PDP-11 memory question

From: Jerome Fine <jhfine_at_idirect.com>
Date: Mon Jan 7 10:11:23 2002

>Tom Leffingwell wrote:

> >On Sun, 6 Jan 2002, Pete Turnbull wrote:
> > Many of the QBus processors have a set of terminators on the CPU card, and
> > the QBus spec allows for the use of short bus length with light loads and
> > no extra terminators. The definitions of "short" and "light" are subject
> > to variation at short notice :-)
> I guess this explains why my system has no BDV11 board in it. Although it
> doesn't completely work either.

Jerome Fine replies:

I may have had a compatible set of boards for the system when I ran a VT103
with the same 4 * 4 backplane - upgraded as I mentioned to 22-bits for the
addresses so that more than 256 KBytes could be used. What I did forget to
mention again was that an RD51 drive was placed under the tube inside the
VT103 and run on the same internal power supply.

> > This sounds familiar. And to be honest, if you are running anything other
> > than RT11SJ, you probably have a lot more than 28KW of memory anyway, so
> > the saving is minimal. My advice would be not to bother trying to squeeze
> > another 2KW out of the system.
> I'm running RT11XM, and it doesn't seem like I need an extra 2KW.

If I remember correctly, when the MMU is activated in an 11/23, the IOPAGE
size defaults to 4 KWords or 8192 Bytes. So with RT11XM, the 2KW IOPAGE
is not possible.

Do you know if the application programs are using extended memory?
If so, they gain much more than they loose, especially if they IOPAGE is
not required - and even more so if they can use more than one window.
As I stated in another post, RT11XM allows the user to request all the addresses
from 20000 to 177776 (56 KBytes) to be set up in extended memory
while still keeping PAR0 in the normal physical space from 00000 to 17776.
This restriction is essential when an ISR (Interrupt Service Routine) is
required. Otherwise, if the program uses memory in the normal manner
(MACRO.SAV is a good example), the use of VBGEXE automatically
sets the program up COMPLETELY in extended memory and makes
all 64 KBytes available for code and especially data or tables. And
providing sufficient additional extended memory is available, more extended
memory can still be used as well - although the two are rarely combined since
once a program starts to use extended memory, it usually does everything
for itself.

Note that VBGEXE probably can't be used in RT11XM when the program
uses VIRTUAL arrays since I seem to remember that one of the upper PARs
is required. So there are some limitations.

Sincerely yours,

Jerome Fine
Received on Mon Jan 07 2002 - 10:11:23 GMT

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