問63
頻出度★★☆
DRAM, ROM, SRAM, フラッシュメモリのうち, 電力供給が途絶えても内容が消えない不揮発性メモリはどれか。
平成25年度 春期 ITパスポート試験 問63
テクノロジ系コンピュータ構成要素 › メモリ
This question's explanation and your answers so far.
解説
問 63
Incorrectア
The pair DRAM and SRAM is wrong because both of them are volatile memories. This combination contains no non-volatile memory at all, and it is the combination furthest from the correct answer among the four choices. DRAM (Dynamic RAM) represents one bit by storing electric charge in a capacitor. The stored charge naturally leaks away over time, so even while the power is on, an operation called refresh, which reads the contents again at fixed intervals and writes them back, is required. Naturally, if the supply of power is interrupted, the contents are lost. SRAM (Static RAM) holds one bit in a circuit called a flip-flop. It does not need refreshing, but retention assumes that current keeps flowing through the circuit, so the contents disappear when the power is cut off. It is common to use DRAM as main memory and SRAM as cache memory, but both share the point that their contents do not remain once the power is turned off.
Incorrectイ
The pair DRAM and flash memory is right in that flash memory is a non-volatile memory, but wrong in that it includes DRAM. DRAM is a representative volatile memory widely used as main memory. It stores data as charge in capacitors, so refreshing is indispensable, and its contents disappear once the supply of power stops. The reason unsaved work is lost when you turn off a personal computer is that the main memory used as the working area is DRAM. Conversely, the reason saved contents are still there the next time you turn the power on is that they were written to a non-volatile device such as an SSD. This combination mixes one non-volatile memory with one volatile memory, so it is not a combination that lists only non-volatile memories.
Incorrectウ
The pair ROM and SRAM is right in that ROM is a non-volatile memory, but wrong in that it includes SRAM. The name Static in SRAM means that, unlike DRAM, it keeps its contents without refreshing; it does not mean that the contents remain after the power is cut off. If you mistake this point, you will wrongly regard SRAM as a non-volatile memory, so be careful. SRAM holds its contents in flip-flop circuits and is a volatile memory that assumes current keeps flowing. It is fast, but the circuit needed per bit is large and the price per unit of capacity is high, so it is used where a small capacity with high speed is required, such as cache memory, which fills the speed gap between the CPU and main memory.
- エ
正解 The correct answer is エ, ROM and flash memory. Memory is divided into two types according to whether it can keep its stored contents when the supply of power stops. Memory whose contents disappear when the power is cut off is called volatile memory, and memory that keeps its contents even without power is called non-volatile memory. Sorting the four devices in the question, ROM and flash memory are the non-volatile memories. ROM (Read Only Memory) is read-only memory whose contents are fixed at the time of manufacture or at the time of writing and are retained even without power. For this reason it is used to store the startup program that is always needed the moment the power is switched on (such as the BIOS), and fixed data built into equipment. Flash memory is a kind of EEPROM whose contents can be rewritten electrically; because its storage element, the floating gate, traps electric charge inside, the charge is kept even when the power is cut off, so the contents remain. Devices such as USB flash drives, memory cards like SD cards, and SSDs, whose contents must not be lost while they are carried around, use this flash memory inside. DRAM and SRAM, on the other hand, both belong to the RAM (Random Access Memory) family, and both keep their contents only on the premise that power continues to be supplied, so they are volatile memories. Therefore the combination that lists only non-volatile memories is ROM and flash memory, and エ is correct.
ここがポイント
- The point tested is the distinction between volatile and non-volatile memory. Memory whose contents disappear once the supply of power is interrupted is volatile memory, and DRAM and SRAM belong to this group. Memory that keeps its contents even without power is non-volatile memory, and ROM and flash memory belong to this group. Roughly remembering that the RAM family is volatile and the ROM family is non-volatile (flash memory is a kind of EEPROM that can be rewritten electrically) makes the judgement easier.
- It is also worth remembering the difference between DRAM and SRAM together. DRAM stores data by accumulating charge in capacitors, so it needs refreshing; its structure is simple, which makes large capacities easy and cheap, so it is used as main memory. SRAM stores data in flip-flops, so it needs no refreshing and is fast, but it is expensive and hard to make in large capacities, so it is used as cache memory. Static in SRAM means that refreshing is unnecessary, not that the contents can be retained when the power is turned off, and this is an easy trap to fall into.
用語