[Coco] Is the 6809 flawed?

Arthur Flexser flexser at fiu.edu
Fri Sep 7 17:24:22 EDT 2018

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


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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