DEC PDP 11/23 - OK to use 22-bit memory boards in an 18-bit qbus?

Pete Turnbull pete at
Tue Jul 20 13:54:46 CDT 2010

On 20/07/2010 19:20, Scott M wrote:

> I recently acquired my first DEC PDP.  It is a VT103 (VT100 terminal with 
> a PDP 11/23 inside).

It would originally have been an 11/03 card.

> It came with a pair of cartridge disk drives (HEAVY!) 
> that are RK05 compatible, a DEC RX02 (pair of 8" floppy disk drives), boxes 
> of manuals, 3 DEC operating systems, software, and an extra VT100. 
> While the DEC operating systems are OK, I am actually interested in using 
> the machine to (finally) go through "Lions' Commentary on Unix" (Unix V6), 
> using software downloaded from  (Lions' book here): 
> After I do that, I want to move up to Unix v7, 2.9BSD, and try other Unix 
> distros available for the PDP. 
> My concern is that the 18-bit PDP 11/23 does not have enough memory. 
> It currently has 128KB installed, with a maximum of 256K possible after a 
> memory board upgrade (M8067, MSV11-PK = 256KB).  How far do you estimate I 
> can get with just 128KB memory?  Can I get through Lions' book? 

Probably.  My 11/23 runs Seventh Edition UNIX in 256KB quite happily. 
Modulo some swapping to disk, that is.

> I am looking at adding wires to the backplane to make it a 22-bit machine, 
> and using a quad height qbus memory board, M7551 (available in 1MB, 2MB and 
> 4MB sizes).  However, I am not ready to dive in and start modifying the qbus 
> backplane for 22-bit addressing just yet.  So my question is: 
> Rather than invest in a 256KB memory board designed for the 18-bit qbus, 
> can I install a 22-bit 1MB, 2MB or 4MB memory board in the 18-bit system, 
> and just use the first 256KB for now?

Yes, you can do that.  Just check the power requirements, and whether 
the specific card(s) are compatible with a serpentine backplane.

   Also, is the VT103 backplane
> compatible with a quad-height memory board?  (It only has dual-height 
> cards installed at the present time). 
> Parts inventory:
> - VT103, with: 
>   - 4x4 qbus backplane, 18-bit.  The manual says "two H803s (2x4 connector 
>     blocks, stacked vertically in a 4x4 configuration)".  I know there are 
>     possible issues with "straight" vs. "serpentine" slots, but I have not 
>     got that figured out yet on the VT103. 

It's serpentine.  Every dual slot can hold a dual card; some quad cards 
are compatible and some not.  There's some backplane information at
though that particular version isn't listed.

>   - M8186 KDF11-A CPU (Newer revisions of KDF11-A's are 22-bit capable). 
>     says: "(Prior to etch rev. C, 18-bit addressing only. ...)" 
>     I could not find revision information on the M8186 board, but "146 CA" 
>     is stamped into one of the red plastic handles.  I can email close-up 
>     photos to anyone who wants to take a look and determine if this is a 
>     rev C or later board capable of 22-bit addressing. 
>   - M8043 Quad Serial board.  This doc says it is 22-bit compatible: 

Yes, it is.

What you need to know about I/O devices is that they don't fully decode 
all the memory address bits.  All I/O lives in the highest address page, 
and there's a special enable signal called BBS7 that is active when (and 
only when) the CPU accesses that I/O page.  All sensible QBus I/O cards 
decode that signal and only use the lower address bits to determine the 
address within the I/O page.  Quite a few memory cards use BBS7 to know 
when to be DISabled, or have some other way of disabling the top 8K, so 
as not to interfere with I/O.

>   - M8029 (RXV21) floppy disk board (18-bit DMA only).  -Will this be a 
>     problem with Unix v6 or 2.9BSD if running with 22-bit addressing? 

I'm not sure either of those actually have drivers for an RXV21.  That 
card will work OK in a 22-bit system without clashing with any memory 
addresses, but is only useful if the driver (exists and) understands to 
put the DMA buffers in the lowest 256KB.  Some OS versions do exactly 
that: they tell the card to transfer data to/from low memory and then 
the CPU moves it if necessary (later version of RT-11 do that, for example).

>   - M8208 "VT103 Maintenance Module" (Unknown if 22-bit compatible). 
>   - Xylogics C510 (formerly called "Wizard 1") cartridge disk controller 
>     board, with 18-bit addressing, connected to a pair CDC 9427H "Hawk" 
>     disk drives (one disk drive is DOA, but the other is OK). 
>   - 3rd party memory board (128KB), Christlin Industries, Inc. 

Dunno about any of those, sorry.

Pete						Peter Turnbull
						Network Manager
						University of York

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