Understand the difference between RAM and ROM, the purpose of each, and the need for virtual memory and cache.

What this dot point is asking

AQA wants you to explain the difference between RAM and ROM, the purpose of each, and why cache and virtual memory are needed.

RAM

When you open a program, it is copied from secondary storage into RAM because RAM is much faster to access than a hard disk or SSD. More RAM lets more programs and larger data sets stay loaded at once, reducing the need to fall back on slower virtual memory.

ROM

ROM is essential because when a computer is first switched on, RAM is empty and the CPU needs instructions immediately. ROM provides those fixed start-up instructions, which load the operating system from secondary storage into RAM.

RAM versus ROM

Cache and virtual memory

Cache works because programs tend to reuse the same instructions and data repeatedly (for example inside a loop). Keeping those in fast cache means the CPU does not have to wait for the slower trip to RAM each time. There is a hierarchy of speed and size: registers (fastest, smallest), then cache, then RAM, then secondary storage (slowest, largest).

The memory hierarchy

The different stores form a hierarchy that trades speed against size and cost. At the top are the CPU registers and cache: tiny but extremely fast, holding the data the processor is using right now. Below them is RAM: larger and still fast, holding the running programs. Below that is secondary storage (hard disk or SSD): far larger and cheaper per gigabyte, but much slower, holding everything permanently. Virtual memory sits at the bottom of the "main memory" idea, borrowing slow secondary storage when RAM runs out. The general rule is that as you move down the hierarchy storage gets bigger and cheaper but slower, which is why frequently used data is kept as high up as possible.

Why both volatile and non-volatile memory are needed

A computer needs both kinds. Non-volatile ROM is needed at switch-on, because RAM is empty and the CPU must be told how to start; ROM holds those fixed boot instructions. Volatile RAM is needed while working, because it is fast and can be written and rewritten as programs run, even though it forgets everything at power-off. Non-volatile secondary storage is needed to keep the user's files and installed programs between sessions. No single type does all three jobs, which is why a real computer combines ROM, RAM, cache and secondary storage.

Try this

Q1. State one difference between RAM and ROM. [2 marks]

• Cue. RAM is volatile and loses contents without power; ROM is non-volatile and keeps its contents.

Q2. State the purpose of cache memory. [1 mark]

• Cue. To store frequently used data close to the CPU so it can be accessed quickly, reducing waiting.