Systems Reference Library - All about the IBM 1130 Computing ...

Device end, when subsequently provided by the device,causes an interrupt condition to be generated.When command chaining is specified after animmediate operation and no unusual conditions havebeen detected during the execution of the command,no interrupt condition is generated. The subsequentcommands in the chain are handled normally, andchannel end for the last operation in the chain causesthe program to be interrupted. The non-0 bytecount required for immediate operations does notresult in a program check, incorrect length indication,or suppression of command chaining.Pending InterruptionIf a start I/0 addresses an I/0 device having a pendinginterrupt condition due to device end or attention,or if a start I/0 addresses a control unit having apending channel end or device end for the device,then the channel status and unit address-status portionsof the CSW are set. The unit status field containsthe busy indicator, identifies the interrupt,and may contain other indicators provided by thecontrol unit or I/0 device. The pending interruptcondition is cleared and unit status pending in thechannel status field is turned on, causing an interruptto the program. The remaining indicators inthe channel status field are off.The operation is not initiated, and the channelis free as soon as the initiation sequence is completed.Thus the channel and I/0 device are immediatelyavailable for initiation of another operation.Device or Control Unit BusyIf a start I/O addresses an I/0 device that is busy,a control unit that is busy, or a control unit that hasa pending channel end or control unit end for a deviceother than the one addressed, then the channelstatus and unit address-status portions of the CSWare set. The unit status field contains the busyindicator or, if the control unit is busy, the busyand status modifier indicators. Unit status pendingin the channel status field is turned on, causing aninterrupt to the program. The remaining indicatorsin the channel status field are off.The operation is not initiated, and the channel isfree immediately after the initiation sequence.Termination With Data TransferFor operations involving data transfer, either thechannel or I/O device can control the timing of thechannel end condition. If command chaining is notspecified, or if chaining is suppressed because ofunusual conditions, channel end causes terminationof the operation at the channel. The status indicatorsin the associated CSW indicate channel end andunusual conditions, if any.The I/0 device can signal channel end any timeafter the initiation of the operation. Channel end mayoccur prior to any actual data transfer.The channel signals the device to terminate datatransfer whenever any of the following conditionsoccur:1. The storage areas specified for the operationare exhausted or filled. This condition occurswhen the channel has decreased the count in thelast CCW associated with the operation to 0.A count of 0 indicates that the channel hastransferred all information specified by the program.2. A program or unit check condition is detected.This condition is due to errors and causes prematuretermination of the operation.3. An XIO control (halt I/0) is executed. Executionof a halt I/O automatically disconnects the devicefrom the channel.If command chaining is specified, the deviceexecuting the operation remains connected to thechannel until the last command of the chain has beenexecuted. Any unusual conditions cause commandchaining to be suppressed and a terminating conditionto be generated. The unusual conditions can be detectedby the channel or the I/O device. If the channelis aware of the unusual condition by the timechannel end is signaled for the operation, the chainis terminated as if the operation in which the unusualconditions occurred were the last operation in thechain.Termination With Halt I/OExecution of a halt I/0 terminates the current I/0operation at the addressed selector channel, controlunit, or I/0 device. If the channel is not busy,modifier bits 8 through 15 of the halt I/0 identify thecontrol unit and 1/0 device to which the halt I/0applies.When the channel is available, and the controlunit is busy, the addressed device is signaled toterminate the current operation. Halt I/0 does notaffect the state of the control unit when both channeland control unit are available.If halt I/0 is issued when the channel is executinga data transfer, the data transfer is terminated andthe device performing the operation is immediatelydisconnected from the channel. In this case,212