Re: creating a new Apple IIgs ROM 03 checksum

[Michael J. Mahon <mjmahon@aol.com>]

Michael J. Mahon <mjmahon@aol.com> wrote:
> barrym95838 <barrym95838@yahoo.com> wrote:
>>> 
>>> Couldn't this magic word be subtracted after all the other summing was
>>> 
>>> complete?  SEC, SBC abs-long, and you're done?
>>> 
>> 
>> Hi, Jerry
>> 
>> I strongly agree that adding it with everything else in the inner loop
>> and subtracting it when done is the more desirable choice, since that
>> strategy is just as compact, and much faster than 32768 php/cpy/bne/plp
>> combos!  I know, we're only talking about a few hundred milliseconds in
>> this particular case, but the benefit of being aware of this is that it
>> can help in other projects, ones that may have much longer loops and/or
>> tight performance constraints.
>> 
>> Of course, if we were coding for a "more modern" processor, some of us
>> might just let it go as is, and let brute power take over.  I don't
>> usually include myself in that group, since I will likely have an 8-bit
>> mentality for life! ;-)
>> 
>> Mike
> 
> It also acknowledges that the "twiddle" value can be anywhere, not just
> where it happens to be located currently. 
> 
> In fact, a program seeking to identify an unmatched ROM might not only

Should be "unpatched"--(autocorrect). ;-(

> require the checksum to be as expected, but might also require the original
> twiddle value to be unchanged.  In this case, one would have to leave the
> original twiddle value and add another elsewhere to correct the checksum. 
> 
> The main point is that it is completely unnecessary to know where the
> "original" twiddle value is, or even if there is one. Just add a word that
> causes the checksum to be unchanged. 
> 
> -michael - NadaNet 3.1 and AppleCrate II: http://home.comcast.net/~mjmahon