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IB DP Computer Science Study Notes

2.1.2 Primary Memory

Primary memory, or main memory, is a critical component of a computer's architecture, acting as the principal storage area that directly communicates with the central processing unit (CPU). This memory is fast and volatile, primarily used to store data and instructions that the CPU needs immediate access to during operation. It's distinct from secondary memory, which refers to long-term data storage devices like hard drives. Primary memory is key for understanding a computer's performance and processing capabilities.

Nature of Primary Memory

Primary memory's main function is to store data and instructions that are in active use. This allows the CPU to access necessary information quickly, facilitating efficient task execution. Unlike secondary memory, primary memory is volatile and loses its contents when the power is switched off.

Characteristics of Primary Memory

  • Volatility: Data is not retained after power loss.
  • Speed: Faster access compared to secondary memory.
  • Temporary Storage: Holds data and instructions currently being processed.
  • Direct CPU Access: The CPU can directly retrieve and store data in primary memory.

Random Access Memory (RAM)

RAM is a vital type of primary memory in computers. It serves as a temporary holding area for data and instructions that the CPU may need to access quickly.

Features of RAM

  • Volatile Memory: Loses its information without power.
  • Read and Write Capability: Permits both storage and retrieval of data.
  • Types: Mainly divided into Dynamic RAM (DRAM) and Static RAM (SRAM). DRAM, which needs regular refreshing of data, is common and cheaper, while SRAM is faster and more expensive but used less due to its cost.
  • Usage: Stores the operating system, applications, and currently used data.

Importance of RAM

  • System Performance: More RAM typically means better multitasking and speed.
  • Upgradability: It's often possible to enhance a computer's performance by increasing its RAM capacity.

Read-Only Memory (ROM)

ROM is different from RAM in that it is non-volatile and primarily used for storing firmware or permanent software.

Characteristics of ROM

  • Non-volatile Memory: Retains its contents without power.
  • Pre-programmed Data: Often contains firmware or software that shouldn't be modified during normal operation.
  • Primarily Read-Only: Data stored in ROM is not meant to be modified frequently.

Types of ROM

  • PROM: Programmable ROM, which can be written to once.
  • EPROM: Erasable Programmable ROM, which can be erased with UV light and reprogrammed.
  • EEPROM: Electrically Erasable Programmable ROM, which can be erased and reprogrammed electrically.

Uses of ROM

  • Bootstrapping: Contains firmware/BIOS necessary for booting the computer.
  • Storing Permanent Instructions: Ideal for storing essential system instructions and firmware.

Comparison between RAM and ROM

Key Differences

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Detailed Exploration of RAM

Understanding RAM Capacity

  • Measured in Gigabytes: RAM size is a crucial factor in determining how many applications a computer can handle simultaneously.
  • Impact on Performance: Insufficient RAM can lead to slow performance and system lag, especially when running multiple programs or high-end software.

RAM in Modern Computing

  • Gaming and High-Performance Tasks: Games and professional applications like video editing software require substantial RAM for optimal performance.
  • Mobile Devices: Smartphones and tablets also use RAM. The concept is similar, but the technology and architecture might differ due to size and power constraints.

In-Depth Look at ROM

The Evolution of ROM

  • ROM's inception traces back to the early days of computing, where modifying the stored instructions was rare or required physical replacement.
  • With advancements like EPROM and EEPROM, updating firmware became more feasible, albeit still not as frequently as RAM.

ROM's Role in Hardware

  • Embedded Systems: ROM is crucial in embedded systems like microwave ovens and washing machines, storing the necessary instructions to operate.
  • Security: Because ROM cannot be easily altered, it's considered more secure against malware compared to RAM.

Concluding Remarks on Primary Memory

Understanding RAM and ROM's distinct roles and functionalities enhances comprehension of a computer system's overall architecture. RAM's volatile nature and high speed facilitate efficient processing and multitasking, playing a critical role in a computer's performance. Conversely, ROM's stability and reliability in storing essential boot instructions and firmware underscore its importance in the fundamental operations of a computer system. Both types of memory are indispensable, each complementing the other to ensure a smooth, efficient computing experience.


The speed of RAM is generally faster compared to ROM. This difference is primarily due to their respective technologies and roles in a computer system. RAM is designed to be rapidly written to and read from, facilitating quick access for the CPU to currently active data and applications. Its architecture, whether DRAM or SRAM, is optimised for speed to minimise latency during data access. In contrast, ROM is primarily intended for storing firmware or essential booting instructions, where speed is not as critical. The construction of ROM, being pre-programmed or not requiring frequent updates, does not necessitate the high-speed architecture of RAM. The speed of ROM is sufficient for its purposes — being read upon system boot or in specific operations where data permanence is more crucial than quick access.

Insufficient RAM in a computer system can lead to several noticeable performance issues. When the RAM is fully utilised and the system requires more memory to function, the operating system begins to use part of the hard drive or solid-state drive as a "swap space" or virtual memory. This swap space acts as an overflow for RAM, but it's significantly slower, leading to a noticeable decrease in system performance and responsiveness. Applications may load and respond more slowly, multitasking becomes less efficient, and the system might experience increased lag or "stuttering". This is particularly evident in resource-intensive tasks like gaming, video editing, or running multiple applications simultaneously. Moreover, excessive use of swap space can lead to increased wear and tear on hard drives, particularly SSDs, which have a limited write lifecycle.

A computer primarily uses ROM during the boot process and in scenarios where unchanging, essential instructions or data need to be reliably stored. ROM contains the firmware or BIOS (Basic Input/Output System), which includes the fundamental instructions that allow the computer to start up, perform self-checks, and initiate the loading of the operating system from secondary storage. Since ROM is non-volatile, it ensures that these crucial instructions are always available and uncorrupted, regardless of power state or system errors. This reliability is essential for system stability and integrity. Additionally, ROM finds applications in various embedded systems (like in microwaves, cars, or industrial machines) where a stable, permanent set of instructions is required for the device's fundamental operations, offering a high level of dependability for essential tasks.

Although ROM (Read-Only Memory) is primarily designed for permanent storage and typically comes pre-programmed from the factory, there are certain types of ROM that can be modified or updated under specific conditions. Programmable ROM (PROM) can be written once after manufacturing; once written, the data can't be erased or changed. Erasable Programmable ROM (EPROM) can be erased and reprogrammed, but this requires exposure to strong ultraviolet light, after which it can be reprogrammed using a special device. Electrically Erasable Programmable ROM (EEPROM) allows for data to be erased and reprogrammed electrically, making it more versatile for tasks that require regular but infrequent updates, like firmware updates in various electronic devices. The ability to update these types of ROM enables firmware revisions and functionality upgrades in hardware without needing to replace physical components.

Primary memory, especially RAM, is considered volatile because it loses its stored data when the power supply is interrupted or turned off. This characteristic has significant implications for data storage and processing in computers. Since any information stored in RAM is lost upon power loss, it's not suitable for long-term data storage, which is instead handled by non-volatile memory like hard drives or SSDs. The volatility of primary memory is beneficial for temporary data storage and quick access, allowing the CPU to rapidly retrieve and process data that's actively being used. This leads to quicker data processing and ensures that sensitive data is not left stored in memory after power off, which can be a security measure. However, it also means that unsaved work can be lost in cases of unexpected power outages, underscoring the importance of regularly saving data in non-volatile storage.

Practice Questions

Explain the difference between RAM and ROM in the context of their use in primary memory.

RAM (Random Access Memory) and ROM (Read-Only Memory) are both crucial types of primary memory in computer systems, but they serve different purposes. RAM is a volatile type of memory, meaning it loses its data when the power is cut off. It's used for temporary storage of data and instructions that the CPU needs immediate access to, which makes it fundamental for multitasking and running applications efficiently. On the other hand, ROM is non-volatile and retains its information even without power. It's mainly used for storing critical firmware or permanent software, like the BIOS in a computer, which is essential for booting and running the hardware. ROM's contents are pre-programmed and, unlike RAM, not meant to be changed frequently. This clear division in functionality and characteristics between RAM and ROM underscores their respective roles in a computer's architecture.

Describe two different types of RAM and discuss their characteristics and uses.

Two different types of RAM commonly used in computing are Dynamic RAM (DRAM) and Static RAM (SRAM). DRAM stores each bit of data in a separate capacitor within an integrated circuit, which makes it relatively dense and inexpensive. However, this design means that the stored data must be refreshed thousands of times per second, leading to slower access times compared to SRAM. DRAM is widely used in personal computers and servers where high capacity at a lower cost is crucial. In contrast, SRAM does not need to refresh its data regularly, which allows for faster access to the stored data. This is because SRAM stores the data in a flip-flop, holding each bit in a stable state until it's changed, leading to faster, more efficient performance. Due to its higher speed, SRAM is used in smaller capacity applications like cache memory in CPUs, where speed is more critical than capacity.

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Written by: Alfie
Cambridge University - BA Maths

A Cambridge alumnus, Alfie is a qualified teacher, and specialises creating educational materials for Computer Science for high school students.

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