Computer Memory | Lucent For GK - UPSC PDF Download

Introduction

Computer memory is an essential subsystem of a computer that stores data and instructions required for processing. It supports operations ranging from the immediate short-term storage needed by the processor to long-term archival storage. Memory devices can be volatile or non‐volatile and are chosen to balance access speed, cost, capacity and permanence. Proper organisation of memory and an understanding of its characteristics are fundamental for students of civil, computer and electrical engineering because memory design and usage affect system performance, real‐time control, data acquisition and large‐scale computation.

Memory Hierarchy

Modern computer systems use a memory hierarchy to combine components of different speeds and capacities so that the processor experiences an effective storage system that is both fast and large. The hierarchy arranges storage from the fastest and smallest at the top (registers and cache) to the slowest and largest at the bottom (tape, cloud storage). Data and instructions are kept nearer to the CPU when they are needed frequently, while less frequently used data are stored in larger, slower media. The goals of the hierarchy are to minimise average access time and to make cost per bit practical for system requirements.

Parameters of Memory

• Storage capacity: The total amount of information a memory can hold, measured in bits, bytes, words or higher units (KB, MB, GB, TB).
• Access modes: The way data locations are accessed - random access (any location in roughly the same time), sequential access (in order, e.g., tape), or direct (by a computed address or pointer).
• Access time (latency): The delay between a request (read/write) and when the data become available at the processor or I/O interface.
• Throughput (data transfer rate): The rate at which data can be moved to or from the memory, typically measured in bytes per second or multiples thereof.
• Physical characteristics: Whether the device uses electronic, magnetic, optical or mechanical principles; these affect speed, durability and cost.
• Permanence of storage: Whether data are retained without power (non‐volatile) or lost when power is removed (volatile).

Types of Memory - Overview

Memory in a computer system is typically classified into two broad categories:

• Primary memory (also called main memory or internal memory) - directly accessible by the CPU and used for active computation.
• Secondary memory (also called auxiliary memory) - used for long‐term storage, not directly accessed by the CPU for execution; data must be transferred to primary memory first.

Primary Memory

Main memory is the working storage a processor uses to hold running programs, active data and intermediate results. Primary memory is typically volatile, meaning its content is lost when power is removed; this makes backup and non‐volatile secondary storage important for persistence.

• Cache memory: A small, very fast memory placed close to the CPU that holds frequently used instructions and data to reduce average access time.
• RAM (Random Access Memory): A larger volatile store used to hold programs and data while they execute.

Random Access Memory (RAM)

RAM allows both reading and writing of data in any order (random access). It holds program code, input data and temporary results during execution. RAM technologies differ in internal structure, refresh requirements and speed:

• Dynamic RAM (DRAM): Each memory cell is typically one transistor plus one capacitor. Capacitors leak charge and therefore DRAM requires periodic refresh cycles to retain information. DRAM is dense (high capacity per chip), relatively inexpensive and used for main system memory.
• Static RAM (SRAM): Uses multi‐transistor circuits per bit and does not require refresh as long as power is supplied. SRAM is faster and more expensive than DRAM and is commonly used for CPU caches.

Cache Memory

• Cache is a small, fast buffer that stores copies of data from frequently used main memory locations so that the CPU can access them more quickly.
• It sits between the CPU and RAM and is organised in levels (L1, L2, L3) where L1 is the smallest and fastest and typically integrated on the processor die.
• Cache uses specialised organisation (lines, tags, sets) and replacement policies (for example, LRU - least recently used) to decide which data to keep.
• Cache sizes are typically measured in kilobytes (KB) to a few megabytes (MB), with sizes such as 256 KB for small caches up to several megabytes for modern multi‐level caches.

Read‐Only Memory (ROM)

ROM is a class of non‐volatile memories that retain their contents without power. ROM typically stores firmware such as boot code or constant data that should not change during normal operation. Different ROM types provide varying degrees of programmability and reusability.

• Programmable ROM (PROM): Manufactured blank and programmed once by burning fuses; once programmed its contents are permanent.
• Erasable Programmable ROM (EPROM): Can be erased by exposing the chip to strong ultraviolet light and then reprogrammed; used where occasional reprogramming is required.
• Electrically Erasable Programmable ROM (EEPROM): Can be erased and reprogrammed electrically at the byte or block level; commonly used for BIOS and small non‐volatile storage.

Tit‐Bits (Key Short Notes)

• Flash memory is a semiconductor‐based, non‐volatile, rewritable memory used in devices such as digital cameras, mobile phones, USB flash drives and SSDs.
• Virtual memory is an OS technique that allows programs to run as if they have more main memory than physically available by swapping pages between RAM and secondary storage; this permits execution of programs larger than physical memory.
• Buffer is a temporary storage area used to compensate for differences in rate or timing between producers and consumers of data, for example between I/O devices and the CPU.

Secondary Memory

Secondary memory devices provide large, persistent storage for data and programs that are not currently in use by the CPU. Secondary memory is typically non‐volatile and must be read into primary memory (RAM) for the CPU to execute files held there.

• Secondary memory is used for backups, archival storage, program installation, and holding large data sets that do not need to reside in RAM continuously.
• It offers much larger capacity at a lower cost per bit but with higher access latency compared with primary memory.

Types of Secondary Memory Devices

Magnetic storage

• Hard Disk Drive (HDD)
• Floppy Disk
• Magnetic Tape

Optical storage

• Compact Disc (CD)
• Digital Versatile Disk / DVD
• Blu‐ray Disk

Solid state storage

• USB pen/flash drive
• Memory card (SD, microSD, etc.)
• Solid State Drive (SSD) - uses flash memory as a faster alternative to HDDs for block storage.

Hard Disk Drive (HDD)

• HDD is a non‐volatile random‐access storage device that stores data magnetically on rapidly rotating platters.
• Data is read and written by read/write heads that move across the platters on an actuator arm; multiple platters and heads are mounted on a spindle.
• Storage is organised in concentric tracks and radial sectors. A set of tracks at the same radius across all platters forms a cylinder.
• HDDs provide high capacity at modest cost but have mechanical latency due to rotational delay and head seek time.

Floppy Disk (Diskette)

• Floppy disks are removable magnetic storage media with limited capacity and slow access compared with modern drives.
• The medium is a thin plastic disk coated with magnetic material (e.g., iron oxide) and enclosed in a protective envelope with a read/write slot.
• Floppies were commonly available in 8‐inch, 51⁄4‐inch, and 31⁄2‐inch formats and are largely obsolete for modern computing.

Magnetic Tape

• Magnetic tape uses plastic film coated with magnetic material to store data sequentially and is often used for high‐capacity backups and archival storage.
• Data are represented by magnetised and demagnetised regions along the tape; access is typically sequential which makes random reads slower than for disks.
• Magnetic tape formats vary in width and length and can offer very large capacities at low cost per bit and good longevity when stored properly.

Compact Disc (CD)

• CDs are optical discs widely used for storage of audio, software and data; they store data in physical pits on the disc surface that are read by a laser.
• Files are stored in contiguous sectors and CDs are inexpensive for moderate amounts of data.

CD types:

• CD‐ROM (Read‐Only): Data written during manufacturing; cannot be modified by users.
• CD‐R (Recordable): Can be written once by the user; data cannot be erased.
• CD‐RW (Rewritable): Can be written, erased and rewritten multiple times.

Digital Video Disk (DVD)

• DVD (Digital Versatile Disc or Digital Video Disk) is an optical disc format introduced by industry consortia including Philips, Sony, Toshiba and Panasonic in 1995.
• DVDs have a higher storage capacity than CDs while keeping the same physical disc size. Typical capacities range from 4.7 GB (single layer, single sided) to several gigabytes for dual layer or double‐sided variants, up to about 17.08 GB for special multi‐layer formats.
• DVDs are commonly used for video, software distribution and data backup; some DVD types are writable or rewritable while others are read‐only.

Blu‐ray Disc

• Blu‐ray discs use a blue/violet semiconductor laser with a shorter wavelength than the red laser used in DVDs, allowing higher data density and larger capacity per layer (about 25 GB per layer).
• Blu‐ray supports multiple formats: BD‐ROM (read‐only), BD‐R (recordable), BD‐RE and BD‐RW (rewritable).

Pen / Thumb Drive (USB Flash Drive)

• A pen drive or USB flash drive is a portable solid‐state storage device using flash memory with a USB interface for easy connection to computers and embedded systems.
• They are small, removable and rewritable with storage capacities spanning a wide range (for example, from hundreds of megabytes to multiple terabytes in modern products).
• USB flash drives are commonly used for transferring files, quick backups and bootable media; they have no moving parts and are therefore more robust than magnetic disks for many portable uses.

Memory Cards

• Memory cards (such as SD, microSD) are compact, re‐recordable solid‐state storage media used in devices like digital cameras, smartphones and laptops.
• They offer portability, light weight, reusability and a wide range of capacities and performance classes suitable for different applications.

Secondary Memory Devices: Storage Method and Capacity

Basic Units of Memory Measurement

• Bit - binary digit (0 or 1), the smallest unit of information.
• Byte - typically 8 bits; used as the basic addressable unit in many architectures.
• Multiples: KB (kilobyte ≈ 1024 B), MB (megabyte ≈ 1024 KB), GB, TB, and higher units used to quantify storage capacity.
• Memory word - a fixed number of bits processed as a unit by a processor (for example, 16‐bit, 32‐bit, 64‐bit word sizes).

Cloud Computing and Storage

Cloud computing delivers computing services - including storage, processing and platforms - over the internet. Cloud storage provides scalable, remote, persistent storage that is managed by cloud providers and accessed over networks. For engineers, cloud storage offers advantages for large data sets, collaborative work, backups and scalable compute for simulations and data analytics.

• Public cloud: Services offered by third‐party providers to multiple customers (for example, storage, virtual machines).
• Private cloud: Cloud infrastructure operated solely for a single organisation, providing greater control over data and security.
• Hybrid cloud: A combination of public and private clouds allowing workloads to move between environments as needed.
• Service models: IaaS (Infrastructure as a Service), PaaS (Platform as a Service) and SaaS (Software as a Service).

Next‐Generation and Emerging Memory Technologies

• FeFET / FeRAM: Ferroelectric memories that aim to provide non‐volatile, fast access with low energy consumption; research and early products exist.
• Nano‐tube RAM: Research into carbon nanotube or similar nanoscale structures is ongoing with the goal of high density and speed.
• Phase Change Memory (PCM): Uses material phase changes (amorphous/crystalline) to store data; versions have reached commercial markets.
• Resistive RAM (ReRAM): Non‐volatile memory with potential for high speed and endurance; positioned for AI and embedded applications.
• Spin‐Orbit Torque MRAM (SOT‐MRAM): A type of MRAM designed to replace SRAM for certain applications due to non‐volatility and fast speeds.

Tit‐Bits (Summary Notes)

• Data transfer rate indicates how fast data can be read from or written to a storage device; it is an important performance metric for disks and networks.
• Root directory is the top‐level directory on a file system; it contains entries and metadata describing files and folders stored on the disk.