PDP 11/45 light chaser

[Megan]

> 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 

Try increasing the delay at 1010.

>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? 

pc = pc + (2*branch_offset)

the branch branches back to the instruction at 1004.  The comment is
obviously wrong.  I would suspect that the comment was for code that
this code was adapted from.

btw - the macro code at 1006 should be 'mov #delay,r1'.

>If 1002 is a DEC R0 instruction, then where's the constant "1" for the 
>mov#1,r0 stored in the object? Humm?

1002 is the initial value to load into R0... there is no 'DEC R0' in
the code above.  The BR at the end branches back to 1004

                                        Megan Gentry
                                        Former RT-11 Developer

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