A DECB is constructed as part of the expansion of READ and WRITE macros and is used to pass parameters to the control program, help control the read or write operation, and receive indications of the success or failure of the operation. The DECB is named by the READ or WRITE macro, begins on a fullword boundary, resides below the 16MB line, and contains the information shown in the following illustration:
| Offset from DECB Address (Bytes) | Field Contents BSAM and BPAM | Field Contents BISAM | Field Contents BDAM |
|---|---|---|---|
| 0 | ECB | ECB | ECB1 |
| +4 | Type | Type | Type |
| +6 | Length | Length | Length |
| +8 | DCB address | DCB address | DCB address |
| +12 | Area address | Area address | Area address |
| +16 | Address of status indicators. Status indicators reside below the 16MB line. | Logical record address | Address of status indicators. Status indicators reside below the 16MB line. |
| +20 | Key address | Key address | |
| +24 | Exception code (2 bytes) | Block address | |
| +28 | Next address |