Data transfers between input/output (I/O) devices and a central processing unit (CPU) take place under instruction or base cycle steal control a byte at a time where the I/O device attachments connect to ports and the ports connect to the CPU. Data transfer can be synchronous or asynchronous. The port involved in the data transfer sends out a device address and command information simultaneously on port data bus out and command bus out, respectively, to the I/O attachments. The addressed I/O device can respond any time within a predetermined time interval. If an I/O device does not respond within the time interval, a blast condition generated by the port causes the I/O attachments to clear the busses between it and the port. During execution of an I/O instruction, the CPU clock is first held in a particular time state while phase clocks and port clocks continue to run and synchronization between the port and I/O attachment is taking place. Upon completion of the synchronization sequence, the port generates an advance time signal to the CPU to advance the CPU clock. The CPU clock runs and the storage clock can be activated depending upon the type of I/O instruction being executed, the CPU clock runs until it reaches a second particular time state and then remains at that particular time state until the port again generates an advance time signal to the CPU. The activity taking place as the CPU clock is advancing depends upon the type of I/O instruction, but generally a data transfer occurs, and the data is entered into or transferred from local storage registers or main or control storage. The extended second particular time state is used for a de-synchronization sequence between the port and I/O attachment.