Date   : Tue, 20 Dec 2011 06:07:42 +0000
From   : jgh@... (J.G.Harston)
Subject: ADFS directories - "reserved" bytes

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.
I've done a quick check editing the final byte on 8-bit ADFS and it
doesn't make a difference. Presumably a non-zero byte is checked by
32-bit ADFS for validity.

PRM 2-212 describes how to calculate the check byte, it's a bit fiddly.
On writing it's probably safest to always write it as a zero, and on
reading it's probably safest to ignore it.

-- 
J.G.Harston - jgh@...      - mdfs.net/jgh