[syslinux] Structure of VBR in FAT32?
Ady Ady
ady-sf at hotmail.com
Thu Jan 4 09:22:16 PST 2018
> > FWIW, the VBR in FAT32 doesn't have to be 512 bytes; FAT32 has the
> > possibility to use more than one sector for boot purposes. In fact,
> > Microsoft's FAT32 VBR uses at least 3 sectors (IIRC, typically, 32
> > sectors)
>
> I did not know that. I just finished writing a go-lang library
> implementation of fat32 (mainly to manipulate image files), I missed
> that. How do you tell if it is the "usual" single sector of 512 bytes
> or more? Wouldn't there need to be some flag at some location? Or is
> the assumption that fat32 can skip any number of sectors until it
> reaches one with the magic signature 0x55AA and it takes all sectors
> from 0 until that one as the VBR?
I'm not sure I understand the question, but I'll try to give you some
generic answer. Perhaps by chance, it will be relevant to your
thoughts/doubts/question.
The possibility of using multiple sectors as VBR in FAT32 volumes is
originated in the filesystem structure itself (i.e. when the "format"
command is executed).
JFYI:
_ In dosfstools' mkdosfs, see parameters:
-R number-of-reserved-sectors
-b sector-of-backup
If the (boot) code in the first sector of the VBR includes some
instruction to continue reading/executing (boot) code located in a
certain (reserved) sector, that's what the boot process will do.
You might want to read Wikipedia's articles about "FAT32" and "Design
of the FAT file system", and/or the FAT32 VBR page in "The Starman's
Realm" of Daniel Sedory.
>
> > moving ldlinux.sys within a FAT32 boot volume could
> > potentially affect the boot process when using SYSLINUX. In such
> case,
> > executing the command line installer would be the simplest/quickest
> > solution.
>
> But even so, it installs the first 512 bytes in the VBR, then those
> bytes need to find the next bytes (ldlinux.sys). Even if I use the
> installer, and then move the files, unless the actual blocks remain the
> same, it will be lost, right?
What happens when you try to do that?
Regards,
Ady.
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