Back in the days of DOS, I wrote a scripting program to demonstrate my company's software. This scripting program would start the application and then pop up windows on top of the application, describe some of its features using animated type, then close the window, feed keystrokes to the application, wait for the application to display the proper screen, and then pop up more windows. It had its own scripting language and even a script editor so I could interrupt the script, edit it, and then resume running it. The best part is that it ran on top of an unmodified version of our application.
The whole demo application was written in C and assembly language. It hooked timer and keyboard interrupts in order to interact with the application. I wrote everything, including the windowing library.
