PDP 11/45 light chaser

[Tony Duell]

> 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.

THe easiest way on an 11/45 (or an 11/70?) is to store the value at the 
address used for the switch register (777570 IIRC) and set the rotary 
knob to the right of the data lights appropriately. 

The first PDP11 program I ever wrote was something like 

       mov #0,r0           ; Initialise counter
loop:  move r0.(#177570)   ; Store counter in lights register
       inc R0              ; increment the counter
       reset               ; wait a bit
       jmp loop            ; round again, jumos are easier to hand assemble
                           ; than branches, but even so I got caught out
                           ; the first time -- you use one more level of 
                           ; indirection than you might think! 

-tony