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Re: Abstract File Identifier offset by 1 byte


From: Thomas Schmitt
Subject: Re: Abstract File Identifier offset by 1 byte
Date: Sat, 17 Apr 2021 10:32:51 +0200

Hi,

Tom Lisjac wrote:
> I noticed that the offset for theĀ Abstract File Identifier
> field appears to be 1 byte early.
> [...]
> https://wiki.osdev.org/ISO_9660#The_Primary_Volume_Descriptor

It looks like the table entry for Abstract File Identifier in OSDev wiki
is wrong.

Other than the specification in ECMA-119
  http://www.ecma-international.org/publications/files/ECMA-ST/Ecma-119.pdf
which is the free version of ISO 9660, the OSDev wiki tells byte offsets
rather than "Byte Positions" (BP).
The difference is that BP starts at 1 whereas byte offset starts at 0.
So the start numbers OSDev wiki have to be 1 lower than those in ECMA-119.

This is the case with most of the fields in the descriptions of the
Primary Volume Descriptor. E.g.
ECMA-119:
  41 to 72  |  Volume Identifier  |  ...
OSDev:
  40  |  32  |  Volume Identifier  |  ...

But in the case of Abstract File Identifier the start numbers are the same:

ECMA-119:
  740 to 776  |  Abstract File Identifier  |  ...
OSDev:
  740  |  36  |  Abstract File Identifier  |  ...

The size in OSDev wiki is 1 too small and thus corrects the bug for the
next start value. Accordingly the size of the preceding line Copyright
File Identifier is one to high.
This confines the bug to the surroundings of Abstract File Identifier.
The neigboring start numbers in OSDev wiki show the expectable difference
of 1 towards ECMA-119.

I have an account at wiki.osdev.org and now corrected the table.
  https://wiki.osdev.org/index.php?title=ISO_9660&diff=25811&oldid=23724


Have a nice day :)

Thomas




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