4.3BSD Quasijarus

[Ethan Dicks]

On Sat, Dec 27, 2008 at 8:29 PM, Steven M Jones <classiccmp at crash.com> wrote:
> Ethan Dicks wrote:
>>
>> The 11/730Z configuration (same backplane as for the 11/725) only has
>> room for five 1MB boards.
>
> My question is whether the Nebula was limited to addressing 5MB, or is it a
> matter of open/correctly-wired slots? Any way to get past 5 memory boards if
> you have a BA11, for instance? I don't have the machine or my UNIBUS VAXen
> handbook handy...

Yeah... that would be a great question to have answered... from what I
recall from 20 years ago, there's 5 slots in a KA730 backplane wired
to work with memory, but only enough to refresh 5MB of 1MB cards.  You
could stuff two 4MB cards in a late-model 11/750 with a newer memory
controller (plus six 1MB cards), but the 11/750 could *not* be
expanded more because it's VAX address space was limited to 24 bits of
RAM out of 32 bits total (16MB total address space, like an MC68000).

I haven't closely examined the KA730, but *if* one could fill the
entire address space below 8MB with RAM, one could theoreticaly make a
new-build 8MB single card for it, perhaps with a fresh address line
run to a memory slot.   I would be surprised to hear that one could
exceed 1MB/slot in an unmodified KK730 backplane.  These days, I'd
personally recommend sets-of-four SRAMs to make a 32-bit-wide
datapath, making 2MB trivial to put one one card, *if* the memory
slots would easily support that.  I've run wires on a 256K-card
VAX/11-750 backplane.  I don't mind finding a way to support 8MB in
one to four cards on an 11-730Z backplane, even if it necessitates
fresh backplane wires to pass along extra multiplexed address bits.
Encoders and decoders are plenty cheap compared to limited backplane
realestate.

-ethan