[Simh] Subtle H316 TTY interrupt bug?

[Bob Armstrong]

Bob -

  Thanks for checking this out.  Did you actually pull out the H316
schematics and work it out?  I'm impressed - I was too lazy to do that!

  FWIW, I've "fixed" my copy of simh by the simple expedient of using
INT_NODIS to disable interrupts for one instruction after any OCP 4 that
switches to output mode.  (This is the same interrupt flag the ENB
instruction uses to defer interrupts for one instruction.)  It's a fairly
trivial mod to the code - in file stddev.c, routine ttyio(), modify the code
like this -

        if (fnc && (tty_mode == 0)) {                   /* input to output?
*/
            if (!sim_is_active (&tty_unit[TTO])) {      /* set ready */
                SET_INT (0,INT_TTY);
		    // [RLA] Fix the "dummy cycle" problem (see above!)...
		    CLR_INT (0,INT_NODEF);  // [RLA]
	    }
            tty_mode = 1;                               /* mode is output */


Admittedly this is not exactly what the real hardware does, but it's a close
enough approximation to solve the immediate problem and it's easy to
implement.  I don't believe it has any bad side effects.

  FWIW, I'm trying to resurrect some real H316 software that uses exactly
the OCP 104/ OTA 4 sequence described, so it's not entirely an academic
concern.  

  Thanks again for double checking this.

Bob Armstrong