Know what the different types of memory are used for and why they are…

The idea

Memory in a computer system is not one thing: each type exists because of a different job and a different trade-off between speed, cost and volatility. Registers sit inside the CPU and hold the operands and results of the instruction currently executing, so they must be the fastest store. Cache (often split into levels) holds recently or frequently used instructions and data close to the CPU to reduce the delay of fetching from slower RAM. RAM is the main memory that holds the programs and data currently in use, including the operating system, and is volatile, so its contents are lost when power is removed. ROM is non-volatile and holds fixed start-up instructions such as the bootstrap loader. Read-only and flash memory may hold firmware.

What earns the marks

• Registers are located in the CPU and hold the data and instructions being processed at that moment; they are the fastest form of storage.
• Cache stores frequently or recently used instructions and data close to the CPU to speed up access and reduce fetching from RAM.
• RAM is volatile main memory holding the operating system, programs and data currently in use; contents are lost when power is switched off.
• ROM is non-volatile memory holding fixed instructions such as the bootstrap/start-up routine used when the computer is switched on.
• The memory hierarchy exists because no single memory type is simultaneously fast, large, cheap and non-volatile, so types are combined to balance speed, capacity and cost.

Common mistakes and how to put them right

• Saying RAM is non-volatile or keeps data permanently.

  Put it right by stating RAM is volatile and loses its contents when power is removed.

• Confusing cache with secondary storage.

  Put it right by explaining cache is small, fast memory close to the CPU, not a disk for long-term files.

• Claiming ROM stores the user's programs.

  Put it right by stating ROM holds fixed start-up instructions such as the bootstrap loader.

Exam tips

• Link each memory type to a specific job (registers to current operands, cache to frequently used data, RAM to running programs, ROM to start-up) rather than giving a vague definition.
• When asked why a type is required, refer to the speed/capacity/cost trade-off in the memory hierarchy.
• Check the volatility of each type before selecting an answer, since volatile versus non-volatile is a common discriminator.