Learn in the textbookMemory Hierarchy and DRAM Standards & Modules
DefinitionSecondary storage (補助記憶) is non-volatile, large-capacity storage that keeps its contents when power is off and holds programs and data for the long term.
ExplanationDevices such as HDDs, SSDs and optical discs belong here, and to run a program its contents are first loaded into main memory. The memory hierarchy puts the register, which holds values being calculated inside the CPU, at the top and runs register → cache memory → main memory → secondary storage, with each lower level slower but cheaper per unit of capacity. Secondary storage is the slowest level of all, yet it has the largest capacity.
AnalogyA filing cabinet: getting papers in and out takes time, but it holds a lot, and its contents stay put even when you switch off the room's lights.
Memory hookOn a PC spec sheet, "Memory 16GB" usually means main memory, and "Storage 512GB" usually means the capacity of secondary storage.