Which one among the following could be a permanent memory of a computer ?

Correct — A, (a) Hard Disk. 'Permanent memory' in this vocabulary means NON-VOLATILE storage — storage whose contents survive the machine being switched off — and of the four options only the hard disk is memory of that kind.

A hard disk records data as magnetised regions on rotating platters. Magnetisation needs no power to persist, so a file written today is still there after the machine has been off for a month. That is what makes the hard disk, along with solid state drives, optical discs and flash drives, SECONDARY storage: large, non-volatile, slower than main memory, and the place where programs and data live when they are not being worked on. The permanence is relative rather than absolute — a disk can be erased, overwritten or physically fail — but as against the memory that empties itself at every power cut, it is the permanent one.

The distinction being tested is worth stating in full, because it is the backbone of this whole block. VOLATILE memory loses its contents when power is removed: the processor's registers and cache, and the RAM that holds whatever is currently running. NON-VOLATILE memory keeps them: read-only memory holding firmware, and secondary storage such as hard disks and solid state drives. The reason for having both is speed against cost and permanence — RAM is fast and expensive and forgets, storage is slower and cheap and remembers.

Had read-only memory appeared among the options it would also have been permanent memory. It does not, and of what is offered only the hard disk retains anything.

Why the others are wrong

• (b)CPU — The central processing unit is the part that executes instructions, not a place where data is kept. It does contain a little very fast memory — registers and cache — but that memory is volatile and is measured in kilobytes and megabytes, existing only to feed the arithmetic. Nothing in a processor survives the power being cut.
• (c)RAM — The classic trap in this item, because random access memory IS memory in the strict sense and is what a specification sheet means by 'memory'. It is volatile: its contents are lost the moment power is removed, which is why unsaved work disappears in a power failure. RAM is the computer's working space, holding the operating system and the running programs; it is temporary by design.
• (d)Motherboard — A circuit board, not a memory device. It carries the processor, the memory modules, the expansion slots and the chipset that connects them, and it does hold a firmware chip with the start-up instructions — which is genuinely non-volatile. But the board itself stores none of the user's data, and describing it as a permanent memory confuses the container with the component.

Computer memory is arranged as a hierarchy trading speed against capacity and cost. At the top are the processor's registers and cache — tiny, extremely fast, volatile. Below them is main memory, RAM, which is volatile and holds what is currently running. Below that is secondary storage — hard disks, solid state drives, optical media, flash drives — which is non-volatile and holds everything else, and beyond it tertiary or archival storage such as tape. Read-only memory sits apart: non-volatile, written once or rarely, and used for firmware such as the instructions a computer follows when it is switched on. Data has to be brought from storage into RAM before the processor can work on it, and written back to storage to be kept.

Key facts

• Non-volatile memory retains its contents without power; volatile memory does not.
• Hard disks store data magnetically on rotating platters and are non-volatile secondary storage.
• RAM is volatile primary memory and holds the operating system and running programs.
• Read-only memory is non-volatile and holds firmware such as the start-up instructions.
• The processor's registers and cache are the fastest and smallest memory, and are volatile.
• Data must be loaded from secondary storage into main memory before the processor can operate on it.
• Solid state drives, optical discs and flash drives are also non-volatile secondary storage.
• The motherboard is the board that interconnects these components, not a memory device.

Common traps

• Answering RAM because the word 'memory' is in its name.
• Treating the motherboard or the processor as storage.
• Forgetting that read-only memory is also permanent memory, which matters when it appears among the options.
• Reading 'permanent' as unchangeable; the term here means only that the contents survive without power.

• practice — not a real PYQ

  Which one among the following memories loses its contents when the power supply is switched off ?

  • (a)Hard disk
  • (b)RAM
  • (c)ROM
  • (d)Optical disc

  Answer(b) RAM