Primary storage is the memory the CPU can use directly. It comes in two kinds with opposite strengths: RAM, which the CPU can change but forgets when the power goes off, and ROM, which never forgets but cannot be changed.
Primary storage is storage that is directly accessed by the CPU. RAM (random access memory) holds the programs and data currently in use; it can be read and written, and it is volatile, so its contents are lost when the power is off. ROM (read only memory) holds instructions that must never be lost, such as the start-up (bootstrap) instructions; it can only be read, and it is non-volatile. A computer needs both: ROM to start up, RAM to run programs.
Fig. 1: RAM and ROM are both primary storage, accessed directly by the CPU
Section 01
Primary storage.
When you open an app, it is copied from secondary storage (such as an SSD) into primary storage so the CPU can fetch its instructions. The CPU cannot run a program straight from secondary storage.
Primary storage: memory that the CPU accesses directly, using the address and data buses. RAM and ROM are both primary storage.
Speed: it is much faster to access than secondary storage, which keeps the CPU supplied with instructions.
Secondary storage (HDD, SSD, optical discs) is not directly accessed by the CPU: data must be loaded into RAM first.
Examiner trap
Cache and registers are also inside or close to the CPU, but in this topic the two named types of primary storage are RAM and ROM.
Section 02
RAM and ROM.
Learn the comparison as pairs. Exam questions often ask for differences, and each difference needs both sides.
RAM
ROM
Full name
Random access memory
Read only memory
Volatile?
Volatile: contents lost when power is off
Non-volatile: contents kept without power
Read / write
Can be read from and written to
Can only be read (contents cannot normally be changed)
What it holds
The operating system, programs and data currently in use
The start-up (bootstrap) instructions, firmware such as the BIOS
Size
Usually large, e.g. 8 GiB to 32 GiB
Usually small
Contents change?
Constantly, as programs are opened and closed
Fixed when the chip is made
Your turn
Give two differences between RAM and ROM.
RAM is volatile, ROM is non-volatile; RAM can be read and written, ROM is read only; RAM holds programs and data in use, ROM holds the start-up instructions.
Examiner trap
"RAM is temporary" on its own is vague. Say volatile: its contents are lost when the power is turned off.
Stage 1 · Learn
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What is primary storage?
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What does it mean that RAM is volatile?
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What is usually stored in ROM?
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Why can the CPU not start up using only RAM?
Section 03
Why a computer needs both.
Neither kind can do the other's job, so every computer, from a laptop to a washing machine, has some of each.
ROM at start-up: when the power comes on, RAM is empty. The CPU needs instructions to start, so the bootstrap program is kept in ROM, where it survives with no power and cannot be accidentally deleted or changed.
RAM to work: the bootstrap loads the operating system from secondary storage into RAM. Programs and data being used must be in RAM because the CPU needs to read and write them quickly.
Embedded systems: a washing machine's fixed control program is in ROM; the current wash settings are held in RAM.
Worked example Where is it stored?
State whether each is stored in RAM or ROM: (a) a document being edited; (b) the boot-up instructions; (c) the web browser you are using.
(a) Being edited means it is in use and changing: RAM.
(b) Needed with no power and must never change: ROM.
(c) A program currently running: RAM.
Answer: RAM, ROM, RAM
Examiner trap
ROM does not store the operating system in a PC. ROM holds the start-up instructions, which load the operating system into RAM.
Stage 2 · Exam
Exam-style questions
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Which statement about ROM is correct?
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Which is stored in RAM while a computer is being used?
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Which pair are both types of primary storage?
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A washing machine stores the user's chosen wash settings. Where are they most likely to be stored while the wash runs?
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Primary storage, mastered.
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