The Break panel provides a means of enabling or disabling what is referred to as "PER mode" operation. The method of enabling and disabling PER mode is discussed later; this section explains the difference between the two modes of operation.

PER is an acronym for "Program Event Recorder". It is a feature of the z/Architecture hardware, and is only available in EC (Extended Control) mode.

When PER mode is enabled through the Break window (the default is for IDF to disable it), IDF puts the processor in EC mode before turning control over to the target program. This lets IDF use the PER instruction fetch, register alteration, and storage alteration features to monitor your program's execution. Single-stepping and breakpoints are available in either mode. Register stops and address stops (storage alteration stops) are available only in PER mode.

Whenever possible, IDF implements breakpoints by means of inserting an invalid opcode at the break address. In the case of a read-only DCSS, this is not possible. In this case IDF needs to use PER instruction fetch events to implement breakpoints. This can only be done when you have set PER=Y.

When you debug your program, allowing for these benefits and disadvantages leads to better results and fewer surprises.

You are not committed to either mode for the entire debugging session. Whenever IDF reaches a breakpoint and is therefore able to display the Break window, you can change modes.