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Re: exFAT12

Cross-posted to 3 groups.

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"Robert Wessel" wrote:
> "Mike Gonta" wrote:
>>"Robert Wessel" wrote:
>>> "Mike Gonta" wrote:

> Oh, this is actually supposed to be MS exFAT.  I've never spent much
> time looking at the internals of that, so I didn't recognize it.

{code]
org 0x7C00
boot_sector:
  jmp boot
  nop
file_system_name: db "EXFAT   "
[/code]

One of the requirements of exFAT is that the beginning of the volume
boot sector has "EXFAT   " in what is the OEM field in FAT.

> I've never bothered since the MS format is licensed and proprietary.
> See, a little documentation would go a long way.

My documentation is the patents.

> And if this is really straight exFAT,

The format generated is.

> why call it exFAT12?  Or have you taken some liberties with the
> format?

This is a simple single boot sector boot loader which loads a named
file from the root directory of an exFAT volume in the same vain as
FAT12. It only supports ASCII (7 bit) and 8 dot 3 filenames in a limited
root directory. I've taken the "liberty" of preallocating 15 root
directory clusters (which are the same size as the supported 512 byte
sectors). While "non standard", it's perfectly OK. One of the features
of exFAT is the supported ability to preallocate contiguous file
clusters. There is of course nothing stopping the user's system from
allocating (when necessary) more clusters which will then cause the file
to be linked in the FAT and not completely contiguous, however this is a
case of "too many files" for a small simple system. As such the boot
loader only reads the 15 contiguous root directory sectors and ignores
the FAT completely. Unlike FAT32 long file names all file names in exFAT
are only "long" with a minimum of 3 file directory entries (which will
accomodate the first 15 Unicode wide characters - 8 dot 3 is 12
characters) to a maximum of 19 entries for the max 255 characters. With
8 dot 3 the 15 preallocated root directory sectors (in this case) can
accomodate 79 files (one each of 3 entries are used by the volume label,
the allocation bitmap and the upcase table).

> I believe MS has claimed a minimum size of 1MB for exFAT, so a 1.44MB
> floppy should not be an issue on that basis.

Windows will not support exFAT on floppy disks. Also an image which is
not the same size as the actual device is not supported unless it is a
partitioned image (of which Windows only supports the first partitioned
volume). To keep the code simple the boot sector boot loader contains a
self referencing MBR table so that the 1.44MB image when mounted or
transferred to a flash drive is supported by Windows.

>>> And this is a small lump
>>~220 LOC
>>> of largely undocumented assembler,
>>One third of which contains comments and/or descriptive labels.

> Which is mostly not documentation.

Which is mostly "documentation" for "self documenting" assembly language
code.

http://mikegonta.com/exFAT12
https://redd.it/5ftj93
http://forum.osdev.org/viewtopic.php?f=2&p=268070#p268070


Mike Gonta
look and see - many look but few see

http://mikegonta.com

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Thread

exFAT12 "Mike Gonta" <mikegonta@gmail.com> - 2016-11-30 19:50 -0500
  Re: exFAT12 Robert Wessel <robertwessel2@yahoo.com> - 2016-12-01 02:29 -0600
    Re: exFAT12 "Mike Gonta" <mikegonta@gmail.com> - 2016-12-01 06:10 -0500
      Re: exFAT12 Robert Wessel <robertwessel2@yahoo.com> - 2016-12-01 06:16 -0600
        Re: exFAT12 "Mike Gonta" <mikegonta@gmail.com> - 2016-12-01 08:57 -0500
          Re: exFAT12 Rod Pemberton <NeedNotReplyHere@xrsevnneqk.cem> - 2016-12-01 16:15 -0500
            Re: exFAT12 "Mike Gonta" <mikegonta@gmail.com> - 2016-12-01 18:14 -0500
  Re: exFAT12 JJ <jj4public@vfemail.net> - 2016-12-01 17:23 +0700
    Re: exFAT12 "Mike Gonta" <mikegonta@gmail.com> - 2016-12-01 06:28 -0500
  Re: exFAT12 "Mike Gonta" <mikegonta@gmail.com> - 2016-12-06 18:18 -0500
    Re: exFAT12 "Mike Gonta" <mikegonta@gmail.com> - 2016-12-07 09:57 -0500

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