Which of the following correctly lists computer memory types from highest to…
2017
Which of the following correctly lists computer memory types from highest to lowest speed ?
Answer: C. CPU Registers; Cache Memory; Main Memory (RAM); Secondary Storage — ConceptThe memory hierarchy ranks storage by how quickly the processor can reach it. Access speed falls as a level sits further from the CPU datapath, while…
- A.
Secondary Storage; Main Memory (RAM); Cache Memory; CPU Registers
- B.
CPU Registers; Cache Memory; Secondary Storage; Main Memory (RAM)
- C.
CPU Registers; Cache Memory; Main Memory (RAM); Secondary Storage
- D.
Cache Memory; CPU Registers; Main Memory (RAM); Secondary Storage
Show answer & explanation
Correct answer: C
Concept
The memory hierarchy ranks storage by how quickly the processor can reach it. Access speed falls as a level sits further from the CPU datapath, while capacity and cost per bit move in the opposite direction. The standard fastest-to-slowest ordering is: registers inside the CPU, cache built from SRAM, main memory built from DRAM, and finally secondary storage reached over an I/O interface.
Application
Place each of the four named levels by where it physically sits and how long a single access takes:
Level | Where it sits | Typical access time |
|---|---|---|
CPU Registers | Inside the CPU datapath | Under 1 nanosecond (a single clock cycle) |
Cache Memory | SRAM on or beside the CPU die | A few nanoseconds |
Main Memory (RAM) | DRAM across the memory bus | Tens of nanoseconds |
Secondary Storage | SSD or hard disk over an I/O interface | Microseconds to milliseconds |
Reading the table from top to bottom gives the highest-to-lowest speed order: CPU Registers; Cache Memory; Main Memory (RAM); Secondary Storage.
Cross-check
Two independent checks confirm this ordering:
Capacity and cost per bit run in the reverse direction: registers are the smallest and the costliest per bit, while secondary storage is the largest and the cheapest per bit. A correct speed ordering must therefore be the exact reverse of the capacity ordering.
Each level exists to hide the latency of the level below it. Cache hides DRAM latency and DRAM hides disk latency, which is only possible if every level is faster than the one it fronts.
Contrasting the rejected sequences by value: starting with Secondary Storage and ending with CPU Registers inverts the entire hierarchy; placing Secondary Storage ahead of Main Memory (RAM) ranks an I/O-attached device above DRAM; and placing Cache Memory ahead of CPU Registers ranks on-die SRAM above the single-cycle registers it feeds.