Storage, Memory & File Systems
RAM, ROM, cache, Flash, NAND SSD, HDD, FAT32 vs NTFS, magnetic tape, registers, memory hierarchy for UPSSSC AGTA.
Memory Hierarchy
Computer memory is organized in a hierarchy — faster memory is smaller and more expensive, while slower memory is larger and cheaper.
| Level | Memory Type | Speed | Size | Cost |
|---|---|---|---|---|
| 1 (Fastest) | Registers | Fastest | Bytes | Most expensive |
| 2 | Cache (L1, L2, L3) | Very fast | KB–MB | Very expensive |
| 3 | RAM | Fast | GB | Moderate |
| 4 | SSD / HDD | Moderate | TB | Cheaper |
| 5 (Slowest) | Magnetic Tape | Slowest | TB–PB | Cheapest |
Key Rule: As you go DOWN the hierarchy → speed decreases, capacity increases, cost per GB decreases.
Primary Memory (Main Memory)
Primary memory is directly accessible by the CPU. It is fast but limited in size.
RAM — Random Access Memory
RAM is volatile memory — data is lost when power is turned off. It stores currently running programs and data.
| Type | Full Form | Key Feature |
|---|---|---|
| SRAM | Static RAM | Faster, no refresh needed, used in cache, expensive |
| DRAM | Dynamic RAM | Slower, needs periodic refresh, used as main memory, cheaper |
| DDR | Double Data Rate | Transfers data on both edges of clock signal |
DDR Generations:
| Generation | Speed | Common Use |
|---|---|---|
| DDR3 | Up to 2133 MHz | Older computers |
| DDR4 | Up to 3200 MHz | Current standard |
| DDR5 | Up to 6400+ MHz | Latest, high-performance |
ROM — Read Only Memory
ROM is non-volatile memory — data is retained even when power is off. It stores permanent instructions like the BIOS/boot program.
| Type | Full Form | Feature |
|---|---|---|
| ROM | Read Only Memory | Factory-written, cannot be changed |
| PROM | Programmable ROM | Written once by user, then permanent |
| EPROM | Erasable PROM | Erased using ultraviolet (UV) light, then rewritten |
| EEPROM | Electrically Erasable PROM | Erased electrically, byte by byte — used in BIOS chips |
| Flash Memory | — | Type of EEPROM, erased in blocks — USB drives, SSDs |
RAM vs ROM Comparison
| Feature | RAM | ROM |
|---|---|---|
| Volatility | Volatile (loses data on power off) | Non-volatile (retains data) |
| Purpose | Temporary storage for running programs | Permanent storage for boot instructions |
| Speed | Faster | Slower |
| Size | Larger (4–64 GB typical) | Smaller (few MB) |
| Modifiable | Read and write | Read only (mostly) |
| Example | DDR4 RAM module | BIOS chip on motherboard |
Cache Memory
Cache memory is ultra-fast memory placed between the CPU and RAM. Its purpose is to bridge the speed gap between the very fast CPU and the slower RAM.
| Level | Location | Size | Speed |
|---|---|---|---|
| L1 Cache | Inside CPU core | 32–128 KB | Fastest |
| L2 Cache | Inside CPU (per core or shared) | 256 KB–1 MB | Fast |
| L3 Cache | Shared among all cores | 4–64 MB | Moderate |
- CPU first checks L1 → L2 → L3 → RAM (in order)
- Cache Hit = data found in cache (fast)
- Cache Miss = data not in cache, must fetch from RAM (slower)
- SRAM technology is used for cache (no refresh needed)
Registers
Registers are the smallest and fastest memory units, located inside the CPU itself.
| Register | Purpose |
|---|---|
| MAR (Memory Address Register) | Holds the address of memory location being accessed |
| MDR (Memory Data Register) | Holds data being transferred to/from memory |
| Accumulator | Stores intermediate results of ALU calculations |
| Program Counter (PC) | Holds address of the next instruction |
| Instruction Register (IR) | Holds the currently executing instruction |
Secondary Storage
Secondary storage is non-volatile — retains data permanently even without power. It is slower but much larger than primary memory.
HDD — Hard Disk Drive
HDD stores data on magnetic spinning platters with a read/write head that moves across the surface.
- Magnetic storage technology
- Moving parts → prone to physical damage
- Capacity: typically 500 GB – 4 TB
- RPM (Revolutions Per Minute): 5400 or 7200 RPM
- Cheaper per GB than SSD
SSD — Solid State Drive
SSD stores data on NAND Flash memory chips — no moving parts.
- Flash-based storage (NAND technology)
- No moving parts → shock-resistant, silent, faster
- Capacity: typically 128 GB – 4 TB
- Much faster boot and load times than HDD
HDD vs SSD Comparison
| Feature | HDD | SSD |
|---|---|---|
| Technology | Magnetic platters | NAND Flash chips |
| Moving Parts | Yes (spinning disk + head) | No moving parts |
| Speed | Slower (80–160 MB/s) | Much faster (500–7000 MB/s) |
| Durability | Fragile (sensitive to drops) | Shock-resistant |
| Noise | Audible spinning noise | Silent |
| Power Usage | Higher | Lower (better for laptops) |
| Cost/GB | Cheaper | More expensive |
| Lifespan | Longer (mechanical wear) | Limited write cycles |
| Boot Time | 30–60 seconds | 10–15 seconds |
Flash Memory
Flash memory is a type of EEPROM that can be erased and rewritten in blocks. It is the technology behind USB drives, SD cards, and SSDs.
| Device | Capacity | Use Case |
|---|---|---|
| USB Flash Drive (Pen Drive) | 4 GB – 256 GB | Portable file transfer |
| SD Card | 2 GB – 1 TB | Cameras, phones, tablets |
| MicroSD | 2 GB – 1 TB | Smartphones, drones |
| CompactFlash | Up to 512 GB | Professional cameras |
- Based on NAND technology
- Non-volatile — retains data without power
- No moving parts — durable and portable
Optical Storage
Optical storage uses laser beams to read and write data on reflective discs.
| Type | Full Form | Capacity | Laser Color |
|---|---|---|---|
| CD | Compact Disc | 700 MB | Red (infrared) |
| DVD | Digital Versatile Disc | 4.7 GB (single) / 8.5 GB (dual) | Red |
| Blu-ray | — | 25 GB (single) / 50 GB (dual) | Blue-violet laser |
Disc Types:
- ROM (Read Only) — factory pressed, cannot be written (commercial movies, software)
- R (Recordable) — write once (CD-R, DVD-R)
- RW (ReWritable) — can be erased and rewritten (CD-RW, DVD-RW)
Magnetic Tape
Magnetic tape is the oldest and cheapest storage medium, still used for backup and archival storage.
- Sequential access only — must read data in order (like a cassette tape)
- Cannot directly jump to a specific location (unlike HDD/SSD which have random access)
- Very high capacity (up to several terabytes per cartridge)
- Used by large organizations for data backup and disaster recovery
- Extremely low cost per GB
Storage Units
| Unit | Abbreviation | Size |
|---|---|---|
| Bit | b | Smallest unit (0 or 1) |
| Byte | B | 8 bits = 1 Byte |
| Kilobyte | KB | 1024 Bytes |
| Megabyte | MB | 1024 KB |
| Gigabyte | GB | 1024 MB |
| Terabyte | TB | 1024 GB |
| Petabyte | PB | 1024 TB |
| Exabyte | EB | 1024 PB |
Exam Tip: 1 Byte = 8 Bits. Remember the sequence: B → KB → MB → GB → TB → PB → EB (each × 1024).
File Systems
A file system is a method used by the operating system to organize, store, and retrieve files on a storage device.
FAT32 vs NTFS vs ext4
| Feature | FAT32 | NTFS | ext4 |
|---|---|---|---|
| Full Form | File Allocation Table 32 | New Technology File System | Fourth Extended File System |
| Developer | Microsoft | Microsoft | Linux community |
| Max File Size | 4 GB | No practical limit (16 TB) | 16 TB |
| Max Volume | 2 TB | 256 TB | 1 EB |
| Security | No file permissions | Full permissions + encryption | Full permissions |
| Journaling | No | Yes (crash recovery) | Yes |
| OS Support | Windows, Mac, Linux, cameras | Windows only (read on Mac/Linux) | Linux only |
| Use Case | USB drives, SD cards | Windows hard drives | Linux systems |
Other File Systems:
- FAT16 — older version, max 2 GB file, used in very old systems
- APFS (Apple File System) — macOS, iOS; optimized for SSD and Flash storage
- exFAT — designed for flash drives, no 4 GB limit like FAT32, cross-platform
Exam Favourite: FAT32 cannot store files larger than 4 GB. NTFS supports file permissions and encryption.
Cloud Storage
Cloud storage stores data on remote servers accessed via the internet, instead of local hard drives.
| Service | Provider | Free Storage |
|---|---|---|
| Google Drive | 15 GB | |
| OneDrive | Microsoft | 5 GB |
| iCloud | Apple | 5 GB |
| Dropbox | Dropbox Inc. | 2 GB |
Advantages: Access from anywhere, automatic backup, sharing, no physical damage risk Disadvantages: Requires internet, privacy concerns, subscription costs for more storage
Storage Unit Conversion Table
| Unit | Equivalent | Approximate Decimal |
|---|---|---|
| 1 Nibble | 4 bits | — |
| 1 Byte | 8 bits | — |
| 1 KB | 1024 Bytes | ~1 Thousand Bytes |
| 1 MB | 1024 KB | ~1 Million Bytes |
| 1 GB | 1024 MB | ~1 Billion Bytes |
| 1 TB | 1024 GB | ~1 Trillion Bytes |
| 1 PB | 1024 TB | ~1 Quadrillion Bytes |
| 1 EB | 1024 PB | ~1 Quintillion Bytes |
Exam Favourite: 1 Nibble = 4 bits. 1 Byte = 8 bits = 2 Nibbles. 1 Terabyte = 1024 GB = approximately 1 trillion bytes.
DDR RAM Generations
| Generation | Data Rate | Voltage | Typical Speed | Year Introduced |
|---|---|---|---|---|
| DDR1 | 200–400 MT/s | 2.5V | 200–400 MHz | 2000 |
| DDR2 | 400–1066 MT/s | 1.8V | 400–800 MHz | 2003 |
| DDR3 | 800–2133 MT/s | 1.5V | Up to 2133 MHz | 2007 |
| DDR4 | 1600–3200 MT/s | 1.2V | Up to 3200 MHz | 2014 |
| DDR5 | 3200–6400+ MT/s | 1.1V | Up to 6400+ MHz | 2020 |
Each DDR generation doubles the data rate, uses lower voltage (more energy efficient), and is physically incompatible with previous slots — DDR4 RAM cannot fit in a DDR3 slot.
SSD Types: SATA vs NVMe
| Feature | SATA SSD | NVMe SSD |
|---|---|---|
| Interface | SATA III (same as HDD) | PCIe (direct to CPU) |
| Speed | Up to 550 MB/s | Up to 3500–7000 MB/s |
| Form Factor | 2.5-inch drive | M.2 stick (small) |
| Performance | 3–5× faster than HDD | 5–7× faster than SATA SSD |
| Price | Moderate | Higher |
| Best For | Budget upgrades | High-performance, gaming, workstations |
Exam Tip: NVMe (Non-Volatile Memory Express) connects via PCIe and is significantly faster than SATA SSD. Both use NAND Flash technology.
RAID — Redundant Array of Independent Disks
RAID combines multiple physical disks into a single logical unit for improved performance, redundancy, or both.
| RAID Level | Method | Feature | Min. Disks |
|---|---|---|---|
| RAID 0 | Striping | Fastest speed, no redundancy — if one disk fails, all data lost | 2 |
| RAID 1 | Mirroring | Data duplicated on two disks — full redundancy, 50% usable space | 2 |
| RAID 5 | Striping + Parity | Good balance of speed and redundancy — can survive 1 disk failure | 3 |
RAID 0 = speed only (risky). RAID 1 = safety only (expensive). RAID 5 = best of both.
Volatile vs Non-Volatile Memory
| Feature | Volatile Memory | Non-Volatile Memory |
|---|---|---|
| Data on Power Off | Lost | Retained |
| Speed | Faster | Slower |
| Cost | More expensive per GB | Cheaper per GB |
| Purpose | Temporary working memory | Permanent storage |
| Examples | RAM (SRAM, DRAM), Cache, Registers | ROM, Flash, SSD, HDD, Optical discs, Magnetic tape |
Access Time Comparison
| Memory Type | Approximate Access Time |
|---|---|
| Register | ~1 nanosecond (ns) |
| Cache (L1) | ~1–5 ns |
| RAM | ~50–100 ns |
| SSD (NVMe) | ~0.05–0.1 ms (50,000–100,000 ns) |
| SSD (SATA) | ~0.1 ms |
| HDD | ~5–10 ms (5,000,000–10,000,000 ns) |
HDD is approximately 100,000× slower than cache memory. This is why cache and RAM are essential for performance.
Blu-ray Disc Capacity
| Type | Capacity |
|---|---|
| Single Layer Blu-ray | 25 GB |
| Dual Layer Blu-ray | 50 GB |
| BD-XL (Triple Layer) | 100 GB |
| BD-XL (Quad Layer) | 128 GB |
Blu-ray uses a blue-violet laser (shorter wavelength = more data on same disc area). A single Blu-ray holds 5× more than a DVD.
Key Takeaways
- Memory hierarchy: Registers → Cache → RAM → SSD/HDD → Tape (fastest to slowest)
- RAM is volatile (data lost on power off); ROM is non-volatile (retains data)
- SRAM = cache memory (fast, expensive); DRAM = main memory (slower, cheaper)
- EPROM erased by UV light; EEPROM erased electrically
- HDD = magnetic platters with moving parts; SSD = NAND Flash, no moving parts
- SATA SSD up to 550 MB/s; NVMe SSD up to 3500–7000 MB/s via PCIe
- DDR generations: DDR1 (2.5V) → DDR2 (1.8V) → DDR3 (1.5V) → DDR4 (1.2V) → DDR5 (1.1V) — each doubles data rate, lower voltage, physically incompatible
- RAID 0 = striping (speed, no redundancy); RAID 1 = mirroring (full redundancy); RAID 5 = striping + parity (balanced)
- Volatile memory (RAM, Cache, Registers) loses data on power off; Non-volatile (ROM, Flash, SSD, HDD) retains data
- Access time: Register ~1 ns → Cache ~1–5 ns → RAM ~50–100 ns → SSD ~0.05–0.1 ms → HDD ~5–10 ms
- Blu-ray: single layer 25 GB, dual layer 50 GB; uses blue-violet laser
- 1 Nibble = 4 bits; 1 Byte = 8 bits; 1 KB = 1024 bytes; 1 TB = 1024 GB
- FAT32 max file size = 4 GB; NTFS = no practical limit with permissions
- Magnetic tape = sequential access, cheapest, used for backup
Summary Cheat Sheet
| Concept | Key Details |
|---|---|
| RAM | Volatile, temporary storage for running programs |
| ROM | Non-volatile, stores boot instructions (BIOS) |
| SRAM | Static RAM — used in cache, fast, expensive |
| DRAM | Dynamic RAM — used as main memory, needs refresh |
| PROM | Programmable ROM — write once |
| EPROM | Erasable by UV light |
| EEPROM | Erasable electrically |
| Cache | L1 > L2 > L3; bridges CPU-RAM speed gap |
| HDD | Magnetic platters, moving parts, cheap |
| SSD | NAND Flash, no moving parts, fast |
| Flash Memory | USB drives, SD cards — NAND technology |
| CD / DVD / Blu-ray | 700 MB / 4.7 GB / 25 GB |
| Magnetic Tape | Sequential access, cheapest, backup storage |
| 1 Byte | 8 Bits |
| FAT32 | Max 4 GB file size, no permissions |
| NTFS | No file size limit, permissions, journaling |
| ext4 | Linux file system, permissions, journaling |
| APFS | Apple’s file system for SSD/Flash |
| Cloud Storage | Google Drive (15 GB), OneDrive (5 GB) |
| DDR1→DDR5 | DDR1 (2.5V) → DDR2 (1.8V) → DDR3 (1.5V) → DDR4 (1.2V) → DDR5 (1.1V) |
| SATA SSD | Up to 550 MB/s, 2.5-inch, SATA III interface |
| NVMe SSD | Up to 3500–7000 MB/s, M.2 form factor, PCIe interface |
| RAID 0 | Striping — fastest, no redundancy, min 2 disks |
| RAID 1 | Mirroring — full redundancy, 50% usable space, min 2 disks |
| RAID 5 | Striping + Parity — balanced speed & redundancy, min 3 disks |
| Volatile | RAM, Cache, Registers — data lost on power off |
| Non-volatile | ROM, Flash, SSD, HDD, Optical, Tape — data retained |
| Access Time | Register ~1 ns → Cache ~1–5 ns → RAM ~50–100 ns → HDD ~5–10 ms |
| Blu-ray | Single 25 GB, Dual 50 GB, blue-violet laser |
| 1 Nibble | 4 bits |
| 1 TB | 1024 GB (~1 trillion bytes) |
Knowledge Check
Take a dynamically generated quiz based on the material you just read to test your understanding and get personalized feedback.
Lesson Doubts
Ask questions, get expert answers