[Simh] Is it possible to simulate the first Vaxen I ever used?

[Johnny Billquist]

On 2020-03-23 14:33, Robert Armstrong wrote:
>> Ethan Dicks <ethan.dicks at gmail.com> wrote:
>> Using a PDP-8 as an FEP on any VAX definitely sounds odd.
> 
>    The console front end for the 730 was an 8085 (just like the KS10, FWIW).

Trying to remember. Not sure, but I think the 11/750 might have had an 
8085 as well.

>    The 730 was interesting in that ALL of the CPU microcode was in RAM and was loaded by the CFE at boot time.  It was possible to locally modify the 730 microcode, and DEC even had a set of microcode development tools for the 730.  I've never seen them except in references.

The 86x0 also loaded all microcode from storage by the FE at boot. In 
this case, the RL02. I think I have enough documentation that it could 
be possible to do something with it, but I have never tried...

The 11/750 could patch the microcode, but only a limited amount of it. 
Ultrix (as well as NetBSD) comes with a patch set for the VAX-11/750 
microcode. Not sure exactly how it worked on the VAX-11/78x.

>    This is relevant because for years I've heard a persistent rumor that the PDP-8/WPS-8 group at DEC had a 730 with microcode that had been hacked to include a PDP-8 compatibility mode, which they used for development.  It was faster than real -8 and supported timesharing to boot.
> 
>    I wonder if this is the source of the original poster's memory?  Can anybody confirm or deny this rumor?

Paul already commented this, and my understanding matches his actual 
observations. This was all on an 11/60. Said to have been the fastest 
PDP-8 around.

   Johnny

-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email: bqt at softjar.se             ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol