Why 'Memory' Isn't 'Temporary Storage': Decoding RAM, ROM, and Storage Classifications

Memory vs Storage: Core Definitions

Early computers used Memory and Storage interchangeably until technology diverged: Memory (内存/记忆体) is directly readable/writable by the CPU, while Storage (外存/存储体) retains data without power and requires loading into Memory for access.

Classification Hierarchy

Memory - 内存/记忆体
├─ ROM - Read Only Memory - 只读内存
│  ├─ PROM - Programmable ROM - 可编程只读内存
│  ├─ EPROM - Erasable Programmable ROM - 可擦除可编程只读内存
│  └─ EEPROM - Electrically Erasable Programmable ROM - 电可擦除可编程只读内存
└─ RAM - Random Access Memory - 随机访问内存
   ├─ SRAM - Static RAM - 静态随机访问内存
   └─ DRAM - Dynamic RAM - 动态随机访问内存
      ├─ SDRAM - Synchronous DRAM - 同步动态随机访问内存
      ├─ DDR - Double Data Rate SDRAM - 双倍率同步动态随机访问内存
      │  ├─ LPDDR - Low Power DDR - 低功耗双倍率同步动态随机访问内存
      │  └─ GDDR - Graphics DDR - 图形用双倍率同步动态随机访问内存
      └─ HBM - High Bandwidth Memory - 高带宽内存

Storage - 外存/存储体
├─ Magnetic Tape - 磁带
├─ Floppy Disk - 软盘
├─ HDD - Hard Disk Drive - 硬盘
├─ Optical Disc - 光盘
│  ├─ CDROM - Compact Disc Read-Only Memory - 只读光盘
│  └─ DVD - Digital Versatile Disc - 数字通用光盘
└─ Flash Memory - 闪存
   ├─ Flash Disk - U盘
   ├─ eMMC - embedded Multi-Media Card - 嵌入式多媒体存储卡
   └─ SSD - Solid-State Drive - 固态盘

Naming Quirks and Clarifications

CDROM includes "Memory" in its name because Philips and Sony coined the term; the author speculates their English may have been imprecise.

Flash Memory is categorized under Storage despite semiconductor similarities to Memory because it retains data without power — a naming conflict arising from new technology.

Magnetic disk evolved from magnetic tape; both floppy and hard disks use magnetic coating, whereas optical discs and SSDs have no magnetic relation.

Computers typically use SSD for external storage; phones and tablets use eMMC — both Flash Memory but in different form factors (drive vs. soldered).

Key Component Types

Cache (缓存) is usually SRAM located inside CPU/GPU.

Buffer (缓冲) appears everywhere: HDDs, SSDs, DDR internals, NICs, keyboards/mice; typically implemented as SRAM.

Computer memory today is mainly DDR4/DDR5; laptops, phones, tablets use LPDDR.

VRAM (显存/显卡内存) is memory used by GPUs, predominantly GDDR.

Main Memory (主存) refers to CPU-attached memory, distinguished from VRAM.

Unified Memory (统一内存) lets CPU and GPU share a single memory pool: Apple M-series and RTX Spark use LPDDR; PS5 uses GDDR.

Marketing Terminology Debunked

运存 (运行内存/运算内存) is a term invented by Chinese phone vendors. The author argues it is misleading: memory is powered continuously and has no compute capability — only CPU/GPU perform computation.

Terminology Ambiguity in Chinese

In Chinese, "存储" (storage) is often used loosely — e.g., "stored a float" may refer to registers or memory. When concepts clash, the author advises not to overthink; precision depends on context.