Which type of memory is directly accessible by the CPU and is faster but more expensive per bit than secondary storage?

• A. Cache memory
• B. Hard disk
• C. Virtual memory
• D. Optical storage

Answer: (A)

The key idea is the memory hierarchy around the processor. Cache memory sits closest to the CPU and is designed to be extremely fast, using rapid memory cells (often SRAM). This proximity and speed mean the CPU can access cached data with very low latency, which is why it’s the fastest option per bit. However, cache is small and expensive to produce, so its total capacity is limited. That’s why the system uses it as a tiny, ultra-fast buffer between the CPU and much larger, slower memory.

If the needed data isn’t in the cache (a cache miss), the CPU must fetch it from slower main memory or beyond, which introduces delay. Secondary storage like hard disks or SSDs is even slower to access and is much cheaper per bit, so it isn’t directly used by the CPU at the same speed as cache. Virtual memory is a technique that uses disk space to extend RAM, but it’s not itself a fast, directly CPU-accessible memory type. Optical storage is likewise slow and not typical for direct CPU access.