Question about memory boards used on VAX 11/7xx

From: Ethan Dicks <>
Date: Thu Dec 30 16:34:17 2004

On Mon, 27 Dec 2004 10:52:24 -0500, Patrick Finnegan
<> wrote:
> On Monday 27 December 2004 05:35, Pierre Gebhardt wrote:
> > Hi all,
> >
> > I got 2 Memory Boards (National semiconductor) several weeks ago
> > (each 1MB). Somerwhere on the net, someone says, that these can be
> > used in VAX 11/7xx. Can I use these boards in my PDP 11/24, too ?

No. There are 1MB boards for the 11/24, but they are somewhat odd,
IIRC. I have an 11/24 at home with a KT24 (M7134?) Physical Address
eXtension (PAX) board that lets you put more than 256K of memory in
the 11/24, giving it a slight edge over the 11/34, at least in terms
of max memory (the 11/34C might be able to crunch numbers a bit
faster). ISTR paying about $300 for it back in the late 1980s when I
was trying to fiddle with 2BSD. If you don't have one, you are
limited to 18 address bits. I found one advertised for $60 at this
place ->
I reasonably confident that the 11/24 and 11/44 used the same memory.
For 256K boards, it should be the M8722 (MS11-MB). Grubbing around on
seems to suggest that the 1MB version is the M8743 (MS11-PB), but I
don't have my hardware close at hand to check my own machines. Max is
4MB (as with any 22-bit address bus).

( see: for general
PDP-11 stuff )

> I doubt it. The 11/24 uses UNIBUS memory, and the 11/78x and 11/750
> both have a special memory bus and controller (or multiple
> controllers).

In particular, the 11/750 has only one controller (unlike what's
possible in the 11/78x line). There are 3 that I know of - the
original one can only handle 256K boards (4116 DRAMs), the more common
one can handle 1MB boards (4164 DRAMs, typically), and the newest one
can handle, I _think_, two 4MB boards and enough 1MB boards to take
the 11/750 up to 14MB total. There is a wire addition to the
backplane to go from 2MB total to 8MB total (extra address line). I
would expect some sort of similar change for the next bump, but I have
never done it.

> I don't know enough about the 11/730 (& 11/725) to say
> "definitely no" but it seems quite unlikely that they used straight
> UNIBUS memory.

the KA730 (11/730 and 11/725 CPU) does _not_ use Unibus memory... it
has a set of dedicated 32-bit-wide memory slots on a custom CPU
backplane, _next to_ the 16-bit Unibus slots. The max there, IIRC, is
5MB total memory.

Received on Thu Dec 30 2004 - 16:34:17 GMT

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