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

-----Original Message----- From: cctalk-bounces at classiccmp.org [mailto:cctalk- bounces at classiccmp.org] On Behalf Of Scott M Sent: Tuesday, July 20, 2010 11:20 AM To: cctalk at classiccmp.org Subject: DEC PDP 11/23 - OK to use 22-bit memory boards in an 18-bit qbus? Hello, I recently acquired my first DEC PDP. It is a VT103 (VT100 terminal with a PDP 11/23 inside). 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 http://www.tuhs.org/. (Lions' book here): http://www.amazon.com/dp/1573980137/ 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? 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? Also, is the VT103 backplane compatible with a quad-height memory board? (It only has dual-height cards installed at the present time).