PDP 11/45 light chaser

[Guy Sotomayor]

On Sun, 2005-05-22 at 20:49 -0500, Jay West wrote:
> Jim wrote....
> > Location    Content Op-code            comment
> > 001000    012700    mov #1,r0        load ro with 1
> > 001002    000001
> > 001004    006100    rol                    rotate r1 left
> > 001006    012701    mov delay r1    load register r1 with delay
> > 001010    007777    delay
> > 001012    005301    dec r1                decrement register 1
> > 001014    001376    bne -2                continue to decrement r1 until
> > r1=0
> > 001016    000772    br -12                back to 001002 (dec r0)
> 
> This runs on my /45, but produces a steady light display, not a cylon or 
> "blinking" effect. It's been decades since I looked at '11 assembler. I'm 
> confused about something in the listing above. If the 012700 at location 
> 1000 is a load r0 with 1, I'm guessing the next word (1002) is the constant 
> to be loaded? So, why would the branch at location 1016 go back to the data 
> at 1002? Obviously there's something I'm missing. Guidance? If 1002 is a DEC 
> R0 instruction, then where's the constant "1" for the mov#1,r0 stored in the 
> object? Humm?

Here's the light chaser I use:

       1                                        .asect
       2 001000                                 . = 1000
       3                                start:
       4 001000 005000                          clr     r0
       5 001002 005200                          inc     r0
       6                                loop:
       7 001004 006100                          rol     r0
       8 001006 000005                          reset
       9 001010 000775                          br      loop
      10                                        .end    start
-- 

TTFN - Guy