How the sliding window affects scrolling and database position

Databases potentially are very large. FM/IMS has to work with limited memory, which means that FM/IMS cannot always hold every segment in the database within memory.

Instead, FM/IMS works with a sliding window that provides you with a look into the database. The top of this window is shown on your IMS data panel by Top of window line. You see this on the IMS data panel if you enter the TOP command. The sliding window holds approximately 500 segments, though this depends on the size of the individual segments, and the structure and size of the database records.

The top of window might also be the top of the database, but there is no immediate way of knowing this. You can position yourself at the top of the database by blanking out the root segment key value in the Database Positioning panel, or by entering the ROOT FIRST command. (The top of the database is determined by the selection criteria that are currently in force.)

If you are looking at data on an IMS data panel, and you keep pressing Up function key (F7), eventually you get to the Top of window line. You cannot go back beyond this line by pressing Up function key (F7).

When the sliding window has reached its maximum size, the top of window moves down (toward the bottom of the database) whenever you do anything that would add to the segments within the sliding window. So if, for example, you enter the BOTTOM command, or press the Down function key (F8), the top of the sliding window moves down. Other commands that might change the contents of the sliding window are CHANGE NEXT, CHANGE ALL, RCHANGE, DOWN, FIND NEXT, RFIND, KEY, ROOT FIRST, and UP MAX.

The TOP command is constrained by the sliding window.