1. A data communications subsystem including a data communications processor, a data communications memory resource using an autonomous memory for autonomous configurations and main host system memory for non-autonomous configurations, and a basic control module which connects to remote peripheral units, said data communications subsystem operating in connection with a main host system having a main central processor and main memory, said data communications subsystem comprising: (a) said data communications processor being initiated by said main host system and including: (a1) a local internal memory for storage of data transfer routines; (a2) a plurality of cluster-interface hubs, each of which hubs is connected to either: (i) a plurality of adapter-cluster module controllers, or (ii) said basic control module having a basic control interface unit and a plurality of front-end controllers; (a3) means to initiate, stop or interrogate each of said plurality of front-end controllers in the subsystem; (a4) means to assemble and convey, to a selected front-end controller, an address of specific instruction-commands regarding data-transfer operations; (a5) means to sense any halt in the main host system and thereupon to operate the data communications subsystem in an autonomous self-running mode during such halt period wherein input messages from peripherals and output messages from the main system are stored on disk files, and wherein said output messages are transferred to their destinations even when the main system is off-line, and said input messages are transformed to the main system from disk files when the main system is on-line again; (b) said autonomous memory having direct access said basic control interface unit, said autonomous memory storing control data and information data dedicated to data transfer operations for use of said front-end controllers; (c) said basic control module including: (c1) a basic control interface unit, under control of said data communications processor, providing means to connecting said main host memory in non-autonomous subsystem configurations or means connecting said autonomous memory in autonomous subsystem configurations; (c2) a plurality of front-end controllers each of which connects, via transmission lines, to one or more peripheral units, each of said front-end controllers being linked to said basic control interface unit for access to main host memory or autonomous memory, each of said front-end controllers including: (c2a) means to receive data communication transfer instructions, after initiation by and receipt of instruction address-data from said data communications processor, said data transfer instructions being accessed from said main host memory, or said autonomous data communications memory; (c2b) means to execute data transfer instructions for data transfers from/to peripheral units connected to each of said front-end controllers; (c2c) program routine means for providing the specific line control disciplines and protocol required by the peripheral units connected to each of said front-end controllers; (c2d) connection means to one or more peripheral units.