Reanimating a PDP-11/23

Ian King IanK at
Fri Dec 4 21:31:09 CST 2009

Just to expand slightly on Dave's concise response and avoid further churn: the PDP-11 departed from the earlier practice of special I/O instructions and implemented memory mapped I/O.  The top 4 kW is reserved for I/O devices, with a special signal being generated on the LSI-11 bus that 'enforces' this.  So a PDP-11 without memory management will allow up to 28 kW; an 18-bit bus will allow up to 124 kW.  I'll let you do the math for a 22-bit memory management system.  

I'm glad things are coming together!  Cheers -- Ian 
From: cctalk-bounces at [cctalk-bounces at] On Behalf Of Dave McGuire [mcguire at]
Sent: Friday, December 04, 2009 2:57 PM
To: General Discussion: On-Topic and Off-Topic Posts
Subject: Re: Reanimating a PDP-11/23

On Fri, December 4, 2009 5:52 pm, Pontus wrote:
>> Then, assuming the cable really is correct (have you tried swapping Tx
>> and Rx?), is plugged in to the correct
> This was the somewhat obvious error I missed when checking the cable. I
> discovered it by trying another cable which isn't keyed. So I
> accidentally put it in upside down, which incidentally flips Tx and Rx.
> Thanks for putting up with me :)
> So now I get this:
> 28
> Which confuses me, I have a 32kB (or is it kW ?) worth of memory
> installed (M8044-DD) configured to start at address 0. Shouldn't it say
> 32 instead of 28? How does the memory detection work anyway?

  4K for the I/O page..


Dave McGuire
Port Charlotte, FL

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