PDP 11/45 light chaser - test-drb@ccmp.vtda.org

24 May 2005

The drawback of the light chaser program that uses the RESET instruction
is that the heads load and unload of the RX02 drive in my PDP-11/40.
That clunck-clunck sound is not pretty for a longer while ...

- Henk, PA8PDP.

...
  -----Original Message-----
 From: cctalk-bounces at classiccmp.org
 [mailto:cctalk-bounces at classiccmp.org]On Behalf Of Paul Koning
 Sent: dinsdag 24 mei 2005 15:40
 To: cctalk at classiccmp.org
 Subject: Re: PDP 11/45 light chaser

 >>>> "Tony" == Tony Duell
<ard at p850ug1.demon.co.uk> writes:  
 > I'm not much of a Unibus expert but I
seem to remember there's
> something odd about what's displayed in the lights, and how you
> get it to change.  
  Tony> THe easiest way on an 11/45 (or an 11/70?) is to store the
  Tony> value at the address used for the switch register (777570 IIRC)
  Tony> and set the rotary knob to the right of the data lights
  Tony> appropriately.

 Definitely, if you have a machine with a display register (which is
 how the write-only register at 777570 is referred to).

 The "data paths" setting roughly matches what you get on an 11/40,
 which doesn't have that switch.  In that case, the lights display the
 contents of R0 during a WAIT (and apparently also during a RESET).
 The fact that this happens for WAIT is the basis of the "spinning
 lights" idle code used in various operating systems (RSTS for one,
 though not RT11 which uses the display register instead).

        paul 