Speaking of 11/73s

From: Jerome Fine <jhfine_at_idirect.com>
Date: Thu Sep 2 17:35:09 1999

>Jacob Ritorto wrote:

> Allison,
> I remember you mentioning the PMI modification that makes the
> 11/73 into an 11/83 on INFO-pdp11 a few years ago. Could you please give
> step-by-step instructions on doing that here? Does 2.11BSD have the
> smarts to use this, or is it a wholly-hardware-incarnated thing? I notice
> my '73 lagging a bit on memory-intensive stuff.

Jerome Fine replies:

FIRST, you require a Qbus backplane with as many slots as the memory
and CPU will occupy. In general, I have heard that you should NOT mix
boards with different sizes of PMI memory - Since I have two MSV11-JE
boards (2 MByte PMI memory - the maximum DEC made was a 2 MByte
memory board - I am told that Chrislen made a 4 MByte board - anyone have
it at a reasonable price, especially if you are not using it anymore), that is not
a problem. The DEC MSV11-JD was a 1 MByte memory board. These
two boards were to be used ONLY for the Qbus. On an 11/84 system,
DEC managed to require the use of either the MSV11-JB (1 MByte) or
the MSV11-JC (2 Mbyte). I have been told to NEVER mix these boards
between an 11/83 and an 11/84 system. Don't know why.

Now, assuming you have 2 (or 1) MSV11-JE memory, you MUST place it
in an ABCD slot above the CPU board which must be the KDJ11-BA,B,C
(M8190-"blank",YB) which is the quad version of the 11/73. PMI will not
work with the KDJ11-AA,B,C which is the dual version of the 11/73 (M8192)
or at least if it does (I have never bothered to try), it will operate as ordinary
Qbus memory regardless of where it is placed in the backplane. I also them
place the 11/73 CPU board as the very next board right after the PMI memory
and the CPU board MUST also occupy an ABCD slot. If you are using a
BA23 with 3 ABCD slots (all BA23s that I know of have exactly 3 ABCD
slots), the the first 2 slots MUST be the PMI memory and the 3rd slot MUST
be the quad 11/73 CPU. If you have only one PMI memory board, make it
the first slot and the CPU the 2nd slot. I presume (I have never had 3 * 1 MByte
boards of PMI memory) that on a BA123 with 4 ABCD slots, it is possible to
have 3 PMI memory boards at the top and the CPU in the 4th slot.

If you place the PMI memory under the CPU, it still works as normal Qbus
memory. Someone (either Allison or Megan Gentry) once explained how PMI
memory works, but I forget. I have never tried to use one board above and
one board below - has anyone ever done so?

Based on a VERY limited set of benchmarks, PMI memory seems to speed
up the system (CPU wise) by about 20%. Moving to a KDJ11-EF (11/83)
seems to add another 13% for a total of about 33% between an 11/73 at
15 MHz without PMI memory and an 11/83 with PMI memory. So if
you want to increase the speed, the major boast comes from PMI memory
and works with all quad 11/73 that I have tested PMI memory with; just
place it on top with memory and CPU underneath, all in ABCD slots.

I have been told that PMI memory will not function as PMI memory with
an 11/53.

The increase in speed is purely a hardware function and does not depend
on the OS or anything else, although I have seen RT-11 able to "know"
that PMI memory is or is not present - have not ever bothered to find
out how.

Sincerely yours,

Jerome Fine
Received on Thu Sep 02 1999 - 17:35:09 BST