Date   : Tue, 05 Feb 2013 16:40:37 +0000
From   : jgh@... (J.G.Harston)
Subject: 6502 coding request

J. E. Klasek wrote:
> Congratulations! My first tryout was multiplying by 5/3, times 5 is 
> easy and
> 1/3 is multiply with 0.01010101010 with a two digits periodic ...

At some point I'll also need 100ths->60ths, which is *0.6, which a 
quick bit of scribbling turns out to be 0.1001100110011etc, so I should 
be able to combine both bits of code into one routine with an entry 
recurring multipland(sp?).

>> LSR wksp+5:ROR wksp+4:ROR wksp+3  :\ add=add/2
>   CLC                               :\ clean add
>> LDA wksp+0:ADC wksp+3:STA wksp+0  :\ num=num+add

I found that by omitting the CLC I got more accurate results due to bit 
0 falling out of the rotate and getting added on. I ran a test from 
&000000 to &444444 and got exact multiples of 100 about 98% of the time, 
and off by -1 or +1 occasionally,  off by -2 about once in 100,000. I'll 
stick the code in mdfs.net/Info/Comp/6502/ProgTips at some point. It's 
good enough for timing where 15.00s or 15.01s make little difference, 
and most timing is TIME>something, so if testing eg TIME-T%>1500 then 
juddering from 1500 to 1502 is still going to succeed.

-- 
J.G.Harston - jgh@...      - mdfs.net