OS/8 FORTRAN IV

[Vincent Slyngstad]

From: Charles Dickman: Wednesday, December 28, 2016 7:43 PM
> What happens when a PDP8/e executes an IOT to a non-existent device?
> My PDP8/e is skipping when it executes a printer flag test for a
> device that is not present.

IOT instructions for non-existent devices should be a no-op.
Skipping for a non-existent device is a bug.

> I tried the following FORTRAN IV program with OS/8 F4 on simh and it
> worked fine.
> 
> C     HELLO WORLD PROGRAM IN FORTRAN IV
> C
>      WRITE(4,100)
> 100   FORMAT(" HELLOW WORLD!")
>      END
> 
> even with hello spelled wrong.
> 
> When I tried it on real hardware it just hung.
> 
> It looks like it is stuck in an interrupt loop.

That may be a different problem.  It is known that the Fortran 
runtime is vulnerable to interrupts from unexpected devices, 
which is why many of the Omnibus devices can be configured 
to default to an "interrupt masked" condition on power up.

> I don't have any printer devices installed, so it is strange that is
> skips on the printer char done flag. When there is no device, what
> does the IOT do? I would expect a NOP with maybe AC corruption, but
> not a skip.

It should do exactly nothing, falling through to the next instruction.

    Vince