Memory and storage Revision Notes

Types of Secondary Storage

Secondary storage devices are crucial for persistent data storage. They are non-volatile, meaning they retain data even when power is off. Key types include Hard Disk Drives (HDDs), Solid State Drives (SSDs), Optical Discs (CDs, DVDs, Blu-ray), and Magnetic Tapes. Each type offers different trade-offs in terms of speed, capacity, cost, and durability, making them suitable for specific applications. For example, SSDs are fast but more expensive per GB, while HDDs offer greater capacity at a lower cost.

Hard Disk Drives (HDDs)

HDDs are traditional storage devices consisting of spinning platters coated with magnetic material and read/write heads. They offer large storage capacities (terabytes) at a relatively low cost. However, they are mechanical, making them susceptible to physical shock, and slower than SSDs due to moving parts. They are often used for general-purpose storage, archival data, and powering desktop computers where speed is not the absolute priority.

Solid State Drives (SSDs)

SSDs use flash memory to store data, similar to USB sticks, but with much higher performance. They have no moving parts, which makes them faster, more durable, and resistant to physical shock compared to HDDs. They also consume less power and generate less heat. Although their cost per gigabyte is higher than HDDs, their speed makes them ideal for operating system installations, frequently accessed applications, and laptops where durability and battery life are important.

Optical & Magnetic Tape Storage

Optical storage, such as CDs, DVDs, and Blu-ray discs, uses lasers to read and write data onto a spinning disc. They are relatively inexpensive, portable, and good for distributing software or media. However, they have lower capacities than HDDs/SSDs and slower access times. Magnetic tape storage involves data being written to a long strip of plastic coated with magnetic material. Tapes offer very high capacity at a very low cost per gigabyte, making them ideal for long-term archival storage and large-scale backups, despite their very slow sequential access speed.

Understanding Primary Storage

Primary storage is the main memory of a computer, directly accessible by the CPU. It holds data and instructions that the CPU actively uses. It is characterised by its high speed and volatile nature, meaning data is lost when the power is off. The two key components are RAM and ROM, each serving distinct purposes essential for system operation. This is distinct from secondary storage, which is slower, non-volatile, and used for long-term data retention.

Random Access Memory (RAM)

RAM serves as the computer's temporary workspace. When you open applications, load operating system components, or work with files, their data is moved from slower secondary storage into RAM. This allows the CPU to access this information rapidly, significantly improving performance. The speed and capacity of RAM directly impact the computer's ability to multitask and handle complex operations efficiently. It is volatile; all contents are lost upon power loss.

Read-Only Memory (ROM)

ROM is a type of non-volatile primary storage. It stores critical, unchanging instructions required for a computer to start up, such as the BIOS (Basic Input/Output System) or UEFI firmware. These instructions tell the computer how to initialise its hardware components before the operating system loads. Unlike RAM, data stored in ROM persists even when the computer is turned off and cannot be easily changed or overwritten by the user.

Registers and Cache Memory

Registers are tiny, extremely fast storage locations built directly into the CPU. They temporarily hold data, instructions, and memory addresses that the CPU is currently processing. Cache memory is another type of high-speed memory, smaller and faster than RAM, located between the CPU and main memory. It stores copies of frequently used data from RAM, reducing the time the CPU spends waiting for data and thus improving overall processing speed and efficiency. Cache is typically organised into multiple levels (e.g., L1, L2, L3).

Basic Units: Bits and Bytes

The fundamental unit of data storage is a bit, which can be either 0 or 1. A bit is a binary digit. For practical purposes, bits are grouped into bytes. A byte consists of exactly 8 bits. This grouping allows for the representation of a wider range of values, such as characters or small numbers. Understanding the relationship between bits and bytes is essential for converting between smaller and larger data units in computer systems.

Larger Units and Their Numerical Equivalents

Beyond bytes, data storage is measured in increasingly larger units, which are powers of 2. A Kilobyte (KB) is 1024 bytes (2^10 bytes). A Megabyte (MB) is 1024 KB (2^10 KB). A Gigabyte (GB) is 1024 MB (2^10 MB). A Terabyte (TB) is 1024 GB (2^10 GB). It's important to remember these specific numerical relationships when calculating storage requirements or converting between units, as they are not simply multiples of 1000.

Calculations and Conversions

To convert from a smaller unit to a larger unit, you divide by 1024. For example, to convert bytes to KB, divide the number of bytes by 1024. To convert from a larger unit to a smaller unit, you multiply by 1024. For example, to convert GB to MB, multiply the number of GB by 1024. This conversion process is important for tasks like determining how many photos can fit on a memory card or how long it will take to download a file.

Types of Data Storage

Data storage can be broadly categorised into primary, secondary, and tertiary storage. Primary storage, like RAM and cache, is directly accessible by the CPU and is volatile, meaning data is lost when power is off. It offers the fastest access speeds, essential for active programs and data. Secondary storage, such as Hard Disk Drives (HDDs) and Solid State Drives (SSDs), is non-volatile and used for long-term storage of operating systems, applications, and user files. Tertiary storage, often involving robotic systems managing large archives of optical disks or tapes, is typically used for backups or very large datasets with infrequent access requirements.

Characteristics of Storage Devices

Key characteristics of storage devices include their capacity (how much data they can hold), access speed (how quickly data can be read from or written to), cost per unit of storage, and portability. HDDs offer large capacities at a lower cost per gigabyte but are slower due to moving parts. SSDs are significantly faster, more durable, and consume less power, but typically have a higher cost per gigabyte and lower maximum capacities compared to HDDs. Optical drives (CDs, DVDs, Blu-rays) offer portable, non-volatile storage with moderate capacities, while magnetic tapes are used for very large archival storage due to their low cost per gigabyte and long lifespan, despite slow sequential access.

Storage Mediums and Their Applications

Different storage mediums are suited for specific applications. RAM is critical for running applications and operating system processes due to its speed. SSDs are ideal for operating system drives and applications requiring fast load times, contributing to overall system responsiveness. HDDs are often used for bulk data storage, such as media files or less frequently accessed documents, where cost-effectiveness and capacity are priorities. USB flash drives and SD cards provide portable storage for convenient data transfer. Cloud storage leverages large data centres, abstracting the physical storage infrastructure from the user and offering scalable, accessible storage over a network.

Measuring Storage Performance

Storage performance is measured by metrics such as read/write speed (data transfer rate, often in MB/s or GB/s), latency (the delay before data transfer begins, measured in milliseconds), and Input/Output Operations Per Second (IOPS), which indicates how many data requests a device can handle per second. SSDs typically have much lower latency and higher IOPS than HDDs, resulting in faster overall system performance, especially for tasks involving many small file accesses. Understanding these metrics helps in evaluating and selecting appropriate storage solutions for different computing needs.