[Coco] Is the 6809 flawed?

[Richard Cavell]

If I understand it, ASL can result in an overflow such that the result is mathematically incorrect. 

But LSL doesn’t.  The bits are just bits. Pixels, perhaps.  If one falls off the left hand side, that’s what was expected and its replacement is don’t care.

On 07/09/2018 at 23:18, linville wrote:

> Not a bug. That's pretty much the whole point.
> What a dumb controversy...
> John
> -------- Original message --------From: Arthur Flexser <flexser at fiu.edu> Date: 9/7/18  5:24 PM  (GMT-05:00) To: CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com> Subject: Re: [Coco] Is the 6809 flawed? 
> John Linville, who wrote the article you linked to, clearly does not
> consider this a bug:
> 
> " Did the CPU architects at Motorola get it wrong? *Spoiler alert:*
> *No!* Neither
> did architects for Intel, MIPS, Sparc, ARM, or any of the other CPUs that
> similarly implement a single instruction for both operations. The fact is
> that when operating on two's-complement
> <https://en.wikipedia.org/wiki/Two%27s_complement> numbers,
> logical-shift-left and arithmetic-shift-left are, in fact, exactly the same
> operation."
> 
> Art
> 
> On Fri, Sep 7, 2018 at 4:47 PM, Dave Philipsen <dave at davebiz.com> wrote:
> 
> > Search for the 6809 ASL bug using Google. No one has ever reported such a
> > thing as far as I can tell.
> >
> > Dave
> >
> >
> > On 9/7/2018 3:29 PM, Salvador Garcia via Coco wrote:
> >
> >>   I am not familiar with 6809 assembler, but for this bug to have gotten
> >> past Motorola testing is, as Vizzini would say, Inconceivable!
> >>
> >> Salvador
> >>
> >>
> >>
> >>
> >>      On Friday, September 7, 2018, 1:47:13 PM CDT, Dave Philipsen <
> >> dave at davebiz.com> 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-rea
> >> lly-are-same.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
> >>
> >>
> >
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> >
> 
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