Re: creating a new Apple IIgs ROM 03 checksum

[retrogear <larrygr510@gmail.com>]

On Wednesday, April 24, 2013 6:35:53 PM UTC-6, barrym95838 wrote:
> > I do not know if i understand wrap around correctly but what i can say is: > > - if you set the data bank with pea plb plbthen you can use the ($) zp addressing instead of the long one. Useful for saving cycles. > > I am not an orca expert, are long a.m.x etc. sufficient? No need for a REP #$30? > It seems to me that if the uninitialized value @ $fd is an odd number, then the 16-bit check-sum is guaranteed to be incorrect. If it's an even number, then it might be okay due to wrap-around. I have almost no 65c816 experience, but it seems to me (with all due respect) that Larry's subroutine is a candidate for some revisions and optimizations. If I'm stepping out of line, I apologize, especially since I would have to do some studying before being able to do any better. Mike

No way are you stepping out of line. All discussion is welcome. This is not actually 'my' subroutine. This is the ROM diagnostic routine from the System 601 sources which I modified into a subroutine to calculate the checksum needed to pass the diagnostic. I agree that FD and FE sould be inited to 0. It just happens they are 0. This led to a new discovery. I'm going to start a new thread about getting further with the Twain ROM !!! Larry