RAM, ROM and non-volatile storage

In short: RAM is volatile main memory: fast, but empty after switch-off. ROM is read-only memory that holds data permanently and is only read. Non-volatile storage media (SSD, hard drive, USB stick) keep data without power and serve for storage.

In more detail: “Volatile” means: the contents are lost without power. Computers need both: fast, volatile memory for running work and non-volatile memory for keeping programs, data and the startup software.

In Depth

	RAM	ROM	Non-volatile storage media
Name	Random access memory	Read-only memory	SSD, HDD, USB stick, SD card
Volatile?	yes	no	no
Writing	fast, any number of times	fixed or rarely	possible, limited number (flash)
Speed	very high (~50 GB/s)	slow	SSD 0.5 – 14 GB/s, HDD 0.15 GB/s
Size	8 – 128 GB	KB to MB	256 GB – several TB
Purpose	Programs and data currently being worked on	Startup software (firmware)	Operating system, programs, files
Price per GB	high	–	low

RAM (mostly DRAM): Each cell is a tiny capacitor that must be refreshed constantly. Direct access to every address in equal time. SRAM (cache) is faster and more expensive.

ROM and relatives:

• Mask-programmed ROM: Content burned in during manufacturing.
• PROM, EPROM (UV-erasable), EEPROM (electrically erasable).
• Flash ROM: Common today for BIOS/UEFI firmware, writable (update).

Non-volatile (also mass storage): Data remains without power. Examples: Magnetic hard drive, flash (SSD), optical media (CD, DVD), magnetic tape.

Example of a file: It lies on the SSD, is copied into RAM when opened, edited there, and written back on saving. If you cut the power before saving, changes in RAM are lost.

• Note: The name “random access” is confusing since SSDs also have random access. The difference lies in speed and volatility.
• New technologies: NVRAM, 3D XPoint (discontinued), MRAM combine RAM speed with non-volatility.

See also: RAM, ROM, NAND flash