[Simh] Software, firmware, and friends

[Bob Supnik]

Well, I didn't have to ask for it, because I had it... ;) Without it, 
though, I could not have gotten the minute differences between the J-11 
and the other PDP-11s correct.

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.

In the end, the implementer may choose to abstract the actual 
implementation away. But if he or she doesn't know what the 
implementation actually does - based on software, microcode, schematics, 
and so on - it's more or less a shot in the dark. One reason I never 
wrote additional VAX models, beyond the 780 and CVAX, is that microcode 
is not available for all the other 'big' VAXes (750, 730, 8200, 8600, 
8900, 9000), and key specs, like bus adapters and device controllers, 
not to mention boot firmware, are missing for them as well as the 4000 
and 6000 series.

/Bob

On 3/12/2017 12:00 PM, simh-request at trailing-edge.com wrote:

From: Johnny Billquist<bqt at softjar.se>


> Heck, the J11 also have firmware. I haven't seen anyone ask for that
> yet. Instead people implement a PDP-11, and try to make it behave like
> the J11.