Computer Memory

Volatile Memory:

• Volatile memory loses its stored data when the power supply is interrupted or turned off.
• Examples : 
  • Random Access Memory (RAM)
  • Cache 
  • Registers

Non-Volatile Memory:

• • Non-volatile memory  retains its stored data even when the power supply is removed. It is persistent and does not require continuous power to keep data intact.

• Examples:

• Read-Only Memory (ROM): 

• Flash Memory: USB drives, SSDs (Solid State Drives), memory cards, 

• • Hard Disk Drives (HDDs): 

Associative memory :

• Associative memory is a type of computer memory or data storage mechanism that is used to locate and retrieve data based on its content rather than its physical location or address. 
  • Note: RAM, rely on a specific memory address to access data, While associative memory uses content-addressable memory (CAM) or content-based addressing, allowing it to search for and retrieve data based on the data itself. 
• This is also sometimes referred to as “associative storage” or “content-addressed memory.”

Primary memory : 

• • Primary memory, also known as main memory or primary storage, is a crucial component of a computer’s memory hierarchy. 
  • It serves as the immediate workspace for the CPU (Central Processing Unit) and holds data and instructions that are actively being used or processed by the computer. 
  • Primary memory is volatile, meaning it loses its data when the computer is powered off or restarted. 
  • There are two main types of primary memory:

• Random Access Memory (RAM)

• • Read-Only Memory (ROM)

Random Access Memory (RAM):

• RAM is used to store data and program instructions that the CPU needs to access quickly during the execution of tasks and applications.
• RAM is volatile, meaning that its contents are erased when the computer is powered off or restarted. 
• This characteristic makes it suitable for temporary data storage.
• RAM comes in various forms, including DDR4 and DDR5.
• The speed of RAM is an essential factor in computer performance, and it is measured in terms of data transfer rates, such as megabytes per second (MB/s) or gigabytes per second (GB/s).
•  Types of RAM:
  • Static RAM (SRAM)
  • Dynamic RAM (DRAM)
  • Synchronous Dynamic RAM (SDRAM)
  • DDR SDRAM (Double Data Rate Synchronous Dynamic RAM)
  • Rambus DRAM (RDRAM)
  • LPDDR (Low Power DDR)
  • DDR4 and DDR5 (Double Data Rate 4 and 5)

Static RAM (SRAM):

• Description: SRAM is a type of RAM that stores data using flip-flop circuits. It is faster and more stable than dynamic RAM (DRAM) because it doesn’t require constant refreshing.
• Characteristics:
• Faster access times and lower latency.
• Used for CPU cache and high-performance applications.
• More expensive and consumes more power compared to DRAM.
• Commonly found in CPU cache, and some high-speed registers.

Dynamic RAM (DRAM):

• It stores data as electrical charges in capacitors and requires constant refreshing to maintain data integrity.
• Slower access times and higher latency compared to SRAM.
• Higher capacity and lower cost per bit compared to SRAM.

Read-Only Memory (ROM):

• ROM is non-volatile, meaning its contents are retained even when the computer is powered off.
• ROM contains firmware or software instructions that are permanently programmed into the computer’s hardware. These instructions include the BIOS (Basic Input/Output System) and other essential system-level instructions.
• BIOS stands for “Basic Input/Output System.” 
  • It is the software interface between the computer’s hardware and the operating system (OS). 
  • BIOS plays a critical role in the computer’s boot process and hardware initialization. 
  • When you power on a computer, the BIOS firmware is the first code to execute. 
  • Its primary task is to initialize and test the computer’s hardware components, including the CPU, RAM, storage devices, and peripheral devices like the keyboard, mouse, and display.
• Unlike RAM, users cannot modify the contents of ROM. 
• It is designed to be read-only.
• Types of ROM : 
  • Mask ROM (permanently programmed during manufacturing), 
  • PROM (Programmable ROM) 
  • EPROM (Erasable PROM) 
  • EEPROM (Electrically Erasable PROM)

Mask ROM (MROM):

• Mask ROM is a type of non-volatile memory that is permanently programmed during the manufacturing process. 
• It gets its name because the data is “masked” onto the ROM at the chip fabrication stage, and it cannot be changed or erased by the end-user.

PROM (Programmable Read-Only Memory):

• PROM is a type of non-volatile memory that allows users or manufacturers to program data into the memory after the chip is manufactured. 
• It is “one-time programmable,” meaning the data can be written to the PROM, but once programmed, it cannot be erased or changed.

EPROM (Erasable Programmable Read-Only Memory):

• EPROM is a type of non-volatile memory that can be programmed by the user or manufacturer and subsequently erased and reprogrammed. 
• Erasure typically involves exposure to ultraviolet (UV) light, which clears the data from the memory, making it ready for reprogramming.

EEPROM (Electrically Erasable Programmable Read-Only Memory):

• EEPROM is a type of non-volatile memory that can be electrically erased and reprogrammed, making it more convenient and flexible than UV-erase EPROM. 
• EEPROM allows individual bytes or small sections of memory to be erased and rewritten, without needing to erase the entire memory.

Secondary memory : 

• Secondary memory, also known as secondary storage or external storage, refers to any type of storage in a computer or data storage system that retains data persistently, even when the power is turned off. 
• Secondary memory is Non-volatile stores data for the long term. 
• It serves as a means to store programs, files, and data that can be accessed at a later time. 

Types of Secondary memory :

• Magnetic storage

• Magnetic tape

• Hard Disk Drives (HDDs)

• Floppy Disks

• Optical storage

• CD-ROMs

• DVD-ROMs

• Blu-ray Discs

• Flash storage

• USB Flash Drives

• Solid-State Drives (SSDs)

• Memory Cards

• Fixed storage

Magnetic Storage:

• • Magnetic storage uses magnetic fields to read and write data on a magnetizable medium. Common examples include:
  • Hard Disk Drives (HDDs): HDDs use magnetic disks to store data. They offer high capacity and are commonly used in personal computers and servers.
  • Magnetic Tape: Magnetic tape is often used in large-scale data storage and archival systems. It offers high-capacity sequential data storage, making it suitable for backup and archival purposes.
  • Floppy disks : Floppy disks, also known simply as “floppies,” were a common form of removable storage media. 

• 5.25-Inch Floppy Disk: Capacity: 1.2 megabytes (MB). 

• 3.5-Inch Floppy Disk: Capacity– 1.44 megabytes (MB). 

Optical Storage:

• • Optical storage relies on lasers to read and write data on optical discs. Examples include:

• CD (Compact Disc): used for storing data

• • • Capacity: holds up to 700 MB of data.
    • Variants: 
      • CD-R (recordable)
      • CD-RW (rewritable)
      • CD-ROM (read-only memory)
      •  CD-ROM/XA (extended architecture).

• DVD (Digital Versatile Disc):

• • • Capacity: DVD-5 holds around 4.7 GB, while dual-layer DVDs (DVD-9 and DVD-18) can hold up to 17.08 GB.
    • Variants: 
      • DVD-R (recordable)
      • DVD+R (recordable)
      • DVD-RW (rewritable)
      • DVD+RW (rewritable)
      • DVD-RAM (random-access memory).

• Blu-ray Disc (BD):

• • • Capacity: Single-layer Blu-ray discs hold about 25 GB, while dual-layer discs can store up to 50 GB.
    • Common Uses: Primarily used for high-definition (HD) and ultra-high-definition (UHD) video playback, as well as data storage.
    • Variants: 
      • BD-R (recordable)
      • BD-RE (rewritable)
      •  BD-XL (extra-large) 

• Ultra HD Blu-ray (UHD BD):

• • Capacity: UHD BDs can hold up to 100 GB or 128 GB for triple-layer discs.

Flash Storage:

• Flash storage uses NAND flash memory to store data electronically. 
• It has no moving parts and offers fast access times. Examples include:
• USB Flash Drives: USB flash drives, or thumb drives, are portable storage devices that use flash memory. They are convenient for data transfer and auxiliary storage.
• Solid-State Drives (SSDs): 
• Memory Cards: Memory cards like SD cards and microSD cards are used in cameras, smartphones, and other devices for data storage.

Fixed Storage:

• Fixed storage refers to built-in, non-removable storage in devices like smartphones, tablets, and some laptops.
•  It is typically composed of flash memory and serves as primary storage for the device’s operating system, applications, and user data.

Latency time: 

• Latency time in computers, put simply, is the time it takes for something to happen or for data to be fetched when you request it. 
• It’s like the delay you might experience when you’re waiting for a webpage to load or for a file to open on your computer.
• It is the Measure of Delay.

Access time : 

• Access time is the time it takes for your computer to find and start using the information you want.
• In computers, access time typically refers to the time it takes to find and retrieve data from storage devices like hard drives or solid-state drives (SSDs). 
• That’s why modern computers often use SSDs, which have very short access times compared to traditional hard drives.

Units of Memory in computers

Bit (Binary Digit):

• The smallest unit of memory in a computer.
• Represents a single binary value (0 or 1).

Byte:

• 1 Byte =  8 bits.
• Can represent 256 different values  (28=256).
• Computer memory is measured in bytes only.

Nibble : 

• A group of four bits is called a nibble. This is equal to half a byte.
• 1 Nibble = 4 bits =( ½)Byte

Word Length : 

• The length of a computer word is measured in bits.
• In different types of computers the word length can range from 1 bit to 64 bits.
• In supercomputers the word length range is 64 bits.

 Measurement of computer memory: 

• 1 bit = Binary Digit (0/1)
• 1 Crumb = 2 Bits
• 1 nibble = 4 bits
• 1 byte = 8 bits = 2 nibbles.
• 1 kilobyte (KB) = 1024 bytes = 210  bytes
• 1 megabyte (MB) = 1024 KB = 220  bytes
• 1 gigabyte (GB) = 1024 MB = 230  bytes
• 1 terabyte (TB) = 1024 GB = 240  bytes
• 1 petabyte (PB) = 1024 TB = 250  bytes
• 1 Exabyte (EB) = 1024 PB = 260  bytes
• 1 zettabyte (ZB) = 1024 EB = 270  bytes
• 1 Yottabyte (YB) = 1024 ZB = 280  bytes
• 1 Brontobyte = 1024 YB = 290  bytes
• 1 Geopbyte  = 1024 Brontobyte = 2100  bytes

Memory interleaving

Memory interleaving is a technique used in computer architecture to enhance memory performance by spreading data across multiple memory modules or banks in a way that improves memory access speed and bandwidth.