Abnormal-End Appendage

To determine the method the system uses to handle an abnormal condition use the abnormal-end (ABE) appendage. The following information explains how to use the ABE appendage.

This appendage can be entered on abnormal conditions, such as unit exception, wrong-length indication, out-of-extent error, intercept condition (that is, device end error), unit check, program check, protection check, channel data check, channel control check, interface control check, and chaining check. It can also be entered when an EXCP is issued for a DCB that has already been purged. The following apply:

If IOBECBCC is set to X'41', this appendage was entered because of a unit exception or wrong-length record indication or both. The system previously called the channel-end appendage, if present. For further information on these conditions, see Channel-End Appendage.
If the IOBECBCC is set to X'42', this appendage was entered because of an out-of-extent error. The system previously called the end-of-extent appendage, if present.
If this appendage is entered with IOBECBCC set to X'4B', the tape error recovery procedure (ERP) either encountered an unexpected load point, or found zeros in the command address field of the CSW.
If the IOBECBCC is set to X'7E', the appendage was first entered because of an intercept condition. If it is then determined that the error condition is permanent, the appendage will be reentered with the IOBECBCC set to X'44'. The intercept condition signals that an error was detected at device end after channel end on the previous request.
If the IOBECBCC was set to X'48', the appendage was entered because of an EXCP being issued to an already purged DCB. This applies only to related requests.
If the appendage is entered with IOBECBCC set to X'7F', it might be because of a unit check, program check, protection check, channel data check, channel control check, interface control check, or chaining check. If the IOBECBCC is X'7F', it is the first detection of an error in the associated channel program. If the IOBIOERR flag (bit 5 of the IOBFLAG1) is on, the IOBECBCC field will contain X'41', X'42', X'48', X'4B', or X'4F', indicating a permanent I/O error.
If the ending address is zero or the subchannel status byte in the IOB (IOBCSTAT) shows any of the following errors: program check, protection check, channel data check, channel control check, interface control check, or chaining check, and your abnormal end appendage determines that the ERP has not yet run, then do not modify IOBSTART. This lets the ERP try to recover. If the ERP has completed and one or more of these six bits is on or the address is zero, then the status of the channel program is not known.

To determine if an error is permanent, check the IOBECBCC field of the IOB for a X'4x' completion code.

To determine the type of error, check the subchannel status word field and the sense information in the IOB. However, when the IOBECBCC is X'42', X'48', or X'4F', these fields are not applicable. For X'44', the CSW is applicable, but the sense is valid only if the unit check bit is set.

By using the return address in register 14 to return control to the system, the channel program is posted complete, and its request element is made available. You can use the following optional return addresses:

Contents of register 14 plus 4: The channel program is not posted complete, but its request element is made available. You can post the channel program by using the calling sequence described under the SIO appendage.
Contents of register 14 plus 8: The channel program is not posted complete, and its request element is placed back on the request queue to be retried. Reinitialize the IOBFLAG1, IOBFLAG2, and IOBFLAG3 fields of the input/output block and set the IOBERRCT field to zero. As an added precaution, clear the IOBSENS0, IOBSENS1, and IOBCSW fields.
The appendage can request that a different type of channel program be started. For example:
- The original request was for a zHPF channel program but the device is no longer enabled for zHPF or does not support the zHPF capabilities required by the new channel program.
- The original request was for a non-zHPF channel program but the new I/O request allows a zHPF channel program to be used.
The channel program type may be changed from non-zHPF to zHPF only if the caller passed an IOBE to EXCP. If the channel program type is changed, the appendage must set or reset the IOBEFMT1 and IOBEZHPF bits correctly to reflect the type of channel program. For example, if the original channel program was a format-1 CCW channel program and the new channel program is a zHPF channel program, then the IOBEFMT1 bit must be reset and the IOBEZHPF bit must be set.
The system will call appendages, beginning with the SIO appendage, as if this were a new EXCP or EXCPVR request. Note that the EXCPVR page fix appendage will not be called again.
Contents of register 14 plus 12: The channel program is not posted complete, and its request element is not made available. (Use this return only if the appendage has passed the request queue element to the exit effector for use in scheduling an asynchronous routine.)

Registers 10 through 13 in an ABE appendage can be used without saving and restoring their contents.