[Coco] Is the 6809 is flawed?

[Joel Rees]

2018年9月8日(土) 3:47 Dave Philipsen <dave at davebiz.com>:

> 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-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.”
>

The question to ask is, what would they have had Motorola do?

I might have wanted, for really obscure reasons, them to provide a shift
that duplicates bit zero on a left shift, as a bracket instruction to ASR,
but that would not be ASL.

Overflow is overflow. It's going to happen quickly in an 8 bit register.
With the ASL instruction, it is properly checked and reflected in the V
flag, and until overflow occurs the sign is not destroyed.

Programmers simply have to learn what overflow is.






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