RAM is semiconductor memory whose data can be freely read and written, used to hold programs and data temporarily while the computer processes them.
The storage that the CPU reads and writes directly is called main memory (主記憶), and RAM is used for it. RAM includes DRAM, which must be rewritten periodically (refresh, リフレッシュ) because its stored charge leaks away, and SRAM, which needs no refresh. A small, fast memory placed between the CPU and main memory to bridge their speed gap is called cache memory (キャッシュメモリ).
AnalogyIt is like the top of a work desk. You can freely add to the papers spread on it, but once you clear it at the end of the day (power off), nothing is left on the desk.
How to rememberRandom Access means that any address can be reached in about the same time; the name itself contains no word for "read" or "write".
Exam points
ExamDRAM has a simple structure and offers large capacity at low cost, so it is used for main memory (主記憶).
ExamSRAM is faster than DRAM but costs more per unit of capacity, so it is used for cache memory (キャッシュメモリ), where a small amount is enough.
ExamMemory dedicated to holding what is shown on the screen is called VRAM (graphics memory, グラフィックスメモリ). It is a kind of RAM that is rewritten every time the display changes.
ROM is semiconductor memory that is read-only in principle and keeps its contents even when the power is turned off.
Contents written at manufacture or a similar stage are read out and used, and are not rewritten during normal use. This makes ROM suited to storing data that rarely changes, such as the program used to start up a device.
AnalogyIt is like a printed book. You can read it again and again, but its contents are not rewritten, and the words do not fade while it sits on the shelf.
How to rememberIt stands for Read Only Memory, and the single word Only carries the meaning "just for reading." Both start with R, but the R in RAM is Random, a different word.
Exam points
ExamROM is non-volatile (不揮発性), so its stored contents are not lost even if the power supply is cut.
Mix-upThe ROM family also includes types that can later be erased electrically and rewritten (such as EEPROM), and flash memory (フラッシュメモリ) belongs to this line. Do not assume that ROM means "cannot be written at all."
Volatile memory (揮発性メモリ) is memory whose stored contents disappear once the power supply stops.
It keeps its contents by drawing power continuously, so the data is lost when the power goes off. RAM has this property, which is why unsaved work in progress disappears in a power outage.
AnalogyIt is like a balloon you keep blowing into. Stop blowing and take your mouth away, and the air quickly escapes.
How to rememberThe Japanese word 揮発 means evaporating away, as alcohol or gasoline does. Its characters suggest contents flying off when the power is cut.
Exam points
ExamDRAM and SRAM are both kinds of RAM, and both are volatile (揮発性), so they lose the data they hold when the power is cut.
Mix-upVolatility (揮発性) is about whether contents remain after power-off, which is a separate axis from whether the memory can be rewritten. Some memory can be rewritten and still keep its contents.
Non-volatile memory (不揮発性メモリ) is memory that continues to retain its stored contents even after the power is turned off.
Semiconductor memory
Volatile
DRAM
SRAM
Non-volatile
ROM (read-only in principle)
Flash memory (electrically rewritable)
Because it needs no power to hold its contents, the same data is available after the power is turned back on. Among semiconductor memories, ROM and flash memory have this property.
AnalogyIt is like notes written in a notebook. Close it and put it in your bag, and the same notes are there the next time you open it.
How to rememberThe word adds the negating 不 ("not") in front of 揮発. Not flying off means staying, which the makeup of the word itself tells you.
Exam points
ExamNon-volatility (不揮発性) is not limited to semiconductor memory: magnetic disks (磁気ディスク) and optical discs (光ディスク) are also non-volatile media that keep their contents without power.
Flash memory (フラッシュメモリ) is non-volatile semiconductor memory that can be rewritten electrically and keeps its contents when the power is off.
Data is erased in fixed units (blocks) before being rewritten. It is widely used in portable storage media and in storage devices that replace hard disks.
AnalogyIt is like a notebook written in pencil. You can erase and rewrite many times, and the writing stays when you close it, but erasing too often wears the paper down.
How to rememberIt is said to be named for the way it erases a whole block at once, in an instant like a camera flash.
Exam points
ExamSSDs and USB flash drives (USBメモリ) use flash memory (フラッシュメモリ) as their storage element. An SSD has no moving parts like those in an HDD, so it resists shock better and uses less power than an HDD.
ExamFlash memory (フラッシュメモリ) cells wear with each rewrite, so SSDs that use it have an upper limit on the number of writes.