400

Explanation
Execute channel program (EXCP) processing found
an error while processing an I/O request. One of the following is
true:The address for the data control block (DCB) in the data extent
block (DEB) is not the same as the DCB address in the caller's input/output
block (IOB). This situation can occur if the data set is not open
or if the control blocks are prematurely freed or overlaid.
The program issued an EXCPVR macro for a DCB that was opened for
BSAM, BPAM, or QSAM. Those access methods allow the user program to
issue EXCP or XDAP but not EXCPVR.
BSAM, BPAM, or QSAM detected that control blocks that are internal
to the access method were corrupted. The control blocks are in the
application program protection key, so the application program might
have overlaid them. The address being checked is in register 11.

System action
The system abnormally ends the task and may issue
an SVC dump.

System programmer response
  If the error recurs and the program
is not in error, look at the messages in the job log for more information.
Search problem reporting data bases for a fix for the problem. If
no fix exists, contact the IBM® Support
Center. Provide the JCL, the program and assembly listings for the
job, and the SVC dump.

Programmer response
Verify that the data set was opened and
that the control blocks were handled correctly in the problem program.
 Run the job again.
If the system wrote a dump, look at field
TCBEXCPD at offset X'C0' in the task control block (TCB)
for the current task.  The TCBEXCPD points to the EXCP debugging area
(XDBA). The XDBA contains diagnostic data associated with the EXCP
processor.

Source
Execute channel program (EXCP)