[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 

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).


_ 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?


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