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

[Johnny Billquist]

On 2020-03-23 19:29, Ethan Dicks wrote:
> 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.

Actually, there are also 64K cards. But with 256K cards, one MK11 is 
enough to fully populate the memory of an 11/70. So there was no obvious 
need to make the MK11 capable of taking larger cards.

However, that did not prevent some from doing a hardware hack to allow 
the MK11 to use 1M cards. I used to run an 11/70 with that about 25 
years ago.

But it requires a small custom PCB to be made and inserted into the MK11.

I would suspect for the 11/730, it might just have assumed that it was 
1M cards. Makes the logic a bit simpler, since you then know how to 
address each card with very simple decoding.

>>    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.

Obviously with more memory, it would swap less. But since the machine is 
slow as molass anyway, and if you were to add more card select lines, 
you also would need a larger backplane, which is definitely non-trivial, 
I very much doubt the point of ever doing this...

   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