PDP-11 addressing question and a model round-up

From: Pete Turnbull <pete_at_dunnington.u-net.com>
Date: Mon Feb 14 13:30:16 2005

On Feb 14 2005, 9:21, Vintage Computer Festival wrote:

> So then basically, the difference is the 11/23 I was using before had
> 18-bit addressable memory, and the 11/23+ I'm using now has 22-bit.

Well, um, sort of. ODT on a QBus machine always uses physical memory.
 Moreover, the top bank of 8KB, the I/O space, is selected by a
specific bus signal -- unlike on Unibus. However, early versions of
the KDF11-A had 18-bit ODT while later versions, and all KDF11-Bs, had
22-bit ODT. All would still happily cope with 22-bit memory under
control of the MMU (as far as I remember -- someone please correct me
if I'm mistaken), providing they are in a 22-bit backplane.

The other point worth noting is that if you give "too many" bits in an
address to ODT, it just discards the excess on the left, so in 18-bit
ODT, 17777170 is treated exactly the same as 777170 is, and so is
77777777170. One useful result of this is that, although you can't
backspace in ODT, you can just add more digits, so if you type 777170
when you meant 777172, just turn it into 777170777172.

The upshot of this is that if you put a 22-bit QBus processor in either
an 18-bit or 22-bit backplane, its 22-bit ODT will address the I/O page
at 17760000-17777777. This will always work because access to I/O is
not controlled by decoding the address on the bus, but by the BBS7 (Bus
Bank Select 7) signal. However, if the backplane is only 18-bit,
access to memory -- which is done by address decoding -- above 777777
(18 bits) won't be possible. The four extra address lines, left
floating, will appear to be zeros because the bus is negative logic
(logic 1 = 0V, and 3.5V or greater (or floating) = logic 0).

At the other extreme of PDP-11 bittyness, if you put an 11/03 in any
backplane, its 16-bit ODT will address the I/O page at 160000-177777,
and since there's no MMU for an 11/03 system, 64KB is what you always

If you put an early KDF11-A in an 18-bit backplane, its 18-bit ODT will
regard the I/O page as being at 760000-777777, and the system will only
be able to deal with 18-bit memory. However, IIRC, the same KDF11-A in
a 22-bit backplane will still address I/O as 760000-777777 in ODT, but
when running with an MMU, it will actually be able to map the whole
22-bit range. I think. I no longer have an 18-bit one to verify that.

Pete						Peter Turnbull
						Network Manager
						University of York
Received on Mon Feb 14 2005 - 13:30:16 GMT

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