Software interrupts are the result of an INT instruction in an executed program. It may be assumed as a programmer triggered event that immediately stops execution of the program and passes execution on to the INT handler. The INT handler is typically part of the operating system and will determine the action that should be taken (for example output to execute, file screen etc.) An instance is INT 21h, whereas it is the DOS service interrupt. Whenever the handler is called it will read the value and stored it in AH (sometimes even AL) and then jumps to the right routine.