Personally, I prefer this quote from Hannessy and Patterson: > The level below main memory contains rotating magnetic disks that take millions of clock cycles

Personally, I prefer this quote from Hannessy and Patterson: > The level below main memory contains rotating magnetic disks that take millions of clock cycles to access. Because of the great discrepancy in access time, no one has yet built a virtual memory operating system that writes through main memory to disk on every store by the processor. (This remark should not be interpreted as an opportunity to become famous by being the first to build one!)