Re: [stella] Euchre: a tight fit

["Erik J. Eid" <eeid@xxxxxxxxx>]

I had asked for help in optimizing my Euchre game, which had gone beyond 
its 4 KB limits.  Here was one of the suggestions:

At 10:12 AM 9/4/01 +1000, Andrew Davie wrote:
> You can save a fair bit by junking the LetterImageTable and calculating the
> address instead of looking it up.  This requires the letters to be defined
> in a set order, but that's not a big limitation.  Replace...
>
> GetLetterImage
>     txa
>     asl
>     tax
>     lda LetterImageTable,x
>     sta $00,y
>     lda LetterImageTable+1,x
>     sta $01,y
>     rts
>
> with
>
> GetLetterImage
>     txa
>     asl
>     asl
>     asl
>     adc #<LetterImageSpace
>     sta 0,y
>     lda #>LetterImageSpace
>     adc #0                                            ; can be omitted if
> LetterImage stuff doesn't cross a page
>     sta 1,y
>     rts
>
> This loses 1 byte in the routine, but junks the LetterImageTable, saving a
> total of 65 bytes or so.

Seeing as the messages are formed of six "letters" each, which are indexes 
into the table, the letters are already in a known order, so this should 
work well.

I would like to understand this revised routine a little better, so I have 
a couple of questions (particularly directed at Andrew, but open as always 
to the list):

- Why is it not necessary to include a clc before each adc statement?  I 
got into trouble with a very early version of the program by not doing this.
- What is the purpose of the adc #0 statement?


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