Memory Organisation in Computer Architecture

Memory organization is essential for efficient data processing and storage. The memory hierarchy ensures quick access to data by the CPU, while larger, slower storage devices hold data for the long term. Effective memory management ensures the system operates efficiently, providing programs with the memory they need and preventing unnecessary delays in processing.

Memory Hierarchy

Types of Memory in a Computer System

Auxiliary Memory (Non-Volatile)

Devices that provide secondary or backup storage are called auxiliary memory. For example, Magnetic disks and tapes are commonly used auxiliary devices. It is not directly accessible to the CPU, is accessed using the Input/Output channels.

• Hard Disk Drive (HDD): A permanent storage device that holds large amounts of data even when the computer is turned off. It is slower than RAM but offers much more capacity.

• Solid-State Drive (SSD): A faster alternative to HDDs with no moving parts. SSDs provide faster read/write speeds compared to HDDs.

• Optical Discs and USB Flash Drives: Optical discs and USB flash drives are other forms of secondary memory used for storage, though they are less common in modern high-speed systems.

Main Memory (Volatile)

The memory unit that communicates directly within the CPU, Cache memory is called main memory. It is fast memory used to store data during computer operations. Main memory is made up of RAM and ROM, majority part consists of RAM.

RAM Random Access Memory

• DRAM: Dynamic RAM, is made of capacitors and transistors. It is slower and cheaper than SRAM.
• SRAM: Static RAM, retains data, until powered off.

ROM Read Only Memory

Read Only Memory, is non-volatile and is more like a permanent storage for information. It also stores the bootstrap loader program, to load and start the operating system when computer is turned on. PROM (Programmable ROM), EPROM (Erasable PROM) and EEPROM (Electrically Erasable PROM) are some commonly used ROMs.

Cache Memory

The cache memory is used to store program data that is currently being executed in the CPU. Whenever the CPU needs to access memory, it first checks the cache memory. If the data is not found in cache memory then the CPU moves onto the main memory. 

Registers

These are small, ultra-fast memory locations within the CPU used to hold data that is being processed. Registers are crucial for executing instructions efficiently.

Tertiary and Offline Memory

• Tertiary memory refers to storage devices used for backups and archives, like magnetic tapes.
• Offline memory is storage that is not directly accessible by the computer (e.g., external hard drives, optical discs) but data can be retrieved when connected.

Other Types of Memory Based on Storage Time

Volatile Memory: This loses its data, when power is switched off.

Non-Volatile Memory: This is a permanent storage and does not lose any data when power is switched off.

Memory Organization

• Program Load: When a program is executed, it is loaded from secondary storage (HDD/SSD) into main memory (RAM). It may also be loaded partially into cache memory to speed up execution.
• Accessing Data: The CPU accesses data through registers and cache for quick computations. If the data is not in the cache, it will fetch it from RAM. If it’s not in RAM either, it will fetch it from secondary storage.
• Swapping and Virtual Memory: If the system runs out of physical RAM, parts of the program (pages) may be swapped out to secondary storage. This process, known as paging, is managed by the operating system’s memory manager.