Learn what Overlays in Operating System are with this complete beginner’s guide. Understand how overlays save memory and improve system
Have you ever wondered how old computers with very little memory managed to run big programs? . That’s where the concept of Overlays in Operating Systems comes in!
What is an Overlay?
In simple terms, Overlays are a memory management technique used when a program is too large to fit entirely into main memory (RAM).
Instead of loading the entire program at once, only a part (or module) of the program that’s currently needed is loaded into memory.
When that part is done, it’s replaced by another part.
Think of it like this:
Imagine your computer’s memory is a small box.
You can’t put everything inside at once, so you only keep what you need right now and swap it when needed.
That’s exactly what overlays do!
How Do Overlays Work?
Here’s how it works step-by-step:
- A large program is divided into smaller modules.
- Each module performs a specific function.
- Only the required module is loaded into memory when it’s needed.
- When that module finishes its task, it is replaced (overlaid) by another module.
This swapping process happens automatically — the programmer just defines which modules depend on which others.
Example of Overlays
Let’s say you have a big program with three parts:
- Module A – main menu
- Module B – handles file operations
- Module C – processes data
If your RAM can only hold two modules at a time, you can load Module A and Module B first.
When Module B finishes, it can be replaced by Module C — without restarting the program.
This way, you can run large applications even on limited-memory systems.
Why Are Overlays Important?
Overlays were super important in the early days of computing, especially when memory was extremely limited.
Even today, the concept of overlays is used in embedded systems or low-memory devices.
Here’s why they matter:
- Efficient memory usage
- Faster program execution on small memory systems
- No need for expensive hardware upgrades
- Useful in embedded systems and real-time operating systems (RTOS)
The Basic Idea Behind Overlays Is:
The basic idea behind overlays is to load only the part of a program that is needed at a given time into memory, instead of loading the entire program all at once.
When one part of the program (called a module) finishes execution, it is replaced — or “overlaid” — with another module that’s required next.
This way, large programs can run on systems with limited RAM, making memory usage efficient and cost-effective.
In short, overlays help execute big programs in small memory spaces by swapping program parts as needed.
Example
Imagine your computer memory is a small cupboard.
You can’t keep all clothes (program modules) inside at once, so you take out one and put in another when needed — that’s how overlays work!
How Overlays Work: Example of Overlays – Assembler with Two Passes
To understand how overlays work, let’s take a simple real-world example — an assembler that performs two passes while converting assembly code into machine code.
Step-by-Step Explanation
An assembler generally needs to go through the source code twice:
- Pass 1:
- It reads the entire source code.
- Collects all labels and symbols.
- Stores their addresses in a symbol table.
- This table is required for the second pass.
- Pass 2:
- It uses the symbol table created during the first pass.
- Converts mnemonics into machine code.
- Produces the final object file.
The Overlay Concept Here
If the system’s memory is small, it might not be possible to keep both Pass 1 and Pass 2 programs in memory together.
So, we divide the assembler into two overlays:
- Overlay 1: Pass 1 of the assembler
- Overlay 2: Pass 2 of the assembler
When Pass 1 finishes, the memory space it used is freed and then overlaid by Pass 2.
This allows both passes to run one after another without exceeding memory limits.
Simple Illustration
| Memory Region | Loaded Module | Purpose |
|---|---|---|
| Main Memory | Pass 1 | Creates symbol table |
| Main Memory (after overlay) | Pass 2 | Uses symbol table to generate machine code |
So basically:
- Pass 1 is loaded → executes → removed.
- Pass 2 is loaded in the same space → executes → program completes.
This is the practical working of overlays — using the same memory space for different program parts at different times.
What Is an Overlays Driver?
In simple terms, an Overlays Driver is a software component that helps the operating system or an embedded platform load and manage device tree overlays dynamically — without needing to rebuild or reboot the entire system.
Let’s understand what that means step by step
What Are Device Tree Overlays?
In Linux and embedded systems like BeagleBone, Raspberry Pi, or STM32, the Device Tree (DT) describes the hardware — such as GPIOs, I2C, SPI, UART, audio codecs, and other peripherals.
A Device Tree Overlay is a small file that modifies or extends the base device tree at runtime.
It’s like saying —
“Hey system, I just connected a new hardware module (like a sensor or display). Please update the configuration to recognize it.”
This update is applied using an Overlays Driver.
What Does the Overlays Driver Do?
The Overlays Driver is responsible for:
- Loading the overlay (.dtbo) file into the system.
- Merging the overlay with the existing base device tree.
- Updating hardware configurations dynamically.
- Unloading overlays when they’re no longer needed.
This makes your system modular and flexible, especially for embedded development.
Example: BeagleBone Black Overlay Driver
On platforms like BeagleBone Black, the Overlays Driver (part of the Linux kernel) allows users to enable or disable hardware features using overlays.
For example:
sudo dtoverlay=BB-UART1
This command loads the UART1 overlay through the Overlays Driver, enabling UART1 pins on the BeagleBone header — without rebooting or changing the base device tree.
Why Overlays Drivers Are Useful
Enable new peripherals dynamically
Avoid full kernel recompilation
Save development time
Improve flexibility in hardware testing
Useful for prototyping and modular system design
Real-World Use Cases
- Enabling GPIO, SPI, or I2C dynamically in Linux-based embedded boards
- Configuring sensors or displays on BeagleBone / Raspberry Pi
- Testing custom hardware during development
Key Advantages of Overlays
Saves memory space
Reduces cost by avoiding larger RAM requirements
Allows running of large programs in smaller memory
Keeps system performance optimal
Limitations of Overlays
However, overlays also come with a few drawbacks:
Complex to implement manually
Requires careful planning of modules
Can cause performance overhead due to frequent swapping
Modern Use of Overlays
While overlays are not commonly used in modern desktop computers (thanks to huge RAM sizes), they are still relevant in embedded systems, where memory and storage are limited.
For example:
- Automotive ECUs
- IoT devices
- Microcontroller-based systems
In these cases, overlays help run large firmware efficiently within a small flash memory.
Final Thoughts
So, to sum it up —
Overlays are a smart way to handle large programs when memory is small.
They load only what’s needed and replace it when done, saving space and keeping things running smoothly.
It’s a simple yet powerful idea that still finds its place in embedded and real-time systems today.
Frequently Asked Questions (FAQs) on Overlays in Operating System & Overlays Driver
1. What are overlays in operating system?
Overlays are a memory management technique where only the required part of a program is loaded into memory at a time. When one part finishes, it’s replaced (overlaid) by another, allowing large programs to run in small memory spaces.
2. Why are overlays used in operating systems?
Overlays are used to save memory and run big programs on systems with limited RAM. They make programs more memory-efficient by loading only what’s needed at any given moment.
3. How do overlays work with an example?
In an assembler with two passes, Pass 1 and Pass 2 can’t fit in memory together. So after Pass 1 finishes, it’s replaced by Pass 2 in the same memory space. This is a simple example of how overlays work.
4. What is an overlays driver in Linux?
An Overlays Driver is a software component that allows the Linux kernel to load and manage device tree overlays dynamically. It helps modify hardware configurations like GPIO, SPI, or I2C without rebooting the system.
5. What are device tree overlays?
Device Tree Overlays are small files (.dtbo) that extend or modify the base device tree in embedded Linux systems. They help the OS recognize new hardware modules like sensors, displays, or communication ports dynamically.
6. Why are overlays drivers important in embedded systems?
Overlays drivers make embedded systems flexible and modular. They let developers enable or disable hardware features at runtime — saving time, avoiding full kernel recompilation, and simplifying testing.
7. What is the main advantage of overlays?
The main advantage of overlays is efficient memory utilization. They help run large programs or manage hardware on systems with limited memory — perfect for embedded devices and real-time operating systems (RTOS).
8. Are overlays still used today?
Yes ✅. Although modern computers have plenty of memory, overlays are still used in embedded systems, IoT devices, and real-time applications where memory is limited and efficiency is crucial.
9. What is an example of overlays driver command in Linux?
In Linux-based boards like BeagleBone, you can enable a peripheral using:
sudo dtoverlay=BB-UART1
This command loads the UART1 overlay dynamically through the overlays driver.
10. How are overlays different from paging?
Paging divides memory into fixed-size pages and loads them automatically, while overlays are manually managed by programmers to load specific program parts as needed.
Mr. Raj Kumar is a highly experienced Technical Content Engineer with 7 years of dedicated expertise in the intricate field of embedded systems. At Embedded Prep, Raj is at the forefront of creating and curating high-quality technical content designed to educate and empower aspiring and seasoned professionals in the embedded domain.
Throughout his career, Raj has honed a unique skill set that bridges the gap between deep technical understanding and effective communication. His work encompasses a wide range of educational materials, including in-depth tutorials, practical guides, course modules, and insightful articles focused on embedded hardware and software solutions. He possesses a strong grasp of embedded architectures, microcontrollers, real-time operating systems (RTOS), firmware development, and various communication protocols relevant to the embedded industry.
Raj is adept at collaborating closely with subject matter experts, engineers, and instructional designers to ensure the accuracy, completeness, and pedagogical effectiveness of the content. His meticulous attention to detail and commitment to clarity are instrumental in transforming complex embedded concepts into easily digestible and engaging learning experiences. At Embedded Prep, he plays a crucial role in building a robust knowledge base that helps learners master the complexities of embedded technologies.
