User's Guide

BSF

DITTO command ---- Back Space Files

Function:

Backspaces a specified number of files on the specified DITTO device. The user has the option to backspace files on the input device or output device.

Parameters:

DEVTYPE
Device type. Specifies which input/output device is to be processed. Valid values are:


  = INPUT:  Input device
  = OUTPUT: Output device

NFILES(0)
Number of files. Specifies the number of files to be backspaced on the input or output device. The default is to backspace to the beginning of the current file.

Examples:

  1. DITTO> bsf devtype=input nfiles=1

    The user has specified to backspace one file from the current position on the input device.

               Before:
                                                   file ptr.
                                                       |
                                                       v
               ------------------------------------------------
               |            |E|            |E|            |E|E|
               |   file 1   |O|   file 2   |O|  file 3    |O|O|
               |            |F|            |F|            |F|F|
               ------------------------------------------------
    
    
    
    
               After:
                          file ptr.
                              |
                              v
               ------------------------------------------------
               |            |E|            |E|            |E|E|
               |   file 1   |O|   file 2   |O|  file 3    |O|O|
               |            |F|            |F|            |F|F|
               ------------------------------------------------
    

  2. DITTO> bsf devtype=output nfiles=2

    The user has specified to backspace two files from the current position on the output device.

               Before:
                                                   file ptr.
                                                       |
                                                       v
               ------------------------------------------------
               |            |E|            |E|            |E|E|
               |   file 1   |O|   file 2   |O|  file 3    |O|O|
               |            |F|            |F|            |F|F|
               ------------------------------------------------
    
               After:
           file ptr.
               |
               v
               ------------------------------------------------
               |            |E|            |E|            |E|E|
               |   file 1   |O|   file 2   |O|  file 3    |O|O|
               |            |F|            |F|            |F|F|
               ------------------------------------------------
    

Description/Algorithm:

The parameters are input into the program. From the parameter DEVTYPE, it is determined which DITTO device is to be processed. The specified number of files are backspaced from the current position of the DITTO device.

Nonfatal Error Messages:

    None.

Fatal Error Messages:

    None.

User Note:

    None.