What is the concept of overlay linking?

Overlay linking is a technique used in computer programming to manage memory by loading only necessary program parts into memory.

In more detail, overlay linking is a method used to enable a program to be larger than the computer's available memory. It works by dividing a program into self-contained blocks, or 'overlays', which are loaded into memory as and when they are needed. This technique is particularly useful in systems with limited memory resources, as it allows for the execution of large programs without requiring all parts of the program to be loaded into memory at once.

The overlay structure is typically defined during the linking process. The linker creates an overlay map, which is a plan of how the overlays are to be loaded into memory. This map is then used at runtime to manage the loading and unloading of overlays.

The main advantage of overlay linking is that it allows for the efficient use of memory. By only loading the necessary parts of a program into memory, it reduces the overall memory footprint of the program. This can lead to improved performance, particularly in systems with limited memory resources.

However, overlay linking also has its drawbacks. It can make programs more complex to write and debug, as programmers must carefully manage the dependencies between different parts of the program. Additionally, the process of loading and unloading overlays can introduce additional overhead, which can impact the overall performance of the program.

In modern computing, overlay linking is less commonly used, as advances in hardware have led to an increase in available memory. However, it remains an important concept in computer science, particularly in the context of embedded systems and other environments where memory resources are limited.