[syslinux] Structure of VBR in FAT32?

Gene Cumm gene.cumm at gmail.com
Sat Jan 6 11:34:33 PST 2018

On Sat, Jan 6, 2018 at 12:34 PM, Avi Deitcher <avi at deitcher.net> wrote:
>> > Is the build process documented? Or am I just going to have to plod
>> > through
>> > the makefiles?
>> I don't think so.

> Oh well...

>> > 1. Run installer
>> > 2. Installer loads ldlinux.sys onto the filesystem (and copies
>> > ldlinux.c32,
>> > but irrelevant for now).
>> > 3. Installer calculates which blocks in the filesystem contain
>> > ldlinux.sys
>> > 4. Installer modifies first sector of ldlinux,sys to contain a list of
>> > all
>> > of the other locations of ldlinux.sys
>> Almost.  The blockmap MUST have at least one record in the first
>> sector but the subsequent blockmap records may exist later.  See
>> "SectorPtrs" and "rl_checkpt_overflow".  I believe 192 entries are
>> currently allocated.

> I will look at the code to try and get it. All of this basically is, "how do
> we bootstrap a program that knows how to read FAT filesystem, when it itself
> is on that filesystem and we don't know how to read it yet."
> 192 entries = 192 potential sectors for ldlinux.sys?


>> > 5. Installer puts the first 512 bytes of ldlinux.bin into the boot
>> > sector
>> ... by reading the existing VBR, modifying ONLY the bytes allowed for
>> code, then writing the modified VBR back.

> So as not to ruin the EBPB or signature, e.g.?

EBPB.  Without it, a FAT file system is near impossible to reliably read.

>> > 6. Installer modifies code in VBR (a specific byte?) to indicate which
>> > block
>> > contains first 512 bytes of ldlinux.sys
>> It's 8 bytes for a 64-bit address located at the labels "Sect1Ptr0"
>> and "Sect1Ptr1".  These are NOT at fixed offsets and might move based
>> on revisions of diskboot.inc.

> That works. I will read it.

>> I'd suggest against trying to make a script to install.

> I have been looking at trying to leverage the installation without the
> installer. How did you know? :-)

There's only 2 reasons to know this depth of information.  1) Coding.
2) Install outside the installer.

Why do you want to install the VBR and ldlinux.sys without the
syslinux/extlinux installer?  You realize the installer can be
recompiled without rebuilding SYSLINUX itself?  In theory, I believe
you could do a syslinux installer for some arm9 Linux OS.

>> If you do that, you'd need to have detailed knowledge of how some of these
>> structures work and how they've changed over time.

> That was what I was afraid of...

TL;DR: Your work might only install a certain version.  Without the
history, you might fail other versions.

>> extlinux lets the Linux kernel do all of the heavy lifting of
>> installing the file then finding its blocks then performs the same
>> modifications as syslinux does on FAT12/FAT16/FAT32.  extlinux has
>> less knowledge about ext2/ext3/ext4 and FAT12/FAT16/FAT32 file systems
>> (basically 0 outside the FAT32 VBR format) than syslinux does about
>> FAT12/FAT16/FAT32.

> "kernel do all of the heavy lifting", i.e. it doesn't actually try to modify
> sectors and find blocks, but instead relies on the mounted filesystem and
> driver? Ah, is *that* why extlinus requires the filesystem to be mounted to
> run install?


> Thanks again to both of you!


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