Two Kinds of Computer Memory — Memory and Storage | IGCSE Computer Science

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

RAM vs ROM and primary vs secondary storage comparisons appear regularly.

Every working computer keeps data in two distinct places:

Primary storage: the fast, working memory directly accessible to the CPU. Used while the program is running.
Secondary storage: the slower, long-term storage that holds programs and files when they are not in use, and that keeps its contents when the power is off.

A useful first comparison:

Feature	Primary storage	Secondary storage
Speed	Very fast	Much slower
Capacity	Small (typically gigabytes)	Large (often terabytes)
Cost per gigabyte	High	Low
Volatile?	RAM is volatile; ROM is not	Always non-volatile
Directly accessible by CPU?	Yes (the CPU reads from and writes to primary storage every cycle)	No (data must be loaded into primary storage first)
Typical examples	RAM, ROM, cache, CPU registers	Hard disk drives, SSDs, USB flash drives, CDs, DVDs, Blu-rays

Computers need both. Primary storage is fast enough to keep up with the CPU, but it is too expensive (and usually too volatile) to hold everything forever. Secondary storage is cheap and persistent, but too slow for the CPU to use directly during the fetch-decode-execute cycle.