Memory and storage · GCSE Computer Science

Primary storage

RAM (random access memory) holds the programs and data that are in use. It is volatile: everything in it is lost when the power is turned off. It can be read from and written to.
ROM (read-only memory) holds the start-up (boot) instructions. It is non-volatile and normally can't be changed.
Cache and registers inside the CPU are also primary storage.

When RAM is full, part of the secondary storage is used as virtual memory. Data that isn't needed right now is moved there to free up RAM.
Moving data between RAM and virtual memory is slow, so the computer slows down. Adding more RAM reduces the need for virtual memory.

Secondary storage keeps data when the power is off (it is non-volatile), such as files and installed programs.
Magnetic storage (a hard disk drive) has a large capacity and is cheap per gigabyte, but it has moving parts, so it is slower and can be damaged if dropped.
Solid state storage (an SSD, USB stick or memory card) is fast, and has no moving parts, so it is durable and silent. It costs more per gigabyte.
Optical storage (CDs, DVDs and Blu-ray discs) is cheap and portable, but it has a small capacity, is slow and scratches easily.

Compare capacity, speed, portability, durability, reliability and cost.
Cloud storage keeps files on remote servers that are reached over the internet. Files can be reached from anywhere and are backed up, but it needs an internet connection and relies on the provider's security.

Primary storage: Memory the CPU can reach directly, such as RAM, ROM and cache.
RAM: Random access memory: volatile memory that holds the programs and data in use.
ROM: Read-only memory: non-volatile memory that holds the start-up instructions.
Volatile: Loses its contents when the power is turned off.
Virtual memory: Part of secondary storage used as extra RAM when RAM is full.
Secondary storage: Non-volatile storage that keeps data permanently, such as an SSD.
Solid state storage: Storage with no moving parts that uses flash memory.
Optical storage: Storage read by a laser, such as a DVD.
Cloud storage: Storing files on remote servers reached over the internet.