Storage Devices and Memory Hierarchy

Study Notes

Storage Devices

Storage devices are commonly used due to their movability, allowing users to transport data to different computers.
Portable storage requires a compatible software to read the data on the device.

Hierarchy of Storage

Primary Memory: Directly connected to the CPU, crucial for computer operation.
Registers and cache: Located inside the CPU chip, storing and providing data and instructions in real time.
Random Access Memory (RAM): Connects directly to the CPU using a memory bus, volatile memory (data lost when power is off). It provides very fast access to data.
Dynamic RAM (DRAM): Simpler technology with more memory space, including some components in the circuit, is common in desktops and laptops.
Types of DRAM:
- Fast Page Mode (FPM DRAM): Used in 386 and 486 computers, outdated technology with low bus frequency (below 66 MHz).
- Extended Data Output (EDO RAM): Replaced FPM DRAM in 1995, higher bus frequency (66 MHz), more speed compared to FPM.
- Burst EDO (BEDO DRAM): Processes four memory addresses per burst, faster than EDO DRAM.
- Synchronous DRAM (SDRAM): Current standard; operates at high clock speed (400 MHz). This technology is synchronized with the CPU and can handle new instructions before finishing the previous one.
  - Single Data Rate (SDR SDRAM): Uses 168-pin DIMMs, operates at 66, 100, and 133 MHz. Slower than later DDR. Can only accept one command and transfer one word of data per clock cycle.
  - Double Data Rate (DDR SDRAM): Uses 184-pin DIMMs, operates at 133, 166, and 200 MHz, and are faster compared to the SDR SDRAM,
Static Random Access Memory (SRAM): More complex circuitry than DRAM, faster access time but more expensive, used in computers requiring speed but less memory.

Secondary Storage

Hard Disks (HDDs): Most common storage on personal computers, non-volatile memory (data is retained when the power is off), stores programs, systems data, and users' data.
- Hard disks store data magnetically on platters.
- Storage capacity ranges from 80 GB to terabytes.
- Higher storage capacity is better for storing more programs and data.
- Rotation speed affects data access speed (5400 to 7200 RPM).
- Wiring connections use IDE or SATA. SATA uses less cable bulk, faster data transfer.
Solid State Drives (SSDs): Faster than HDDs; no moving parts and use NAND flash memory.
- Advantages: Faster boot times, read and write speeds, lower noise, tolerance for wider temperature range, resistant to shocks. More durable.
- Disadvantages: More expensive than HDDs, limited number of write operations from the NAND flash memory.

Tertiary Storage

Tertiary storage is used for large volumes of data. These data are archived, not frequently used. It take longer access time (few seconds). It is used in some institutions and not home computers for storage.
Tertiary storage examples: Optical jukebox, which uses robotic arms to locate optical disks for accessing data.

Off-line Storage

Off-line storage is not directly connected to or controlled by the computer system. It can be disconnected, transported to another location or used on different computer systems.
Examples:
- Floppy disks: Small capacity storage media accessed by a floppy disk drive (generally not commonly used).
- Optical discs: Use laser technology to store data, accessible by Optical Disk Drive (ODD). Example:
  - Compact Discs (CDs): Common optical disc format with a storage capacity of around 700 MB. - Digital Versatile Discs (DVDs): Higher capacity than CDs (4.7 GB).
  - Blu-ray Discs (BD): High-definition video storage with storage capacity greater than DVD.

Other storage details, components

Memory cards: Different types of flash memory cards (Compact Flash, Memory Stick, MultiMedia Card, Secure Digital, MiniSD, MicroSD) with varying sizes and capacities are used in devices such as digital cameras, cell phones, and other electronic devices.
USB Flash Drives: Portable and durable, more reliable storage compared to floppy disks, have high storage capacity and fast data transfer speed, and are affordable.