💾 Storage, Memory & File Systems

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.

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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

Two older exam-capsule terms also fit here. Semi-volatile memory means memory that behaves like volatile memory for speed, but can retain data briefly or under limited conditions after power is removed. Protected memory means the operating system gives each program its own memory area and prevents one program from freely interfering with another program's memory space, improving reliability and security.

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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)

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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

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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

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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

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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
HVD	Holographic Versatile Disc	A high-capacity optical format often described around 3-5 TB	Holographic storage concept

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)

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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

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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

The core size relationship is straightforward: 1 Byte = 8 bits, and storage units grow step by step as B, KB, MB, GB, TB, PB, and EB, with each level being 1024 times the previous one.

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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

The most practical distinction here is that FAT32 cannot store a single file larger than 4 GB, while NTFS supports features such as permissions and encryption.

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Cloud Storage

Cloud storage stores data on remote servers accessed via the internet, instead of local hard drives.

Service	Provider	Free Storage
Google Drive	Google	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

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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

It is useful to connect the units clearly: 1 nibble = 4 bits, 1 byte = 8 bits = 2 nibbles, and 1 terabyte = 1024 GB, which is roughly one trillion bytes in decimal terms.

Another useful term is octet, which is simply another name for an 8-bit byte, especially in networking and standards language.

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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.

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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

NVMe drives connect through PCIe and are much faster than SATA SSDs, although both store data using NAND Flash memory.

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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.

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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

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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.

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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.

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Summary Points

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)