On a computer, the software which allows the system to boot up, presents the basic user interface (command line or windowing-type interface), and provides a foundation of essential system services for application software to use.
When referring to synths and other music-specific hardware devices (as opposed to general-purpose desktop computers), there is a tendency to refer to all software that runs on the unit as the “operating system”. This is not strictly correct in computer-science terms, but since vendors of such hardware almost never sell or provide application software for this hardware independently, from the user’s point of view there is no difference. Among the hardware synths which contain embedded software (aka firmware), most have the software burned into an EPROM integrated circuit, and the system can be upgraded, if it all, only by swapping out the EPROMs. This can range from a moderately easy to a very difficult job, depending on exactly how the EPROMs are installed. Some synths can accept operating system upgrades via floppy disk, MIDIsystem exclusive messages, or file trasfer via USB.