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 _______________________________________________ Simh mailing list [email protected] http://mailman.trailing-edge.com/mailman/listinfo/simh
