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

[Paul Koning]

> On Mar 23, 2020, at 10:03 AM, Robert Armstrong <bob at jfcl.com> wrote:
> 
>> Paul Koning <paulkoning at comcast.net> wrote:
>> ... extended microcode to run PDP-8 code faster.  But it was an 11/60
> running RSTS/E.
> 
>  So it is true (well, sort of, but with a PDP-11/60 rather than a
> VAX-11/730)??  Do you know any details of how it was implemented?  If it ran
> RSTS then it was obviously still a PDP-11 at the same time it was emulating
> a PDP-8.  Was there a -8 "compatibility" mode, something like the -11 mode
> on the VAX?  

RSTS/E had a general mechanism called "run-time system", vaguely like a cross between a shell and a shared library.  The earliest officially released one (other than BASIC) was the RT11 runtime system, which provides RT11 system call emulation (SJ subset) and a simple V2-style command line interface.  This was done to allow Sysgen to be run on RSTS, starting with V5B.  Later RSX emulation was added via the RSX runtime system, which eventually migrated into the kernel.

Runtime systems can also implement languages or the like; BASIC, TECO, FORTH, and ALGOL are runtime systems and basicaly implement virtual machines for the language in question.

I think that the PDP-8 emulation was done as a runtime system, with WCS microcode to help execute the instructions faster than they could have been emulated in PDP-11 code. I'm fairly sure that Richie Lary, one of the most famous PDP-8 developers in DEC, did much of the work, and presumably the microcode in particular.

	paul