[Coco] Is the 6809 is flawed?

[Gene Heskett]

On Friday 07 September 2018 14:44:38 Dave Philipsen wrote:

> If you haven’t watched episode 73 of CoCoTALK!, you should. A
> contention is brought up that claims the 6809 ASL instruction “does
> not work for signed math”.  Is it true?  After 40 years are we just
> now finding out that the 6809 has a bug and that Motorola engineers
> “messed up here”?  John Linville was the first to notice the flawed
> tutorial and comment about it here:
> https://retrotinker.blogspot.com/2018/09/yes-lsl-and-asl-really-are-sa
>me.html
>
> You can see for yourself by visiting the CoCoTALK! Webpage here:
> http://cocotalk.live/video-episodes/ and viewing episode 73.  You’ll
> find the interesting part around 1:23:00.  Hear for yourself and
> determine whether our favorite CoCo processor, the 6809 is flawed and
> in regard to the Arithmetic Shift Left if you should “just never use
> that in life...never use that command and you’ll be fine.”
>
>
>
> Dave

For twos-complement, it works correctly, in both the 6809 and the 6309. 
I've used both shifts extensively and have never failed to get the 
expected result.

That I think is one of the drivers for the sex instruction, but have yet 
to see the utility in it since the input data is expected to be correct. 
In any event, I'd check for bad data BEFORE I applied the sex 
instruction. There must be a reason since the C compiler uses it just to 
be boring, and is an instruction I usually skip when doing my personal 
optimizations of the C compilers output.

-- 
Cheers, Gene Heskett
--
"There are four boxes to be used in defense of liberty:
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Genes Web page <http://geneslinuxbox.net:6309/gene>