Discussion
In a memory hierarchy, which storage is fastest and smallest?
Answer: B
Correct answer: CPU registers
Registers are the fastest and smallest storage, sitting inside the processor and accessible in a single clock cycle. A processor typically has only a few dozen. The hierarchy descends from there: registers, then cache in several levels, then main memory in DRAM, then secondary storage on solid-state or magnetic disk, then tertiary storage on tape or optical media. At each step capacity rises, cost per bit falls, and speed falls - often by an order of magnitude or more. The hierarchy works only because of locality of reference. Programs concentrate their accesses in small regions of address space, so keeping the active region near the top gives most references the speed of the fast level at the cost of the slow one. Without locality the whole arrangement would be pointless. Volatility changes at the boundary between main memory and secondary storage: everything above loses its contents when power is removed. Registers are managed by the compiler, cache by hardware, and main memory by the operating system.
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