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Device-Dependent Parameters z/OS DFSMSdfp Advanced Services SC23-6861-01 |
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If you do not want to select a specific device until job setup time, specify the device type requiring the largest area; that is, DEVD=DA. The following diagrams illustrate the device-dependent portion of the DCB for each combination of device type specified in the DEVD parameter and data set organization specified in the DSORG parameter. Fields that correspond to device-dependent parameters in addition to DEVD are indicated by the parameter name. When processing concatenated data sets, the system changes the value in DCBDEVT as appropriate as it reaches each new data set. These values are described in z/OS DFSMS Macro Instructions for Data Sets. Figure 1. Device-dependent portion
of the DCB with DEVD=DA and DSORG=PS (or DSORG=PO)
┌───────────────────────────────────────────────────┐ ³ 0 ³ ³ DCBDCBE ³ ├────────────┬──────────────────────────────────────┤ ³ 4 ³ 5 ³ ³ Reserved ³ (DCBFDAD) ³ ├────────────┘ ³ ³ 8 ³ ³ ³ ³ ┌──────────────────────────────────────┤ ³ ³ 13 ³ ³ ³ (DCBDVTBA) ³ ├────────────┼────────────┬─────────────────────────┤ ³ 16 ³ 17 ³ 18 ³ ³ DCBKEYLE ³ (DCBDEVT)³ (DCBTRBAL) ³ └────────────┴────────────┴─────────────────────────┘ The fields in parentheses represent information not associated with parameters of the DCB macro instruction. EOV sets all of these fields. OPEN sets DCBDVTBA and DCBDEVT, but not DCBFDAD. Your program can modify DCBFDAD, DCBKEYLE, or DCBTRBAL as described in the text. When writing on DASD, maintain certain fields of the device-dependent
portion of the DCB. The system uses the information in the following
instances:
Maintain the following fields of the device dependent
portion of the DCB when writing on DASD:
If space is available for output data sets, the system uses the contents of the full disk address (DCBFDAD) field, plus one block, to write a file mark when the DCB is closed or EOV is issued. If the track balance field (DCBTRBAL) is less than eight, the file mark is written on the next sequential track. Note that the track containing your last data block,
as identified by DCBFDAD, might not be the best place for a file mark.
Consider the following:
If the system is to write a file mark, you must maintain the contents of these two fields and set on bit 0 of DCBOFLGS. For further information on DCBOFLGS, see Handling End of Volume and End-Of-Data-Set Conditions. Use the OPEN macro instruction to initialize DCBDVTBA and DCBDEVT. You can use DCBDVTBA or DCBDVTBL with the DEVTAB parameter of the TRKCALC macro (see Performing Track Calculations (TRKCALC macro) for the TRKCALC description). Figure 2. Device-dependent portion
of the DCB with DEVD=DA and DSORG=DA
┌────────────┬─────────────────────────────────────┐ ³ 16 ³ 17 ³ ³ DCBKEYLE ³ Reserved ³ └────────────┴─────────────────────────────────────┘ Figure 3. Device-dependent
portion of the DCB with DEVD=TA and DSORG=PS
┌───────────────────────────────────────────────────┐ ³ 12 ³ ³ (DCBBLKCT) ³ ├────────────┬────────────┬────────────┬────────────┤ ³ 16 ³ 17 ³ 18 ³ 19 ³ ³ DCBTRTCH ³ (DCBDEVT) ³ DCBDEN ³ Reserved ³ └────────────┴────────────┴────────────┴────────────┘ The fields in parentheses represent information not associated with parameters of the DCB macro instruction. They are set by OPEN and EOV. Your program can modify DCBBLKCT as described for the REPOS parameter above. The system uses the contents of the block count (DCBBLKCT) field to write the block count in trailer labels when the DCB is closed or when the EOV macro instruction is issued. For tape cartridges, the system also compares this count with a count calculated from hardware information. OPEN and EOV set this DCB field to zero except when reading standard labeled tape backward. In that case OPEN or EOV set DCBBLKCT to the block count in the trailer label. The I/O process increments this field by the contents of the IOBINCAM field of the IOB upon completion of each I/O request. To indicate to the system that your program is maintaining DCBBLKCT, code foundation block parameter REPOS=Y. See Foundation Block Parameters. Figure 4. Device-dependent portion
of the DCB with DEVD=PR and DSORG=PS
┌────────────┬────────────────────────────────────┐ ³ 16 ³ 17 ³ ³ DCBPRTSP ³ Reserved ³ └────────────┴────────────────────────────────────┘ Figure 5. Device-dependent portion
of the DCB with DEVD=PC or RD and DSORG=PS
┌────────────┬──────────────────────────────────────┐ ³ 16 ³ 17 ³ ³ DCBMODE ³ Reserved ³ ³ DCBSTACK ³ ³ └────────────┴──────────────────────────────────────┘ The following DCB operands pertain to specific devices and can
be specified only when the DEVD parameter is specified.
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Copyright IBM Corporation 1990, 2014
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