optional · CSS study notes
External Memory
CSS Optional — Paper-II, Computer Organization & Architecture
1. Definition
External (secondary) memory is non-volatile storage that retains data without power, used to hold programs and files that are not currently being executed by the CPU.
2. Classification — Magnetic Disk Structure
- Platter — the physical disk coated with magnetic material.
- Track — a concentric ring on the platter.
- Sector — a subdivision of a track; the smallest addressable unit.
- Cylinder — the set of tracks at the same radius across all platters.
- Read/write head — moves across tracks to read or write data.
3. Classification — SSD vs HDD
| Feature | SSD | HDD |
|---|---|---|
| Technology | Flash memory, no moving parts | Magnetic platters, moving head |
| Random access speed | Much faster | Slower (mechanical seek) |
| Cost per GB | Higher | Lower |
| Durability | Limited write cycles | Mechanical wear/failure risk |
4. Classification — Optical Storage
- CD / DVD / Blu-ray — laser-based, read/write via reflectivity changes; mainly legacy or archival use today.
5. Key Formula — Disk Access Time (high-yield)
Disk Access Time = Seek Time + Rotational Latency + Transfer Time
- Seek time — time for the read/write head to move to the correct track.
- Rotational latency — average time for the desired sector to rotate under the head.
- Transfer time — time to actually move the data once positioned.
🗺️ Architecture Diagram — Magnetic Disk Structure
graph TD
D["Disk Platter"] --> T["Tracks
concentric rings"]
T --> S["Sectors
pie-slice subdivisions"]
D --> C["Cylinder
same track, all platters"]
H["Read/Write Head"] -->|Seek| T
H -->|Rotational Latency| S
H -->|Transfer| DATA["Data Read/Written"]
🧮 Solved Numerical — Disk Access Time
Q: Seek time = 8 ms, Rotational latency = 4 ms, Transfer time = 1 ms. Find the total access time.
Total access time = 8 + 4 + 1 = 13 ms
Answer: 13 ms.
🎯 CSS Exam Angle
Disk access time numericals occasionally appear; SSD vs HDD comparison is a very common short/definitional question. See also RAID for multi-disk redundancy/performance schemes.
🔁 Repeated CSS Questions
- Calculate total disk access time given seek time, rotational latency, and transfer time. (Numerical)
- Compare SSD and HDD storage technologies in terms of speed, cost, and reliability.
- Explain the physical structure of a magnetic disk (tracks, sectors, cylinders).
⚠️ MCQ Trap
Rotational latency is the average time for the desired sector to rotate under the head — it is NOT the same as seek time, which is the time for the head to move across tracks.