Memory and Storage

Primary Storage

• The computer’s immediate, high-speed memory that the CPU can access directly.

• It temporarily stores data, instructions, and programs that are currently in use, allowing the processor to access them quickly during execution.

• The main types of primary storage are ROM, RAM, flash and cache memory.

• It’s crucial for system performance, as it determines how quickly data and instructions can be retrieved and processed.

• It’s limited in size compared to Secondary Storage.

Read Only Memory (ROM)

• Non-volatile primary storage that permanently stores essential system instructions needed for the computer to start and operate.

• The data in ROM is written during manufacture and cannot be easily modified or erased.

• It contains critical programs such as the firmware or BIOS, which starts up hardware components and loads the operating system when the computer is powered on.

Random Access Memory (RAM)

• Volatile primary storage used to temporarily store data, instructions, and programs that are currently being used by the CPU.

• It allows for fast read and write access, enabling the processor to quickly retrieve and manipulate information during operation.

• The more RAM a system has, the more data and programs it can handle simultaneously, improving overall system performance and multitasking.

Cache Memory

• Small, high-speed primary storage located close to or within the CPU.

• It temporarily stores frequently used data and instructions, reducing the time the CPU spends accessing data from slower main memory, helping to minimise delays and increase processing efficiency.

• Cache is organised into levels (L1, L2, and L3), with L1 being the smallest and fastest, and L3 being larger but slower.

• By keeping recently or repeatedly used data close to the processor, cache memory significantly improves system performance and reduces bottlenecks.

Flash Memory (Primary and Secondary)

• Non-volatile storage that stores data using electrically erasable and programmable memory cells, allowing it to be rewritten many times.

• Flash memory is faster and more durable than traditional magnetic storage because it has no moving parts, making it ideal for use in SSDs, USB drives, memory cards, and embedded systems.

• It offers quick access times, low power consumption, and high reliability, though it has a limited number of write cycles compared to RAM.

Examples of Flash Memory

• Gaming Consoles

• Digital Cameras

• Memory Cards

• Embedded Systems

• USB flash drives

Secondary Storage

• Non-volatile storage devices used to store data and programs permanently, providing long-term storage for the operating system, applications, and user files, unlike primary storage, which is temporary.

• Secondary storage typically has a much larger capacity than primary memory but is slower to access.

• It plays a crucial role in data retention and backup, allowing users to store and retrieve large volumes of information as needed.

Optical Storage

• Secondary storage that uses lasers to read and write data on the surface of a disc which is stored as microscopic pits and lands that represent binary information, which are read by a laser beam.

• Comes in three forms; read-only, write-once and rewritable.

• Optical storage is portable, durable, and cheap, making it useful for media distribution, backups, and archiving. However, it has slower access speeds and smaller capacity compared to magnetic or flash storage.

• It is gradually being replaced by cloud and solid-state storage, but remains useful for long-term data preservation.

Examples of Optical Storage

• DVDs

• Blu-ray Disk

• CD drives

Magnetic Storage

• Secondary storage that uses magnetised patterns on a disk or tape to represent binary data which is read and written using a magnetic read/write head that detects or alters the magnetic fields on the surface.

• Magnetic storage offers large capacity and is cost-effective for storing vast amounts of data, making it popular for backups, servers, and data centres. However, it has moving parts, which make it slower and less durable than solid-state or flash storage.

Examples of Magnetic Storage

• Hard Disk Drives

• Floppy Disks

• Credit Card Stripes

• Magnetic Tapes

Solid State Storage

• Secondary storage that uses flash memory chips instead of moving parts to store data electronically making it non-volatile, providing much faster read and write speeds than magnetic or optical storage.

• No moving parts makes solid-state storage more durable, energy-efficient, and silent, making it ideal for laptops, mobile devices, and high-performance systems.

• It is generally more expensive per gigabyte and has a limited number of write cycles compared to traditional hard drives.

Examples of Solid State Storage

• Solid State Drives

• USB flash drives

• Memory Cards

• Smart Cards

Paging

• Paging is a memory management method that splits logical memory into fixed-size pages and physical memory into frames of the same size, so a program can be stored in the empty spaces of RAM

• A page table tracks where each page is stored, and a TLB (Translation Look-aside Buffer) speeds this up by caching recent translations, helping avoid fragmentation but causing slower performance if many page faults occur.

Thrashing

• Thrashing happens when a paged system gets too many page faults, so the OS spends most of its time swapping pages instead of running the program

• It occurs when processes need more frames than the system has available, causing a big drop in performance.

TLB (Translation Look-aside Buffer)

• TLB is a small, high-speed, associative cache inside the CPU that stores recently used virtual-to-physical address translations from the page table, reducing memory access time

• When a needed entry is found in the TLB address translation is fast, which is known as a TLB hit, but if it is not the system must access the slower page table in memory, increasing the effective access time which is known as a TLB miss.

Tertiary Storage

• Non-volatile, High-capacity, removable storage systems used mainly for backups, archiving, and long-term data retention.

• It typically involves automated libraries or robotic systems that can load and unload optical discs or magnetic tapes as needed.

• It has slower access times compared to primary and secondary storage.

• Commonly used by large organisations, data centres, and cloud providers for disaster recovery and historical data storage, where speed is less important than reliability and cost efficiency.

Cloud Storage

• Tertiary storage where data is stored on remote servers accessed via the internet, allowing users and organisations to store, back up, and share data without relying on local physical devices, reducing the need for on-site infrastructure and allowing data to be accessed from any location

• Cloud storage is scalable, flexible, and often replicated across multiple data centres, which improves reliability and fault tolerance.

• It is widely used for file hosting, backups, collaborative work, and disaster recovery.

Lossy Compression

• Lossy compression is a data compression method that reduces file size by permanently removing some information, usually details that are less noticeable to humans, such as in images, audio, or video

• It achieves higher compression than lossless methods but cannot perfectly reconstruct the original file, so some quality is lost.

Lossless Compression

• Lossless compression is a data compression method that reduces file size without losing any information, allowing the original file to be perfectly reconstructed

• It is commonly used for text, software, or data files where accuracy is essential, but it usually achieves smaller compression ratios than lossy methods.

Storage Size

• Burger King Makes Great Toilet Paper!

• Bit < Nibble < Byte < Kilobyte < Megabyte < Gigabyte < Terabyte < Petabyte

• A byte stores one character

Storage Characteristics

• Capacity

• Speed

• Portability

• Durability

• Reliability

• Cost

Optical Storage Characteristics

• Third in Capacity

• Third in Speed

• First in Portability

• Second in Durability

• Second in Reliability

• First in Cost

Magnetic Storage Characteristics

• First in Capacity

• Second in Speed

• Second in Portability

• Third in Durability

• First in Reliability

• Second in Cost

Solid State Characteristics

• Second in Capacity

• First in Speed

• Third in Portability

• First in Durability

• Third in Reliability

• Third in Cost