PDP-11/05 returns to life

Tobias Russell toby at coreware.co.uk
Fri Jan 9 05:25:54 CST 2009

I've tried out this chasing lights code on the 11/05:

	Location	Contents	Opcode		Comment
	001000		012700		mov #1,r0	Load 1 into R0
	001002		000001
	001004		006100		rol r0		Rotate R0 left
	001006		000005		reset		Initialise bus (70ms)
	001010		000775		br .-4		Loop back to 'rol r0'

(from http://www.psych.usyd.edu.au/pdp-11/11_40.html)

This runs, but no chasing lights, I just see what looks like PC activity. So perhaps the 
11/05 doesn't display R0 on a RESET.

On the operating system front, I've built an RT11 v4 RX01 image on SIMH, but its refusing
to work with 16KW of memory (using SJ). Does anyone have any tips for minimizing the memory 
requirements on RT-11?

Finally, I have a choice of RX21 + RX02 or RX11 + RX01 on the 11/05. Beyond the added pain of
having to toggle in a longer bootstrap on the RX02, is there any reason not to go with the
RX21/RX02 combo?


On Thu, 2009-01-08 at 22:10 +0100, Johnny Billquist wrote:
> Paul Koning <Paul_Koning at Dell.com> wrote:
>  >>>>>> >>>>> "Henk" == Henk Gooijen <henk.gooijen at hotmail.com> writes:
>  > >> 2. Is there a chasing lights program I can run on the 11/05? Would
>  > >> be nice to see it visibly doing something
>  >
>  > Henk> Search the net. There is a small program (estimate 7 words). It
>  > Henk> uses the RESET instruction (is that 000005?) ... not nice if
>  > Henk> you connect an RX01 or RX02 to the machine as on every RESET
>  > Henk> (INIT) the heads clunk!
>  >
>  > That's a bit ugly...
> Definitely.
>  > A cleaner one is the RSTS null job.  It relies on the fact that a
>  > number of PDP-11s -- including the 11/05 as far as I remember -- will
>  > display R0 in the lights during a WAIT instruction.
>  >
>  > RT11 won't do, that uses the status register so you need a machine
>  > that has it and that displays it.  I forgot what RSX does.  Not sure
>  > you could run that on an -05 anyway.
> RSX uses the side effect of R0 being shown on the data bus on a WAIT 
> instruction as well.
> And yes, I think you could get RSX running on an 11/05. I don't think 
> there was any PDP11 that couldn't run RSX, but it would be a rather 
> specific RSX you would gen for it. I doubt you could boot any generic 
> RSX system on that machine. Instead you would have to run the sysgen on 
> another machine, and just boot the resulting system on the 11/05.
> Think unmapped RSX. That won't use any MMU. Still, you'd probably want 
> the full 56K on that machine.
>  > Sufficiently old RT11s should work on that machine.  So would RSTS
>  > V4A, if you can find an RK05 or RF11 or RP03 disk drive.   :-)
> New versions of RT-11 should do as well, as long as you select a variant 
> that don't need the MMU. Same goes for RSX, except that RSX won't be 
> self-hosting.
>  > RSTS null job looks like this:
>  >
>  >
>  >	SOB	R1,20$		;KEEP WAITING
>  >	BR	10$		; AND AROUND AGAIN...
>  >
>  > You need a periodic interrupt, of course, to break out of the WAIT.
> And RSX is something similar. Fool around a little, set up R0, and WAIT. 
> The clock interrupt would break you out.
> 	Johnny

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