Date : Wed, 25 Oct 2006 12:55:55
From : Steve O'Leary <navalenigma@...>
Subject: (no subject)
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> From: kortink@...> To: bbc-micro@...> Date: Wed, 25
Oct 2006 13:38:23 +0200> Subject: Re: [BBC-Micro] ADFS sector checksums> No,
you start at index 254, not index 0.> [snip]> > Either you start 0 or start
255. There's no difference> in the result unless you never consume a
generated carry> in the second case, i.e. if all your bytes except the very>
last one are zero (if the last one is not, it will generate> a carry, but this
is never consumed).> > I.e.> > FF C=0 .. <+ 00> .. <+ 01> = 00 C=1
<+ 00> = 01 C=0> 00 C=0 .. <+ 00> .. <+ 01> = 01 C=0 <+ 00> = 01
C=0> > i.e. same result, while in the pathetic case :> > FF C=0 .. <+
00> .. <+ 00> = FF C=0 <+ 01> = 00 C=1> 00 C=0 .. <+ 00> .. <+
00> = 00 C=0 <+ 01> = 01 C=0> > i.e. different result.> > Acorn
probably intended to start 0, but some fan of> obscure programming realised that
it's the same as> starting 255 since there's always a non-zero byte>
before the last one is added in (and since Y already> starts 255 and loops 'NE',
TYA then saves one byte,> compared to LDA#0). Whoopee.> > There's one case
where this fails, and that's where> there's no free space left on the
drive, so it's> /possible/ for all but the last byte (at index 0) to> be zero
in sector 1 (while there's always a non-zero> disc size in sector 0).> > >
John KortinkThanks for that detail. Apologies for my emails going a bit
weird for some people when I'm quoting, I did the last one normally and still
went wrong. Although they display OK for me. Something to do with it not
being plain text I guess but at the moment I cannot seem to force a plain
text mode. I'm looking into it. Didn't used to be like this.
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