Introduction to Memory

The Computer’s Short-Term Storage

An architectural overview of computer memory, covering the memory hierarchy, the differences between RAM and ROM, and the role of CPU cache.
Author

Chuck Nelson

Published

October 18, 2025

Computer Memory

1 Purpose

This document provides a foundational overview of computer memory. You will learn about the different types of memory, their roles within the memory hierarchy, and the key technologies like DRAM, SRAM, and ROM that enable a computer to function.

2 What You’ll Learn

By the end of this reading, you will be able to:

  • Describe the memory hierarchy and its purpose.
  • Differentiate between volatile (RAM) and non-volatile (ROM) memory.
  • Explain the difference between SRAM (used for caches) and DRAM (used for main memory).
  • Explain the role of the CPU cache (L1, L2, L3).

This reading maps to the following program and course learning outcomes:

  • Program Learning Outcomes (PLOs):
    • 3. Apply terminology and numeric or system concepts: This document covers RAM, ROM, SRAM, and DRAM, which are fundamental hardware concepts.
  • Course Learning Outcomes (CLOs):
    • 1. Identify hardware and basic network components: This is a core document for identifying and understanding different types of memory hardware.

This exercise will help you develop the following skills and knowledge, which align with the O*NET SOC Code 15-1232.00 for Computer User Support Specialists.

Learning Objective O*NET KSAs Technologies Used
Differentiate between major memory types (SRAM, DRAM, ROM). Knowledge: Computers & Electronics
Skills: Reading Comprehension
Abilities: Information Ordering		 N/A (Conceptual)
Explain the purpose of the CPU cache hierarchy. Knowledge: Computers & Electronics
Skills: Systems Analysis		 N/A (Conceptual)

3 An Introduction to Computer Memory

Computer memory is the essential component that stores data and instructions for immediate use, acting as the bridge between the ultra-fast processor and slower long-term storage. This article provides an introduction to the primary types of memory and how they are organized.

3.1 The Memory Hierarchy

Modern computing utilizes a memory hierarchy—a structure that organizes memory based on a trade-off between speed, capacity, and cost. Memory that is faster is also more expensive and thus smaller in capacity. This hierarchy ensures the CPU has the fastest possible access to the data it needs most urgently.

flowchart TD
    A["CPU Registers<br>(Fastest, Smallest)"] --> B["L1/L2/L3 Cache"];
    B --> C["Main Memory (DRAM)"];
    C --> D["Storage (SSD/HDD)<br>(Slowest, Largest)"];

    style A fill:#D0E7FF,stroke:#333
    style D fill:#FFDDDD,stroke:#333

3.2 Primary Memory: RAM vs. ROM

The two fundamental classes of primary memory are Random-Access Memory (RAM) and Read-Only Memory (ROM). They are distinguished by their volatility.

Volatile Memory (RAM)

RAM is the “working memory” of the computer for currently running programs. It is volatile, meaning its contents are lost when the power is turned off.

  • DRAM (Dynamic RAM): The most common type, used for the main system memory. It stores each bit in a tiny capacitor that must be constantly refreshed to maintain data. Modern systems use DDR SDRAM (Double Data Rate Synchronous DRAM).
  • SRAM (Static RAM): Used for CPU Cache. SRAM uses a more complex structure of transistors to store each bit and does not need to be refreshed. This makes it much faster but also more expensive and less dense than DRAM.

Non-Volatile Memory (ROM and Flash)

Non-volatile memory retains its data without power. It is used for critical boot-up instructions and long-term storage.

  • ROM (Read-Only Memory): Traditionally contained the computer’s BIOS or UEFI firmware required to start the computer.
  • Flash Memory (NAND): A high-density, low-cost form of non-volatile memory that is the basis of Solid-State Drives (SSDs), USB drives, and memory cards.

3.3 The CPU Cache

To bridge the large speed gap between the CPU and DRAM, several levels of extremely fast SRAM are integrated directly into the CPU.

  • L1 Cache: The smallest and fastest cache, located on the CPU core itself.
  • L2 Cache: Larger and slightly slower, often dedicated to a single core.
  • L3 Cache: The largest on-chip cache, often shared across all CPU cores.

When the CPU needs data, it checks the L1 cache first. If it’s not there (a cache miss), it checks L2, then L3, and only as a last resort does it fetch the data from the much slower main memory (DRAM), which causes a significant delay.

4 Reflect and Review

ImportantReflection: 3-2-1

Now that you have reviewed this document, take a moment to reflect on your learning. In your Microsoft Teams Student Notebook, create a new page for this topic and write down the following:

  • 3 types of memory from the memory hierarchy.
  • 2 key differences between SRAM and DRAM.
  • 1 question you still have about how CPU cache works.

This reflection is for your instructor to review and helps solidify your understanding of the concepts.

TipCheck on Learning

Test your understanding with the following questions. These questions provide retrieval practice and reinforce key concepts covered in this reading. In your Microsoft Teams Student Notebook, answer the following:

  1. What is the main difference between volatile and non-volatile memory?
  2. Why is SRAM used for CPU caches instead of the cheaper and denser DRAM?
  3. What does DRAM stand for, and why does it need to be “refreshed”?
  4. What is the purpose of the L1, L2, and L3 cache levels?
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