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1,108,804. Electric digital calculators. INTERNATIONAL BUSINESS MACHINES CORPORATION. 29 March, 1965 [6 April, 1964], No. 13160/65. Heading G4A. In a data processing system, input/output units are controlled in response to respective stored unit control words. A micro-programme-controlled central processing unit (CPU) can communicate with a selected input/output (I/O) unit over a common set of lines comprising two sets of nine lines (for an 8-bit byte and a parity bit) called " bus out " and " bus in," and in and out lines for indicating the presence on the corresponding bus of data (in or out), commands (out), addresses (in or out) or status information (in). Lines are provided to carry timing signals from the CPU to the I/O units. A " select out " line carries a signal from the CPU to the I/O units in turn in priority order. If an I/O unit does not require service it passes the signal to the next I/O unit, otherwise it blocks the signal and sends its address to the CPU to interrupt the current programme, the address indicating the identity of the I/O unit and the operation required. The far end of the " select out " line is connected to a " select in " line returning to the CPU. An " operational in " line is provided to indicate to the CPU that an I/O unit has been selected, and an " operational out " line is provided for interlocking purposes. A " suppress interrupt " (out) line is provided to enable the CPU to prevent the I/O units from interrupting it. The CPU may select a particular I/O unit by sending its address. I/O devices may be time-division multiplexed on to the set of lines to the CPU. The CPU memory stores I/O control words as follows: (a) channel command words CCW, (b) unit control words UCW, (c) channel status words CSW. A CCW has operation, count, flag and data address fields. The data address field gives the first byte address in CPU memory to be used for I/O operations, and the count field gives the number of consecutive byte locations after it which are available. The flag field comprises indications whether or not (a) addresses are to be chained (i.e. the storage area defined by the next CCW is to be used for the current operation), (b) commands are to be chained (i.e. the operation specified by the next CCW is to be performed with the present I/O unit), (e) data from the I/O unit is to be stored in the CPU memory, (d) the programme is to be interrupted. During an I/O operation, fields from the appropriate CCW are transferred to a UCW