What sort of embedded systems are you interested in?  There are "embedded systems" which consist of standard server hardware with some preinstalled software (aka. "server appliances"); there are "embedded systems" which consist of low power boards running general purpose operating systems (e.g., cheap home routers); there are "embedded systems" which run specialized operating systems (e.g., the Martian rovers, running a commercially-available RTOS); and there are "embedded systems" which don't run what you'd recognize as an OS at all (e.g., your car, your TV, my blood glucose tester, etc).