Reading tape files backward

You can also specify that an input file on magnetic tape be processed backward by using the RDBACK parameter. The RDBACK option is supported for:
  • QSAM tape files
  • Records formats of F, FB, and U
  • All types of label processing
  • Single volume magnetic tape files.

The following table describes the tape positioning for files that are opened and closed for RDBACK.

Table 1. Positioning of Files Opened and Closed with RDBACK
Macro Specifies LEAVE on the OPEN or CLOSE Macro FILEDEF TAPn Options Tape positioning
OPEN Yes SL|SUL|AL|AUL|NSL option Assumed position is immediately after the tape mark that ends the trailer label group.
OPEN Yes LABOFF|BLP|no label processing option Assumed position is immediately after the tape mark that ends the data.
OPEN Yes No label processing option The tape is rewound.
OPEN No1 All except LABOFF The tape is rewound.
OPEN No1 LABOFF Assumed position is immediately after the tape mark that ends the data.
CLOSE Yes SL|SUL|AL|AUL|NSL option The tape is left immediately before the header label group.
CLOSE Yes LABOFF, BLP, NL, no label processing option The tape is left immediately before the first data block of the file.
CLOSE No2   The tape is always rewound.

For more information on the RDBACK option, see the DFSMS Macro Instruction for Data Sets.

1 If neither LEAVE nor any other positioning option is specified on the OPEN macro, then positioning depends on the FILEDEF options. If you specify the LEAVE option on the FILEDEF command, positioning is as if you specified OPEN LEAVE.
2 If neither LEAVE nor any other positioning option is specified on the CLOSE macro, then the positioning option is taken from the FILEDEF DISP PASS or KEEP parameter, if any was specified. CLOSE with no options and FILEDEF with the DISP and PASS options behaves like CLOSE LEAVE.