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

[Ethan Dicks]

On Mon, Mar 23, 2020 at 1:30 PM Robert Armstrong <bob at jfcl.com> wrote:
>
> > Johnny Billquist <bqt at softjar.se> wrote:
> >The memory bus on the VAX-11/750 is the same as for the MK11 box for the
> >PDP-11/70, and they shared some memory cards.
>
>   Same memory was used on the 730 too.  The 730 memory cards were always 1MB though - never saw any other size.

Yes.   The 11/70 supported the 256K cards only, the 11/750 supported
either the 256K cards or the 1MB cards (early 11/750s lacked one
address wire on the memory bus - I added it to our machine S/N
BT0000354), and the 11/730 (and 11/725!) supported only the 1MB cards,
but I don't know if that was because of memory controller limitations
or because you could only stuff 5 cards in the box and 1.25MB wasn't
enough to run VMS.

>   I don't know the actual number of physical address bits implemented on the 730, but as a practical matter 5x 1Mb cards was the most you could fit in the chassis.

I remember looking into this a few years back - ISTR the PALs only
generate 5 memory select lines (that go to each memory-capable slot)
and there were no unused pins on that PAL.

It seems possible to rework this, but I don't think it would be a
trivial mod.  An 8MB 11/730 wouldn't be all that much better than a
5MB 11/730.  Some.  I remember using ours as mostly a single-user
machine in the late 80s because by the time a third person logged in,
it started swapping like a fiend.

-ethan