1. A terminal having input/output means for transmission of electronic signals comprising: (a) processor means connected to control the input/output means; (b) main memory means connected to the input/output means for receiving electronic signals therefrom and for storing the signals as data and instructions, and also connected to the processor means for receiving data therefrom and for providing data and instructions thereto; (c) read-only mass memory means having processor instructions permanently stored therein, connected to the main memory means through the processor means for downloading instructions selected by the processor means, to the main memory means for subsequent transfer to the processor means when required; and (d) control means connected to the processor means, to the main memory means and to the read-only mass memory means for receiving main memory means and mass memory means addresses from the processor means, and for controlling the downloading instructions from the read-only mass memory means to the main memory means, through the processor means, and for controlling the flow of data and instructions between the main memory means and the processor means.