Data Control Block (DCB) Fields

The EXCP form of the DCB macro instruction produces a DCB that can be used with the EXCP macro instruction. Code a DCB macro instruction for each data set to be processed by your channel programs. (Notation conventions and format illustrations of the DCB macro instruction are given in z/OS DFSMS Macro Instructions for Data Sets.) DCB parameters that apply to EXCP depend on the following elements of the DCB that are generated:

Foundation block: This portion is required and is always 12 bytes in length. If the DCBMACRF field indicates that a DCB portion before this is missing or short, the system ignores values in those fields.
EXCP interface: This portion is optional. If you specify any parameter in this category, 20 bytes are generated.
Foundation block extension and common interface: This portion is optional and is always 20 bytes in length. If this portion is generated, the device-dependent portion is also generated.
Device dependent: This portion is optional and is generated only if the foundation block extension and common interface portion is generated. Its size ranges from 4 to 20 bytes, depending on specifications in the DEVD parameter. If you do not specify the DEVD parameter and the foundation extension and common interface portions are generated, 16 bytes for this portion are generated.

Some of the procedures performed by the system when the DCB is opened and closed (such as writing file marks for output data sets on direct access volumes) require information from optional DCB fields. Make sure that the DCB is large enough to provide information for the procedures you want the system to handle.

Figure 1 shows the relative position of each portion of an opened DCB. The fields, corresponding to the DCB macro instruction parameters, are also identified, except for DDNAME. (DDNAME is not included in a DCB that has been opened.) The fields in parentheses represent system information that is not associated with parameters of the DCB macro instruction.

Sources of information for DCB fields other than the DCB macro instruction are data definition (DD) statements or dynamic allocation parameters, data set labels, and DCB modification routines. You can use any of these sources to specify DCB parameters. However, if a particular portion of the DCB is not generated by the DCB macro instruction, the system will not accept information intended for that portion from any alternative source.

You can provide symbolic names for the fields in one or more EXCP DCBs by coding a DCBD macro to generate a dummy control section (DSECT). For further information, see Mapping the DCB.

The EXCP DCB does not have a field to contain the current or maximum block size but the DCBE does have such a field.

Figure 1. Data Control Block Format for EXCP (After OPEN)

The fields in parentheses represent system information that is not associated with parameters of the DCB macro instruction.