GB1404703A - Input output systems - Google Patents

Abstract

1404703 Input/output control INTERNATIONAL BUSINESS MACHINES CORP 26 Nov 1973 [2 Jan 1973] 54766/73 Heading G4A Input/output channels 22, for executing I/O programs of channel command words (CCW) assembled by a central processing unit 10, are arranged in groups 12, 14, and unless an I/O instruction specifies a particular channel 22, channel group hardware assigns a non-busy channel 22 from the group to control the requested I/O operation. If an I/O device disconnects during execution of a series of chained CCW's the I/O operation may be resumed subsequently on any one of the channels 22 in the same group. It is also possible, e.g. for diagnostic testing, for a START I/O command to specify a particular channel 22, and in this case the selected channel controls the entire I/O operation. If a HALT DEVICE instruction occurs, channel selection logic determines if one of the channels is assigned to the device and if so routes the halt instruction through that channel. In operation, a unit (channel group) address from a start I/O instruction is translated by table look-up into a unit control word (UCW) address, the UCW is fetched from shared storage 18, 20 and subsequently a channel address word (CAW) is fetched from a predetermined storage location and is entered in the selected channel where it is used for indirect addressing of the first CCW. Since each set of UCW's 18, 20 is common to a group of channels 22 an interlock is provided which prevents any channel accessing a UCW until it has control of the corresponding channel/device interface.