Date   : Wed, 25 Oct 2006 11:56:02 +0100
From   : navalenigma@... (Steve O'Leary)
Subject: ADFS sector checksums

> On  25 Oct 2006 11:58:54 +0200, John Kortink wrote:> > On 25 Oct 2006 
01:28:49 +0100, Jonathan Graham Harston wrote:> > >[...]> >> >For instance:> 
>Sector 0: 07 00 00 00 00 .... 00 00 00 00 20 xx> >FNsum returns &27, actual 
checksum is &27> > Yes.> > >Sector 1: F9 FF 1F 00 00 .... 00 27 E6 00 03 
xx> >FNsum returns &2A, but actual checksum is &2B.> > No, the actual checksum
is &29.> > The proof of the pudding : putting the relevant harddisc> images
on GoMMC : &2B for sector 1, as expected, gives a> 'Bad FS map' error. &29,
as expected, doesn't.
Mmmm... Ignoring the code snippets, doing this on paper gives me 2A if I
add with Carry and 27 if I don't, unless I've added up wrong; Here's my working,
converting the values to decimal;
 
sum = $FF (255), carry is zero
 
add in $F9 (249) = 504 (carry set)
 
sum = 248
 
add in  $FF (255) and 1 for the carry = 504 (carry set)
 
sum =248
 
add in $1F (31) and 1 for the carry = 280 (carry set)
 
sum = 24
 
add in 00 + 1 for the carry = 25 (carry clear)
 
sum = 25  // then we add the remaining zero's until we get to $27
 
add in $27 (39) no carry = 64 (carry clear)
 
sum = 64
 
add in $E6 (230) no carry = 294 (carry set)
 
sum = 38
 
add in $00 with carry = 39 (carry clear)
 
sum =39
 
add in 03 no carry = 42
 
sum =42 or in Hex 2A 
 
 
Now I know I'm miss-interpreting somewhere but can you point out where. 
Also in my code I was clearing sum to 0 before starting the loop but you
set it to 255, even adding that part into the "add all bytes with carry"
routine (which I've done above) I cannot match your result, which as you've
proven appears correct.
 
Thanks for any help.
 
Steve.
 
 
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