CSS Computer Science (Optional) — Paper-I, Section-A, Topic I, p.18-20. Covers: Storage Media and Their Types.

Definition

• Storage media: any physical medium used to record and retain digital data, either temporarily (while in use) or permanently (for later retrieval) — includes registers/cache/RAM as well as HDD, SSD, optical discs, tape, and cloud storage.
• Storage device: the hardware unit that reads/writes data to a storage medium (e.g., a disk drive is the device; the disk/platter inside it is the medium) — a distinction sometimes tested directly.
• Types (by technology): Magnetic, Optical, Solid-State, Cloud — detailed below.
• Classification (by other axes): volatile vs non-volatile, sequential vs random access, primary vs secondary — see "Classification of Storage Media" below.

Memory/Storage Hierarchy (Classification by Speed vs Capacity)

Going down this hierarchy: speed decreases, capacity increases, cost-per-GB decreases.

• Registers
  • Location: inside the CPU itself.
  • Speed/Capacity: fastest of all, but only a few bytes.
  • Use: holds data the CPU is processing right now.
• Cache (L1/L2/L3)
  • Location: on or very near the CPU chip.
  • Speed/Capacity: extremely fast, small (KB-MB range).
  • Use: holds recently/frequently used data to avoid slower RAM access.
• Primary Memory (RAM)
  • Speed/Capacity: fast, moderate size (GB range).
  • Volatility: volatile — cleared when power is off.
  • Use: holds currently running programs/data.
• Secondary Storage (HDD, SSD)
  • Speed/Capacity: slower than RAM, much larger (GB-TB range).
  • Volatility: non-volatile — retains data without power.
  • Use: long-term storage of files, OS, installed programs.
• Tertiary / Off-line Storage (magnetic tape, optical discs, cloud)
  • Speed/Capacity: slowest, largest and/or cheapest per GB, often removable.
  • Use: backup, archival, distribution.
• Exam angle: a classic short-answer question is "arrange registers, cache, RAM, and secondary storage by speed" — the answer order is exactly the list above, fastest to slowest.

Classification of Storage Media

Storage media can be classified along several independent axes — examiners may ask for any one of these:

• By volatility:
  • Volatile: loses data when power is off — RAM, cache, registers.
  • Non-volatile: retains data without power — HDD, SSD, CD/DVD/Blu-ray, magnetic tape, ROM, cloud storage.
• By access method:
  • Sequential access: must read through data in order — magnetic tape.
  • Random/direct access: can jump straight to any location — HDD, SSD, optical discs, RAM.
• By technology (the main classification used below): Magnetic, Optical, Solid-State, Cloud.
• By role: Primary (RAM/cache — directly accessed by the CPU) vs Secondary (HDD/SSD/optical/tape — accessed via I/O controllers, not directly by the CPU).