Understand the different types of memory within a computer:

The idea

A computer uses several types of memory that differ in speed, volatility and purpose. Main memory is directly accessible by the CPU and is usually volatile, meaning its contents are lost when power is removed. Random access memory (RAM) is read and write memory holding the programs and data currently in use. Read only memory (ROM) is non-volatile and holds fixed instructions such as the start-up routine. Cache is small, very fast memory close to the CPU that stores frequently used instructions and data to reduce access time to main memory. Registers are the fastest, smallest storage inside the CPU. Secondary storage such as a hard disk or solid-state drive is non-volatile and keeps data when the computer is switched off.

What earns the marks

• RAM is volatile read and write memory holding programs and data currently in use.
• ROM is non-volatile memory holding fixed instructions such as the start-up routine.
• Cache is small, fast memory close to the CPU that stores frequently used instructions and data to reduce access time.
• Registers are the fastest and smallest storage, located inside the CPU.
• Secondary storage is non-volatile and retains data when the computer is switched off.
• Volatility distinguishes memory that loses its contents without power from memory that keeps them.

Common mistakes and how to put them right

• Assuming RAM keeps its contents when power is removed.

  Put it right by stating that RAM is volatile.

• Confusing cache with main memory.

  Put it right: Cache is smaller and faster and sits close to the CPU to reduce access time.

• Thinking ROM is writable during normal use.

  Put it right: ROM holds fixed instructions such as the start-up routine.

• Treating secondary storage as part of main memory.

  Put it right: Secondary storage is non-volatile and separate from the CPU's directly accessible main memory.

Exam tips

• Read the question for a key property such as volatile, non-volatile, fastest or holds start-up instructions, then match it to one memory type.
• Eliminate options that contradict a known property, for example any option calling RAM non-volatile.
• Check whether the question asks about speed, volatility or purpose before choosing.