Computer Memory and Backing Storage

Main Memory

• Comprises two primary technologies used directly by the CPU during processing
  • RAM (Random-Access Memory)
  • Purpose: holds data and program instructions currently in use while the computer is powered on
  • Size: larger than ROM, measured today in $\text{GB}$ or $\text{TB}$ for high-end servers
  • Volatility: volatile – loses all contents when power is removed
  • Access: read & write; any location can be reached in roughly the same time (random access)
  • ROM (Read-Only Memory)
  • Purpose: stores the firmware needed to start up (boot) the computer
    • Contains the BIOS – Basic Input/Output System (transcript erroneously said “Screen”)
    • Performs Power-On Self-Test (POST): hardware check to verify the presence and functionality of all major components
  • Size: typically smaller than RAM (measured in $\text{MB}$)
  • Volatility: non-volatile – contents persist without power
  • Access: traditionally read-only (modern chips often allow re-flashing but at a much lower write frequency)

Memory vs. Backing Storage

• Memory (RAM + ROM)

  • Stores data currently needed or essential to startup
  • Limited capacity compared to secondary storage
  • High speed; located on or near the motherboard
  • Drawbacks: RAM is volatile; ROM is read-only & small

• Backing Storage (Secondary / Auxiliary Storage)

  • Provides permanent or semi-permanent retention of data
  • Much larger capacities; can range from $\text{GBs}$ to multi-$\text{PB}$ enterprise arrays
  • Slower access speeds compared with main memory
  • Implemented through Magnetic, Optical, Solid-State, or Cloud technologies

Backup (Concept)

• Definition: an extra copy of important files kept separately from the original
• Protects against
  • Malware (virus, ransomware) or malicious deletion by hackers
  • Accidental deletion or user error
  • Hardware failure or file-system corruption of the primary storage device
• Best practice: keep backups off-site or use cloud to reduce single-point-of-failure risk

Overview of Backing-Storage Technologies

• Magnetic: store data by magnetising spots on a surface (e.g.
  fixed/portable hard disks, magnetic tape)
• Optical: use light to read pits/lands burned into discs (CD, DVD, Blu-ray)
• Solid State: rely on non-volatile flash memory (SSD, memory sticks, memory cards)
• Cloud: remote servers accessed through the Internet providing elastic capacity

Magnetic Backing Storage

Typical Devices & Uses

• Fixed Hard Disk (HDD)
  • Permanently installed inside desktops/laptops/servers
  • Primary storage for operating system, applications, user files
• Portable / External Hard Disk
  • USB or Thunderbolt enclosure for data transfer between computers or on-site backups
• Magnetic Tape
  • Cartridge-based serial medium, common in enterprise archival and large-scale backup

Advantages

• HDD (internal)
  • Very fast access speed for magnetic technology (ms seek times)
  • Large capacity (multi-TB common)
• Portable HDD
  • Keeps advantages of HDD while being portable
  • Still provides large capacity
• Magnetic Tape
  • Very large capacity per cartridge (multiple $\text{TB}$)
  • Very low cost per $\text{GB}$ (cheapest archival medium)

Disadvantages

• HDD (internal)
  • Not portable once installed
  • Susceptible to data loss from strong magnetic fields & mechanical shock
• Portable HDD
  • More expensive than internal equivalents (housing + interface electronics)
  • Still vulnerable to magnetic fields & drops
• Magnetic Tape
  • Slow access (serial access – must wind tape to location)
  • Requires separate tape drive/reader
  • Also affected by magnetic fields & can be noisy during operation

Optical Backing Storage

• Common consumer formats (implicit in transcript though not spelled out): CD-R/RW, DVD±R/RW, Blu-ray
• General characteristics (from comparison table later)
  • Portable, inexpensive media
  • Cheaper per GB than solid state but slower than magnetic & solid state
  • Medium robustness; discs prone to scratching/warping
• Specific examples were not listed in the transcript aside from heading – the notes focus on technology class

Solid-State Backing Storage

Portable Solid-State Devices

• Memory Stick / Pen Drive (USB flash drive)
  • Capacities now exceed $1\,\text{TB}$; originally described as “many GB”
  • Ideal for quick file transfer between computers
• Flash Memory Cards (used in cameras, phones, audio players)
  • XD Card (Extreme Digital) – older, small form factor for certain digital cameras
  • SD Card (Secure Digital) – very small, high-capacity; variants: SDHC, SDXC, SDUC
  • CFast Card (CompactFast) – high-performance removable storage for professional photo/video equipment

Solid-State Drive (SSD) vs. Hard Disk Drive (HDD) – Detailed Comparison

Comparative Summary of Secondary-Storage Classes

Cloud Backing Storage

• Definition: a remote, Internet-based storage platform comprising large groups of inter-linked data servers managed by a provider
• Requires Internet connectivity to upload, download, or synchronise data

Transferring Files Between Computers via Cloud

1. Create a cloud-storage account (e.g.
   Google Drive, OneDrive, Dropbox)
2. Log in at the source computer (home, office, etc.)
3. Upload the desired data to the cloud servers – many services can automatically back up selected folders
4. At destination computer, log in to the same account
5. Download (or synchronise) the data to local storage
6. User typically pays a monthly fee proportional to storage used; infrastructure maintenance is handled by the provider

Using Cloud to Share / Sync a Document Among Team Members

1. Owner uploads the document to a shared folder in the cloud
2. Owner grants access (permissions/links) to team members
3. Team members log in to the cloud service
4. Members open the document (often in collaborative web editor)
5. Members edit / update; changes are saved in real time
6. File is synchronised across all devices/accounts
7. Activity can be tracked by the owner (version history, audit logs)
8. Service keeps several distributed copies for redundancy and disaster recovery

Advantages

✓ Accessible anywhere with an Internet connection

✓ Multiple users can share data simultaneously

✓ Supports multi-device access (PC, tablet, phone)

✓ Automatic backups handled by provider reduce management overhead

✓ Scalable capacity – increase quota on demand

Disadvantages

 Greater risk of hacking because servers are online 24/7

 User loses some control over physical location and handling of data

 Requires Internet; no connectivity ⇒ no access

 Difficult to guarantee complete deletion; copies may reside on numerous servers/backups

 Long-term subscription costs may exceed the one-off price of local hardware

Ethical, Practical & Real-World Implications (Implicit)

• Data sovereignty: cloud storage location may be in a different legal jurisdiction affecting privacy laws
• Environmental impact: SSDs lower power use; cloud providers consolidate but consume large energy in data centres
• Business continuity: backups (local + cloud) essential for disaster recovery and compliance
• Cost-benefit trade-offs: choose storage type according to required speed, capacity, mobility, robustness, and budget