Date   : Wed, 21 Dec 2011 08:42:14 -0600
From   : jules.richardson99@... (Jules Richardson)
Subject: ADFS directories - "reserved" bytes

J.G.Harston wrote:
> Jules Richardson wrote:
>> one 'gotcha' that I found is that the following two references
>> claim that byte 0x4ff in a directory block should be 0x00:
> ...
>> ... however, I've got a disk here that has 0x14 in that location for 
>> one
>> single directory on the disk (all other dirs have 0x00 at offset 
>> 0x4ff).
> 
> The RISC OS PRMs calls this the "Check byte on directory". This 
> suggests
> that that one directory has been written to on RISC OS as some point.

Interesting... the disk in question is one of the Pandora source ones, 
although it's possible it went near a RISC OS machine at some point 
(perhaps someone just needed a disk to write to temporarily, although I 
suspect that RISC OS machines with 5.25" drives aren't very common!)

> I've done a quick check editing the final byte on 8-bit ADFS and it
> doesn't make a difference. 

Thanks for checking that! Like you say, sounds like it's best to ignore it 
then and just validate using the 'Hugo' strings etc.

Aside: do you happen to know if there's a reason why ADFS checks the master 
sequence number and 'Hugo' string at the start and end of a directory 
block, and reports a 'broken directory' *if they don't match*?

That almost reads to me as though there was something other than a duff 
sector that could trash a directory block (Perhaps the directory is the 
last thing to be written in certain ops, and users had a habit of yanking 
the floppy out before the directory block had been completely written?)

Maybe I'm just reading too much into it, though...

cheers

Jules