[Simh] Timing dependencies in operating systems

[Bob Supnik]

The RSX TPC issue is just one example of numerous timing dependencies 
that have cropped up over the years. There's a paper on the SimH web 
site about some of my favorites ("Bug, Feature, or Code Rot?"). If I can 
ever find Tim Shoppa and get access to the web site again, I'll update 
the paper.

There's no real solution short of introducing accurate timing and then 
modeling every device with real world delays. I remember how real DEC 
machines worked -- slowly -- so I'm not much of a fan of that idea. 
Further, DEC IO devices were attached to systems with different 
performance levels. A delay that's right for the 0.6mip J11 may be dead 
wrong for the 2.5mp CVAX and vice-versa. (Indeed, between VAX NetBSD, 
VAX OpenBSD, and RSX, there is no single value for the RQ's queuing 
delay that will work properly.)

As particular issues crop up, minimum delays are adjusted to accommodate 
as much of the software base as possible, but no solution that allows 
the simulators and their IO to run faster than real time will be perfect.

/Bob