[Simh] EXTERNAL: Problem booting an emulated VAX-11/780 from a Massbus disk drive

[Alan Frisbie]

I accidentally sent this just to David yesterday instead of the
list.   Here it is to the list:

> In many hangs, the problem turns out to be a timing issue - the
> controller simulation is responding too fast or too slow, based
> on the behavior that the driver expects to see. VMS may be hung
> waiting for an interrupt that has already fired or a register
> value that has already been overwritten - that is, the simulation
> is "too fast" and needs to delay things with a timer.

"Back in the days", I used to work with a company that made
solid-state disk emulators.   I would often get calls from
them to help figure out why a particular drive would pass
all the diagnostics, yet fail to boot, etc.

In almost every case it was a timing issue, as described above.
Bootstrap drivers seemed to be the biggest offenders.   I
think they were written by the same engineers who designed
the real DEC controllers & drives, so obscure (undocumented)
behavior was often depended on.

Emulating the older devices was the hardest because they used
hard-wired logic, which the emulater usually did in firmware.
Thus, delays in the firmware made a *big* difference.   When
DEC started using microprocessors in their own controllers it
got a lot easier, but even then there were occasional problems.

I miss those days.   I had a lot more fun then.

Alan Frisbie