No debug-mode is the selected default by on power-on reset. Monitor debug-mode must be selected after reset by setting DSCR[15]. See CP14 c1, Debug Status and Control Register (DSCR). When a software debug event occurs, as Software debug event describes, and Monitor debug-mode is selected and enabled, then a Debug exception is taken, although Prefetch Abort and Data Abort vector catch debug events are ignored. Debug exception describes debug exception entry. The Prefetch Abort handler can check the IFSR, and the Data Abort handler can check the DFSR, to find out the caused of the exception. If the cause was a Debug exception, the handler branches to the debug monitor target. When the debug monitor target is running, it can determine and modify the processor state and new software debug events can be programmed.