3.3 Data storage

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Secondary Storage

Secondary storage is non-volatile memory used to store data and programs permanently. Unlike RAM, it does not lose its contents when power is removed.

1. Magnetic Storage (HDD)

Hard Disk Drives (HDDs) use magnetic properties to store data on rapidly rotating disks called platters.

How it works:
  • Data is stored on magnetic platters that spin at high speeds (e.g., 7200 RPM).
  • A read/write head sits on an actuator arm and moves across the surface of the platter.
  • The surface is divided into tracks and sectors.
  • The head changes the magnetic polarity of particles on the surface to represent binary 1s and 0s.

2. Optical Storage

Optical storage uses lasers to read and write data on the surface of a disk.

Mechanism:

  • A high-powered laser burns "pits" into the reflective surface of the disk; the flat areas are called "lands."
  • To read data, a low-powered laser shines on the surface.
  • Lands reflect light back to a sensor (binary 1), while Pits scatter the light (binary 0).

Common Types: CD (700MB), DVD (4.7GB), and Blu-ray (25GB+). Blu-ray has higher capacity because it uses a blue laser with a shorter wavelength, allowing for smaller, more densely packed pits.

3. Solid State Storage (Flash Memory)

Flash memory is electronic, non-volatile storage with no moving parts. It is used in SSDs, USB sticks, and SD cards.

How it works:
  • Uses a grid of floating-gate transistors.
  • Electrical charges are trapped within these gates.
  • The presence of a charge (or lack thereof) determines the bit value.

Static vs. Dynamic Flash Technology

While both are non-volatile, they differ in architecture and how they access data:

Feature Static Flash (NOR) Dynamic Flash (NAND)
Data Access Allows "Random Access" (can read individual bytes). Accesses data in "Blocks" or pages.
Read Speed Very fast reading. Fast reading, but slower than NOR for small data.
Write/Erase Speed Slower writing and erasing. Much faster at writing and erasing large amounts of data.
Storage Density Low (Expensive per GB). High (Cheaper per GB).
Typical Use Storing Firmware or BIOS (where code needs to be executed directly). Mainstream storage (SSDs, USB Drives, Smartphones).