For direct access and tape devices, the system considers the channel program completed when it has received both a channel-end and device-end. The channel-end and device end can be presented by the device separately or simultaneously. But for other devices, the system considers the channel program completed when it receives a channel-end condition in the subchannel status word (SCSW). Unless a channel-end (CHE) or abnormal-end (ABE) appendage directs otherwise, the system places a completion code in the IOB and IEDB and then in the ECB (after appendages have been called). The completion code refers to errors associated with channel end. If device and channel end occur simultaneously, errors associated with device end (that is, unit exception or unit check) are also accounted for.

If device end follows channel end and an error is associated with device end, the completion code in the ECB will not indicate the error. However, the status of the device and channel is saved by the system for the device. The next I/O request directed to the I/O device from any address space is marked as intercepted. The error is assumed to be permanent, and the completion code in the ECB for the intercepted request indicates interception. The DCBIFLGS field of the DCB will also indicate a permanent error. Note that, if a write-tape-mark or erase-long-gap CCWis the last or only CCW in your channel program, the I/O process does not attempt recovery procedures for device end errors. In these circumstances, command chaining a NOP CCW to your write-tape-mark or erase-long-gap CCW ensures initiation of device-end error recovery procedures.

To be prepared for device-end errors, you should be familiar with device characteristics that can cause such errors. After one of your channel programs has terminated, do not release buffer space until determining that your next request for the device has not been intercepted. You can reissue an intercepted request.