Main Line Code

For the most part, the "main line" code of a process does not need to be concerned with the I/O to message files using software interrupts. As long as interrupts are enabled, they can occur anywhere in user code. If one occurs during an MPE/iX intrinsic, the interrupt is postponed until you exit the intrinsic and re-enter the process' code.

There are some exceptions. Interrupts can occur during a "generalized" IOWAIT, during an IOWAIT on another message file not using software interrupts, or during a PAUSE.

The use of software interrupts introduces the possibility of a problem that applications normally do not have to think about. Some code is sensitive to interrupts. The problem usually occurs with data that is altered by both the interrupt handler and the main line code.

For example, suppose the main line code decrements a counter and the interrupt handler increments the same counter. The main line code loads the old value and subtracts one from it. Before it is stored back, an interrupt occurs. The interrupt handler loads the old value, increments it, and stores the new value back. The main line code resumes, storing its new value on top of the interrupt handler's new value, and the increment is lost.

One solution is to protect sensitive code by using FINTSTATE (FALSE) to disable interrupts before the operations and FINTSTATE (TRUE) to enable interrupts afterwards.