Application-specific programmable processing systems consist of not only a processor, but also the software that runs on it. In order to support development of such systems, a design environment must support both hardware and software development. Unfortunately, there are no specification languages for processors that are suitable for such dual use. Therefore, we have designed a functional-style language that is specifically intended for describing instruction sets; its functional nature allows it to describe the result that an instruction produces without having to specify the mechanism of operation. This property is key to allowing dual use of the specification, since it will not be biased towards either hardware or software development. We are constructing a design environment based on our language. >