HP 3000 Manuals HP 3000 Manuals
Interrupt Handler The interrupt handler is a special procedure of the process, devoted to completing the I/O request after an interrupt occurs. This procedure is never called explicitly. Instead, MPE XL invokes the interrupt handler when a software interrupt occurs, possibly from the middle of a statement. The procedure declaration can be either without parameters, or have a single INTEGER parameter into which MPE XL puts the file number. The file number parameter of the software interrupt handler is useful if there are several files sharing the same software interrupt handler procedure and some require special handling. For example, each file may have a different buffer address passed to IOWAIT. MPE XL automatically disables software interrupts and CONTROL-Y traps when it jumps to the interrupt handler. A call to IODONTWAIT or IOWAIT is needed to finish the I/O request. The I/O request can be completed immediately, so IOWAIT and IODONTWAIT will work the same way. (IOWAIT always completes immediately.) Check the condition code (a good idea after any intrinsic call). CCG means the interrupt occurred because of an End-Of-File condition. If a CCE is returned, the process has a record. It can be processed here, or a flag can be set to indicate that the record has been received and should be handled during the normal non-interrupt processing. Often, at this point, the next software interrupt read is set up by performing another FREAD, if the process wants to continue to see new records written to this message file. The FREAD does not have to be done here and could be performed elsewhere during normal processing. The process may not want to issue any FREAD at all, if the command just received was to stop immediately and terminate. The last statement in the interrupt handler must be a call to FINTEXIT. This allows the process to pick up where it left off when the interrupt occurred, enable CONTROL-Y traps, and optionally leave software interrupts enabled or disabled. If this call is omitted, the process may never get back to where it was before the interrupt. Exiting with software interrupts enabled is usual, but the process may leave them disabled if the record needs special processing and it does not want any additional interrupts until it is completed. At that time it needs to call FINTSTATE to enable interrupts.