Discussion
Which of these is a volatile memory?
Answer: D
Correct answer: RAM
RAM is volatile: it loses its contents the instant power is removed. That is why unsaved work is lost in a power cut, and why a computer must load the operating system from disk into RAM at every start-up. The other three are non-volatile and retain data without power. ROM holds firmware written at manufacture; a hard disk stores data as magnetic patterns; a flash drive uses floating-gate transistors that hold charge indefinitely. RAM divides in two, and the difference matters. SRAM stores each bit in a flip-flop, needs no refreshing and is fast, so it is used for cache - but it takes several transistors per bit and is expensive. DRAM stores each bit as charge on a tiny capacitor which leaks and must be refreshed thousands of times a second; it is far denser and cheaper, so it is used for main memory. The name random access distinguishes both from sequential media such as tape.
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