MODIFIED DATA TAG (MDT) - IBM Mainframe

MDT is a one-bit attribute character. If it is off (0), it indicates that the terminal user has not modified the field. If it is on (I), then it means that the field has been modified. Only when the MDT is on, will the data of the field be sent by the terminal to the host program. A judicious use of MDT drastically reduces the amount of data traffic thus improving performance significantly. There are 3 ways of setting and resetting MDT. They are:

  • When the user modifies a field the MDT will be automatically set to ON.
  • If CNTL=FRSET is specified in the DFHMSD macro or DFHMDI macro, when the map or mapset is sent to the terminal MDT will be reset to 0 or OFF for all fields of the map or mapset.
  • If FSET is specified in the ATTRB option of the DFHMDF macro, when the map is sent to the terminal, MDT will be set to 1 or ON for this field regardless of whether the field has been modified by the user.

FRSET is very useful if the same map is send over and over again to the terminal for data entry. By specifying FRSET MDT will always reset to OFF whenever the map is sent. Another factor that creates problem is that, if the MDT of output field is set on then the data will be sent to the host computer, which will distort the alignment of the data in the symbolic map and increase the data traffic. So for output fields FSET should not be placed. Another problem is in displaying default values; say current date, in the screens for making the data entry easy. But if the operator does not modify the data, which most probably he won't, it will not be sent to the program. Coding FSET option for those fields thus making the MDT on can solve this problem.


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