Discussion
Which memory is directly accessible by the CPU without going through the bus to main memory?
Answer: C
Correct answer: Registers and cache
Registers sit inside the processor itself, and cache - at least its first level - is fabricated on the same die, so both are reached without a transaction on the external memory bus. That proximity is what makes them fast. The memory hierarchy arranges storage by the trade-off between speed, capacity and cost per bit. Registers are fastest and smallest; L1, L2 and L3 cache follow; then main memory in DRAM; then solid-state and magnetic disk; then optical media and tape at the bottom, slowest and cheapest. Everything below cache is reached over a bus and takes hundreds or millions of cycles rather than one. Only the top three levels are directly addressable by instructions at all. Disk, optical media and tape are secondary storage: their contents must be copied into main memory before the processor can operate on them, which is what an operating system’s input and output subsystem exists to arrange.
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