ISRDDN

When you log on to your mainframe TSO (Time Sharing Option) session, you can use the TSO supplied commands. Apart from those commands, if you know how to code in REXX or CLIST, you can come up with your own commands.

There are also occasions when you use commands written by some 3rd party vendors. For example, Quick Reference - you invoke their product by typing "QW keyword". Sometimes you wonder, how does TSO know these commands exist or where to get the source/load from - the source could be in a lot of different datasets.

When you logon into your TSO session, the system preallocates a bunch of datasets. This is determined by your logon PROC. Usually the system programmer determines the appropriate logon PROC for each user. For example, a developer might need a set of datasets preallocated, while a DBA might need a different set of datasets preallocated. So, when you type QW in the command line, TSO searches in these datasets and executes it.

To find out which are all the datasets that have been allocated to your TSO session, type ISRDDN (in some shops, you may have to type TSO ISRDDN) in the command line and hit enter. You will get all the datasets that are allocated and the corresponding DD names. If you want to find the source of a particular CLIST, REXX, Panel, etc., just type "M membername" (without quotes). e.g. M qw -- will search for the source code or load for QW.