HP 3000 Manuals HP 3000 Manuals
Software Interrupt Initialization As with most other files, the message file must be explicitly opened. Software interrupts are usually used when reading from the file, but there is nothing to stop a process from using them when writing. After the call to HPFOPEN the application can perform normal WAIT or NOWAIT I/O on the file. When software interrupt operation begins, the process calls FCONTROL with a controlcode of 48, passing the plabel of its interrupt handler. FCONTROL with a controlcode of 48 returns the previous value of the plabel. If that value is zero, this means that software interrupts were not armed. If the program is using software interrupts on multiple files, FCONTROL with a controlcode of 48 must be called for each file. Each file can have its own interrupt handler, or more than one file can share the same interrupt handler. When a message file has been opened and FCONTROL has been called for the file with a controlcode of 48, that message file is said to have software interrupts "armed." At this point, the process can start the I/O (in this example, assume it is FREAD). FCONTROL with a controlcode of 48 overrides the HPFOPEN NOWAIT I/O option setting. Regardless of the setting of this bit, an IODONTWAIT or IOWAIT is required to complete any I/O started after the FCONTROL, controlcode 48, software interrupt. A call to IODONTWAIT or IOWAIT before the interrupt occurs does not complete the I/O. A CCL is returned, and FCHECK returns the message, FSERR 79--NO NOWAIT I/O PENDING FOR SPECIFIED FILE.
NOTE If the FREAD intrinsic was called before FCONTROL with a controlcode of 48 was invoked, it would have been handled as a normal, non-software interrupt FREAD.
MPE XL starts out with software interrupts disabled. If the FREAD is satisfied, the software interrupt is postponed until interrupts are explicitly enabled. The process uses FINTSTATE at this point to enable software interrupts for all "armed" files opened by this process. This includes all files currently armed, and any yet to be armed by this process, until FINTSTATE is called to disable interrupts. The call to FINTSTATE can occur anywhere in this sequence, but the other intrinsic calls should be made in the order given.