I/O access method for using flags to selectively control data operation between control unit and I/O channel to allow them proceed independently and concurrently

An access method allows the separation of the command stream being sent to the control unit from the command stream interpreted by the channel. This access method allows the I/O subsystem to, first, reduce the number of interlocks required on the I/O interface. Second, the access method transfers as much of the I/O operation to the device as possible in a single block of data. Third, the access method allows the channel and device to "disconnect" from each other at the end of the transfer of this control information. The access method prepares the information to be transferred to the device in a single command. This is accomplished by means of a new flag in the CCW, using the spare bit in the flag field, called the DS flag. This flag allows the channel to operate as if it were a Chain Command (CC) from the view point of direction of data flow in the channel and a Chain Data (CD) from the view point of transferring control information across the interface. The channel I/O request can, under a single command to a device, control the direction (read or write) of the flow of data separate from the flow of control information.