Abstract: Message switching organization for exchanging data messages between function or signalling units within the memory of a processor controlled telecommunications system. Each such function unit has its own program and its own data storage, and the program of a unit has access only to the data storage of its own unit. Messages transmitted between units are of two types, those including information regarding the destination unit address and those omitting destination unit address. Each unit has a message handler area with its own program and data storage, the message handler acting to supply destination unit address information for messages of the second type.