Primary and Secondary Storage

All computer storage is either volatile or non-volatile. This is the most fundamental distinction in understanding how different storage types behave.

Property	Volatile	Non-volatile
Retains data without power?	No — contents lost immediately	Yes — data persists indefinitely
Typical use	Running programs and current data	Permanent storage
Examples	RAM	ROM, SSD, HDD

Volatile storage loses all its contents the moment the power supply is cut. This is why unsaved work disappears in a power cut — it existed only in volatile RAM.

Non-volatile storage retains data independently of power. Secondary storage devices (SSDs, hard drives) and ROM are all non-volatile — files saved to them persist when the computer is switched off.

The volatile/non-volatile distinction is the starting point for almost every AQA exam question on storage. Identify which category a device falls into before describing any other properties.

RAM (Random Access Memory) is the computer's main working memory. It is volatile and directly accessible by the CPU.

When a program runs, its instructions and the data it is currently working with are loaded into RAM from secondary storage. The CPU reads from and writes to RAM continuously during execution. The contents of RAM change constantly as programs open, close, and swap data.

RAM property	Detail
Volatile?	Yes — contents lost when power is removed
Directly accessible by CPU?	Yes
Speed	Fast (much slower than CPU cache, much faster than secondary storage)
Typical capacity	4 GB – 64 GB in modern computers
Role	Holds currently running programs and data in active use

More RAM allows more programs and data to be held in main memory simultaneously. When RAM is full, the operating system must move data to secondary storage (a process called paging), which slows performance significantly because secondary storage is far slower than RAM.

ROM (Read-Only Memory) is non-volatile memory that stores data permanently. Its contents cannot be modified during normal computer operation.

ROM typically stores the firmware — the low-level instructions the computer needs to start up before an operating system has loaded. When a computer is powered on, the CPU reads from ROM to run the initial boot process (sometimes called the BIOS or UEFI).

ROM property	Detail
Volatile?	No — contents persist without power
Directly accessible by CPU?	Yes (as primary storage)
Modifiable?	Not during normal operation
Role	Stores boot instructions and firmware

ROM is primary storage because the CPU can access it directly — but unlike RAM, its contents do not change as programs run. Think of ROM as the computer's fixed starting instructions and RAM as its scratchpad.