Consider the following : 1. Small memory 2. Less expensive memory 3. Fast memory Which of the above are the characteristics of the cache memory?

Correct — C, 1 and 3 only. Cache memory sits between the processor and main memory to hold the instructions and data most likely to be needed next, so it must be fast — that is its entire purpose, and statement 3 is right. It is also small, typically a few kilobytes to a few tens of megabytes against gigabytes of main memory, and statement 1 is right for the same reason: the very technology that makes it fast makes it costly and physically bulky per bit. That is why statement 2 is wrong. Cache is the more expensive memory per unit of storage, not the less expensive one.

• (a)1 and 2 only — Correct that cache is small but wrong that it is cheap, and it also leaves out speed, which is the property cache exists for.
• (b)2 and 3 only — Right about speed and wrong about cost. Static RAM cells used for cache need several transistors each, against one transistor and a capacitor for the dynamic RAM of main memory, so cache costs more per bit.
• (d)1, 2 and 3 — Would need cache to be cheap as well as small and fast. The three properties are not independent — smallness follows from the expense, so a memory that was cheap would not need to be small.

Computer storage is arranged as a hierarchy, and each level trades speed against cost and capacity. Registers inside the processor are fastest, smallest and dearest; cache comes next, then main memory, then secondary storage such as a solid-state or hard disk, and finally archival media. Cache works because programs show locality of reference — they tend to reuse the same instructions and data, and to use neighbouring addresses. Keeping that working set close to the processor means most accesses are served without waiting for main memory, and cache itself is usually split into levels, L1 being the smallest and fastest.

The item is easier than it looks once the properties are seen as connected rather than as three separate facts. Cache is built from static RAM, which is fast because each cell holds its value in a flip-flop with no refreshing, and expensive because that flip-flop needs several transistors. Being expensive, it is provided in small quantity. So smallness and speed both follow from the same design choice, and cheapness contradicts it — which is why exactly two of the three statements survive.

• Cache memory lies between the processor and main memory and holds frequently used data and instructions.
• It is built from static RAM, which is faster but costlier per bit than the dynamic RAM of main memory.
• Cache is small — kilobytes to tens of megabytes — against gigabytes of main memory.
• Cache works because programs display locality of reference in both time and address space.
• Modern processors use several cache levels, L1 being the smallest and fastest.

• Assuming small means cheap; here small is a consequence of expensive.
• Confusing cache with a buffer; a buffer smooths a rate mismatch, cache exploits locality of reference.
• Forgetting that registers, not cache, are the fastest storage in the machine.

A three-property item where two properties follow from one design decision, so identifying the odd statement out is enough.

No directly related past PYQ was found.

• practice — not a real PYQ

  Cache memory is placed between the

  • (a)processor and main memory
  • (b)main memory and hard disk
  • (c)keyboard and processor
  • (d)hard disk and printer

  Answer(a) processor and main memory — it holds the data and instructions the processor is most likely to need next.

• practice — not a real PYQ

  Which one of the following types of memory loses its contents when power is switched off?

  • (a)ROM
  • (b)RAM
  • (c)Hard disk
  • (d)Flash drive

  Answer(b) RAM — random access memory is volatile, while ROM, hard disks and flash drives retain data without power.