114 lines
4.4 KiB
Plaintext
114 lines
4.4 KiB
Plaintext
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Re: FAT12 vs FAT16
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from the Linux kernel mailing list
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Martin Mares (mj@atrey.karlin.mff.cuni.cz)
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Fri, 22 Aug 1997 13:10:56 +0200
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http://lkml.iu.edu/hypermail/linux/kernel/9708.2/0498.html
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There are the following ways to determine FAT type:
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(1) "FAT12" or "FAT16" string in the boot block. Not reliable and not supported
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in all DOS versions.
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(2) "Sector/cluster count rules" -- described for example in the Tech Help.
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Varies between DOS versions. Even if you know the DOS version, you can still
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be wrong as the disk might have been formatted on a different version.
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(3) Partition type. Seems to be totally ignored by DOS and sometimes very
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unreliable as DOS's format command doesn't alter this during reformatting
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(if you create the partition by one DOS version and format it by another one,
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an inconsistency might arise).
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(4) Number of sectors per FAT -- this value must be correct as the root
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directory position is calculated from it, but it sometimes allows both
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possibilities (although such cases are very rare).
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(5) Number of 0xff's at the start of the FAT. As you can remember, the first
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two cluster numbers are reserved for special purposes and their FAT entries
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are set to 0xff with an exception of the first FAT byte which contains a media
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descriptor. If there are less than 3 0xff's, the FAT must be 12-bit. In other
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case, both types are possible.
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Richard B. Johnson (root@analogic.com)
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Fri, 22 Aug 1997 15:53:45 -0400 (EDT)
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http://lkml.iu.edu/hypermail/linux/kernel/9708.2/0527.html
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No No No No! Page 3-7 of the MS-DOS Technical information (the real
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system integrator's manual) states:
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"For disks containing more than 4085 clusters (note that
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4085 is the correct number), a 16-bit FAT entry is used."
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The number of clusters is calculated from the BPB in the boot record from
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the BYTE Custer size and the rest of the information about the size
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of the media. The number of sectors entry is the total of the media.
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You perform the indicated operations which even provides for the
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reserved sectors (like for the boot record), and come up with the
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number of clusters.
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----- (some code resembling the BIOSParamBlock struct) -----
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This will always work. It must work. This is how MS-DOS knows the size
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of the media.
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I used to get paid for making strange media work on MS-DOS. This will
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always work. The media descriptor doesn't mean anything any more nor
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does the OEM Name, etc. However the other information is updated in
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this table when the disk is formatted. The boot record loading code
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with a 360 kb floppy boot-record BPB is used by FORMAT, etc. It is
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modified before being written to the physical media. This same boot
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record is used for all MS-DOS media and is modified before being written.
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You can copy the boot record from a 1.2 mb floppy to a 1.44 mb floppy
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and you will find that MS-DOS thinks it is now a 1.2 mb floppy. You
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can use format/u to reformat it "as-is", and it is now a 1.2 mb floppy!
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Martin Mares (mj@atrey.karlin.mff.cuni.cz)
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Fri, 22 Aug 1997 23:36:57 +0200
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http://lkml.iu.edu/hypermail/linux/kernel/9708.2/0537.html
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This is correct, but not sufficient for FAT type detection as you can
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have a disk with _less_ than 4085 clusters and 16-bit FAT.
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Have a nice fortnight
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Richard B. Johnson (root@analogic.com)
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Fri, 22 Aug 1997 18:12:30 -0400 (EDT)
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http://lkml.iu.edu/hypermail/linux/kernel/9708.2/0542.html
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Not if it's a disk supported by MS-DOS or created using MS-DOS tools.
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MS-DOS format will not (read cannot) make such a disk. Even with its
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media type/size/heads options, it doesn't have the code necessary to
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produce a disk under any other rules because it calculates the
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clusters not you, and it uses the 4085 cluster rule for writing the FAT.
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Just because, in principle I can make a disk with one cluster and a 16-bit
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FAT, does not mean that it is a MS-DOS disk. MS-DOS will not understand
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such a disk so no other OS should bother with such a deviation either.
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H. Peter Anvin (hpa@transmeta.com)
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23 Aug 1997 00:04:49 GMT
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http://lkml.iu.edu/hypermail/linux/kernel/9708.2/0549.html
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I once came across a DOS floppy formatted with FAT16; I think some (OEM?)
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version of DOS 2.x or 3.x always formatted FAT16. Surprised the
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living daylight out of me...
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Martin Mares (mj@atrey.karlin.mff.cuni.cz)
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Sat, 23 Aug 1997 11:49:03 +0200
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http://lkml.iu.edu/hypermail/linux/kernel/9708.2/0567.html
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Unfortunately, the rules are not the same for different DOS versions :-(
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