[Coco] Mod10 Suggestions

[William Mikrut]

I like how this works from right to left.
The only issue is the LSLA on every number.

The algo is to double every other number, starting with the right most
digit, and sub 9 if the result is 10 or more.

Now if the number is always 16 digits, Brett's 16 bit word seems the
easiest way to go.
If the number is 13 digits long the 16 bit word method won't work, but I am
happy to pretend all numbers are 16 digits!

I am going to try to include a couple things you showed me into Brett's 16
bit chunk method and try a slightly different routine!


On Sat, Feb 18, 2017 at 10:22 AM, William Astle <lost at l-w.ca> wrote:

> On 2017-02-18 12:43 AM, msmcdoug wrote:
>
>> Actually I'm surprised noone has suggested bcd arithmetic on the result
>> to eliminate divide by 10 loop
>>
>
> BCD would certainly give a predictable overall cycle count. It would
> require a significantly different approach, though. The only register you
> can use for BCD arithmetic is A and DAA is only useful after ADDA or ADCA.
>
> I had thought about using BCD but had initially dismissed it due to
> possible complexity. However, upon reflection, the extra cycles to use BCD
> would probably be less than the average cycle time of the modulus loop
> combined or checking for digit overflow during the loop.
>
> I think you could use code that looks something like the following which
> is based off Mr. Mikrut's most recent posted code. (warning: mailer code™
> follows so it may have errors)
>
>         ORG $1200
> CCD     RMB 16
> RESULT  RMB 1
> START   LEAX CCD+16,PCR
>         CLRA
>         LDB #8
> LOOP    PSHS A
>         LDA ,-X
>         LSLA
>         CMPA #10
>         BLO LOOP2
>         SUBA #9
> LOOP2   ADDA ,S+
>         DAA
>         ADDA ,-X
>         DAA
>         DECB
>         BNE LOOP
>         ANDA #$0F
>         STA RESULT,PCR
> ENDPGM  RTS
>
> I'm using the stack for a temporary storage location instead of something
> PCR relative for code size reasons. You could use the "RESULT variable for
> the temporary to eliminate stack usage. That would probably be slightly
> faster at the expense of two more code bytes. This is one of those
> size/speed trade-offs.
>
> DAA has to be used after every addition and only applies to A. Using BCD
> means we can eliminate the mod 10 loop and just mask off the upper digit
> (BCD stores two decimal digits in a byte). That gives a constant time for
> the "mod 10" result and also only takes 2 bytes (and 2 cycles).
>
> I have also eliminated the STATUS variable and just store the result. You
> can test RESULT for non-zero trivially so there's no need for a separate
> STATUS value.
>
> By my calculation, this version is 32 bytes, requires 1 byte of stack
> space, 17 bytes of data space, and runs in a maximum of 351 cycles (and a
> minimum of 336 cycles if none of the doubled digits goes above 9). For this
> analysis, I've assumed 8 bit offsets for the PCR references. 16 bit offsets
> in PCR mode are quite a bit more expensive (4 extra cycles and 1 extra
> byte).
>
>
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