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.
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 | 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.
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MPE/iX 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.
