Method for Minimizing the effect of Channel Turnaround Time on the Gap Size in the Transmission and Processing of Channel Command words in Variable Length Formatted Data
Original Publication Date: 1978-Aug-01
Included in the Prior Art Database: 2005-Feb-21
In IBM System/370 the operating system of each CPU initiates communications with an attached input/output device with a START I/O instruction. The execution of the instruction causes control to be relinquished to a series of channel command words (CCWs). The channel command words are, in turn, sent to a control unit over the channel/control unit interface for accessing the device. For devices of the direct-access storage type (DASDs), data is recorded and retrieved from such DASDs in variable-length record format. The rate at which channel command words can be sent from the CPU to the control unit is determined by channel transmission and control unit execution time which is equal to a corresponding recording distance along a track termed a "gap."