One response to this question might be one of a real estate agent's old favorites: “Location, location, location!” Cache memory is usually part of the central processing unit, or part of a complex that includes the CPU and an adjacent chipset where memory is used to hold data and instructions that are most frequently accessed by an executing program -- usually from RAM-based memory locations. In the classic von Neumann computer, the...
RAM was the “chalkboard” where processors did the math of a program. Placing this data store closer to the processor itself -- so data requests and responses didn’t have to traverse the motherboard bus -- reduced the wait time or latency associated with processing and delivered better than average latency and faster chip performance.
A RAM cache, by contrast, tends to include some permanent memory embedded on the motherboard and memory modules that can be installed by the consumer into dedicated slots or attachment locations. These memories are accessed via the mainboard bus (channels or conduits etched into the motherboard that interconnect different devices and chipsets). CPU cache memory operates between 10 to 100 times faster than RAM, requiring only a few nanoseconds to respond to the CPU request. RAM cache, of course, is much speedier in its response time than magnetic media, which delivers I/O at rates measured in milliseconds.
It should be noted that somewhat slower flash memory is now being used to provide an additional cache at the magnetic media level -- on disk controllers -- in an effort to change the latency characteristics of disk, especially as disks become more capacious and access to data increases. Considerable ink has been spilled to suggest that flash -- or solid-state disks -- will at some point in the future displace magnetic disks altogether as a production storage medium.
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