[Simh] Software, firmware, and friends

[Seth Morabito]

[I seem to have missed the original message from Bob, so replying
via Johnny's message.]

On 2017-03-12 22:51, Bob Supnik wrote:
>In general, I am no longer a fan of "approximate" simulations. If you
>just use the spec and ignore the implementation, then even if some
>particular test case (VMS Vxyz) works, the next piece of software may
>fail. I've seen this repeatedly - how the initial simulation of the RH
>worked with all DEC operating systems but failed with Unix, because a
>critical screw-up in the interrupt logic wasn't implemented faithfully;
>how the 750 simulation ran VMS but failed with BSD, because the UBA was
>a simple clone of the 780 (it's still wrong in critical aspects, as is
>the RH750); how the MicroVAX II & III/QVSS combo failed with Ultrix,
>because Ultrix cheerfully violates the SRM and the hardware just works.
>I'm still trying to work out the mischief that the SDS 940's tape drive
>perpetrates. The devil is in the details.

This has plagued me continuously while trying (and often failing) to get
the 3B2 simulator working. The lack of documentation from AT&T means I
have had to get at everything via reverse engineering, and that means
that sometimes I must make educated guesses, and sometimes those guesses
are wrong. And other times, even when looking at the real hardware under
a logica analyzer, I may misunderstand what I'm seeing, or make wrong
assumptions about what I'm seeing. It is very frustrating, and slow, and
painful.

I'm facing a situation now where I believe I am very faithfully
following the microsequences _as described_ in the WE32100 manual, but I
am seeing results that lead me to believe the manual is incorrect, or
more likely incomplete. (The manual, after all, is targeted at people
trying to write software for the WE32100, not people trying to
implement it, so I can't really blame the authors)

So, I agree completely. The 3B2 simulator is unfortunately an approximate
simulation, rather than fully faithful to the original. And I am
unlikely to ever know HOW approximate it is!

-Seth
-- 
Seth Morabito
web at loomcom.com