Main memory will be considered to be any form of memory that is directly…

The idea

Main memory is the storage the CPU can reach directly over the address and data buses, but the definition deliberately excludes cache and registers, which are faster internal storage. In practice main memory means RAM (and ROM), where programs and data currently in use are held while the processor runs. A useful test is to ask two questions: can the CPU address this location directly, and is it outside the CPU's own cache and registers? If both answers are yes, it counts as main memory.

For example: A program loaded from disk sits in RAM and the CPU fetches its instructions from there; the same instructions may also be copied into cache, but that copy is not classed as main memory under this definition.

What earns the marks

• Identifies main memory as storage directly accessible by the CPU.
• Recognises that cache and registers are excluded from the definition.
• Gives RAM (and/or ROM) as the typical example of main memory.
• Distinguishes direct CPU access from storage reached only via input/output or secondary storage.
• Explains that programs and data in current use are held in main memory while executing.

Common mistakes and how to put them right

• Treating cache as main memory.

  Put it right by noting the definition explicitly excludes cache and registers.

• Assuming any storage the computer uses is main memory.

  Put it right by applying the direct-access test, so a hard disk is not main memory.

• Confusing main memory with secondary storage.

  Put it right by stressing that main memory is directly addressable by the CPU, secondary storage is not.

Exam tips

• Apply the two-part test: directly accessible by the CPU, and not cache or registers.
• Name RAM as the standard example and say why it fits the definition.
• When a distractor mentions cache or registers, rule it out immediately because the definition excludes them.