Figure 1 shows the process and the sequence of filling in the DCB from various sources.

Figure 1. Sources and Sequence of Operations for Completing the DCB

The following items correspond to the boxed numbers in Figure 1.

1. The primary source is your program; that is, the DCB and DCBE macro or compiler. Usually, you should use only those DCB and DCBE parameters that are needed to ensure correct processing. The other parameters can be filled in when your program is to be run.
2. A JCL DD statement or a call to dynamic allocation (SVC 99) creates a job file control block (JFCB). The DD or SVC 99 can supply a data class (DATACLAS) name. The installation data class ACS routine can supply or override the data class name. The resulting data class provides defaults for certain parameters that were omitted from the DD or SVC 99. Parameters from a data class do not override a DD or SVC 99. If the DD statement or call to dynamic allocation has a BLKSIZE value that exceeds 32␠760, that value is in a system control block that is a logical extension to the JFCB, and the JFCB BLKSIZE field has a zero value.
3. When a DASD data set is opened (or a magnetic tape with standard labels is opened for INPUT, RDBACK, or INOUT or is being extended), any field in the JFCB not completed by a DD statement or data class is filled in from the data set label (if one exists). When you open a new DASD data set, the system might previously have calculated an optimal block size and stored it in the data set label. It does that if RECFM, LRECL, and DSORG are available.
   When opening a magnetic tape for output, the OPEN function usually assumes the tape labels do not exist or to apply to the current data set. The exceptions are if you specify DISP=MOD on the DD statement or the dynamic allocation equivalent, or the OPEN macro has the EXTEND or OUTINX option and a volume serial number is present. A volume serial number is present if any of the following is true:

   • The data set is cataloged
   • The VOLUME parameter of the DD statement or dynamic allocation has a volume serial number
   • The DD statement has VOL=REF that refers to a DD statement with a volume serial number that is resolved before the open for the DD statement with VOL=REF

   OPEN does not perform a merge from a data set label to JFCB for a “like” sequential concatenation when making the transition between data sets. If you want a merge, turn on the unlike attribute bit (DCBOFPPC) in the DCB. The unlike attribute forces the system through OPEN for each data set in the concatenation, where a label to JFCB merge takes place. See Concatenating Unlike Data Sets.

4. From the JFCB, OPEN fills in any field not completed in the DCB or DCBE. This completes what is called the forward merge.
5. Certain fields in the DCB or DCBE can then be completed or changed by your own DCB user exit routine or JFCBE exit routine. The DCB and DCBE macro fields are described in z/OS DFSMS Macro Instructions for Data Sets. These exits are described in DCB OPEN Exit and JFCBE Exit.
6. After OPEN calls the user's optional DCB OPEN exit or JFCBE exit, it calls the installation's optional OPEN exit routine. Either type of exit or both can make certain changes to the DCB and DCBE.

   The block size field (BLKSIZE) is in two bytes in the DCB if you are not using large block interface (LBI). Its maximum value is 32␠760. The block size field is in four bytes in the DCBE if you are using LBI. After possibly calling these exits, OPEN tests if the block size field is zero or an exit changed LRECL or RECFM after the system calculated a block size when the DASD data set space was allocated. In either case, OPEN calculates an optimal block size according to the device type if the RECFM is not U.

7. All DCB fields are then unconditionally merged into corresponding JFCB fields if your data set is opened for output. This is the beginning of what is called the reverse merge. Merging the DCB fields is caused by specifying OUTPUT, OUTIN, EXTEND, or OUTINX in the OPEN macro.

   The DSORG field is merged only when it contains zeros in the JFCB. If your data set is opened for input (INPUT, INOUT, RDBACK, or UPDAT is specified in the OPEN macro), the DCB fields are not merged unless the corresponding JFCB fields contain zeros.

8. The open routines use the updated JFCB and associated control blocks to write the DASD data set labels if the data set was open for OUTPUT, OUTIN, OUTINX, or EXTEND. For standard labeled tapes, the open routines write labels only for the OUTPUT or OUTIN options when you are not extending the data set. You are extending if the OPEN option is OUTPUT or OUTIN with DISP=MOD or the OPEN option is OUTINX, EXTEND or INOUT. When extending a standard labeled tape data set, the EOV and CLOSE functions use the updated JFCB and associated control blocks to write trailer labels. If the data set is not closed when your program ends, the operating system closes it automatically.

When the data set is closed, the DCB is restored to the condition it had before the data set was opened (except that the buffer pool is not freed) unless you coded RMODE31=BUFF and OPEN accepted it.