Often it's dealing with interfaces. A couple years ago I worked at a place with a laser cutter, like an old one. You sent instructions to a program that talked

Often it's dealing with interfaces. A couple years ago I worked at a place with a laser cutter, like an old one. You sent instructions to a program that talked to an interface card, which controlled the cutter. The program ran under DOS. It wasn't possible to replace the OS or machine easily, because it was an ISA card and had custom interface software that relied on having direct access.