, other copies of that data in other caches become stale or “stale”. One way to solve this kind of problem is to use a special type of cache bus, sometimes called an inter-core bus. This bus links all the caches together so that each can monitor what the others are doing: if one updates a shared piece of data, the others can immediately reflect the new content.
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