A data event control block is a 20- to 32-byte area reserved by each READ or WRITE macro. It contains the ECB, control information, and pointers to control blocks. It must reside below the 16 MB line. The DECB is described in z/OS DFSMS Macro Instructions for Data Sets and Data Event Control Block. The ECB is described in Event Control Block.

The DECB is examined by the CHECK routine when the I/O operation is completed to determine if an uncorrectable error or exceptional condition exists. If it does, CHECK passes control to your SYNAD routine. If you have no SYNAD routine, the task is abnormally ended.