[Coco] Divide by 10 on a Motorola MC6809(E)

[Don Barber]

Sorry yes arithmetic shift was incorrect, my bad. Should clear the carry bit with the right ANDCC and then do LSRA instead.

Don

Sent from my iPhone

> On Mar 20, 2024, at 3:48 PM, Sean Conner via Coco <coco at maltedmedia.com> wrote:
> 
> It was thus said that the Great Don Barber via Coco once stated:
>> A trick I use to divide 8 bit integers by 10 is to MUL by 205, then shift
>> the A register down by 3 bits (ASRA three times). This works out to
>> 205/2048 which is close enough precision to 1/10 for 8 bit integers.
> 
>  Nice, although when I tried it, it gave very ... odd ... results for
> higher values of A.  Up through 150 it gave the right answers, but starting
> with 160, it started returning (mostly incorrect) negative answers:
> 
>    160    -16
>    170    -15
>    180    -14
>    190    -13
>    ...
>    240    -8
>    250    -9
> 
> change the ASRA to LSRA and the results are what you would expect for
> unsigned division.  Also, after the sequence:
> 
>        ldb    #205
>        mul
>        lsra
>        lsra
>        lsra
> 
> the C flag is 1 for remainders of 5 through 9; clear otherwise.  I found
> that interesting.
> 
>  -spc
> 
> 
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