23. A computer readable memory storing a control program which controls a message processing device for sending and receiving a message composed of one or more elements between different kinds of devices, said control program comprising the steps of: holding a parse tree generated from syntax descriptions defining structure of a message sent or received; as message data to be processed is applied, scanning said parse tree to encode and decode a value and outputting encoded and decoded values; processing a preamble field of a message for use in indicating whether an element exists or not in message data sent or received; and processing an index field of a message for use in indicating what number of element is selected in message data whose one of a plurality of elements is selected to be sent or received.
23. A computer readable memory storing a control program which controls a message processing device for sending and receiving a message composed of one or more elements between different kinds of devices, said control program comprising the steps of: holding a parse tree generated from syntax descriptions defining structure of a message sent or received; as message data to be processed is applied, scanning said parse tree to encode and decode a value and outputting encoded and decoded values; processing a preamble field of a message for use in indicating whether an element exists or not in message data sent or received; and processing an index field of a message for use in indicating what number of element is selected in message data whose one of a plurality of elements is selected to be sent or received. 29. The computer readable memory as set forth in claim 23, wherein said control program in said preamble processing step, stores the parse tree in a continuos region on a storage region, and for decoding a message, when message data is applied, first reads and stores an index field, decodes the contents of the index field to obtain an index value, stores an address of a node being currently scanned in a variable indicative of an address of a node of the parse tree, and scans a node of the parse tree to be referred to by a sum of said variable and a product of the size of a storage region necessary for holding one node of the parse tree and the index value to decode the element in question.