Understand the differences between main memory and secondary storage.

The idea

Main memory and secondary storage differ in purpose, volatility, speed, capacity and cost. Main memory, principally RAM, is directly accessible by the CPU and holds the operating system, programs and data currently in use; it is volatile, so its contents are lost when power is removed, and it is faster but generally smaller and more expensive per unit than secondary storage. Secondary storage, such as a hard disk drive, solid-state drive or optical disc, is non-volatile and holds programs and files long term, including when the computer is switched off; it is slower to access but offers much greater capacity at lower cost per unit. Because main memory is volatile and limited, secondary storage is required to keep data and software permanently, and data must be loaded from secondary storage into main memory before the CPU can process it.

What earns the marks

• Main memory (RAM) is volatile and loses its contents when power is removed, whereas secondary storage is non-volatile and retains data permanently.
• Main memory is directly accessible by the CPU and holds the operating system, programs and data currently in use.
• Secondary storage holds programs and files long term, including when the computer is switched off, and is not directly processed by the CPU.
• Main memory is faster but generally smaller and more expensive per unit than secondary storage, which is slower but offers greater capacity at lower cost.
• Data and programs must be loaded from secondary storage into main memory before the CPU can process them.

Common mistakes and how to put them right

• Treating main memory and secondary storage as interchangeable.

  Put it right by stating main memory is volatile working storage while secondary storage is non-volatile long-term storage.

• Saying secondary storage is directly processed by the CPU.

  Put it right by explaining data must first be loaded into main memory.

• Assuming main memory is larger than secondary storage.

  Put it right by noting main memory is typically smaller and more expensive per unit.

Exam tips

• Compare the two on named criteria (volatility, speed, capacity, cost, CPU access) rather than describing each separately.
• Use a concrete example such as RAM versus an SSD to anchor the comparison.
• State the direction of data flow: programs and files are loaded from secondary storage into main memory before processing.