Hi All,

When the 16meg mmc came it was formatted with FAT12 and when I reformatted
it, the only option in W2K is FAT12.  I was hoping that the format was
FAT16, but that doesn't seem to be the case.

What determines which FAT is used is the number of clusters: FAT12< 4085;
FAT16< 65525; everything above FAT32.

I'm still trying to get my head around the format, but it goes like this for
my 16Meg card.
BootSector is 1 Sector long (512 bytes, Sector 0)
FAT1 is 12 Sectors long
FAT2 is 12 Sectors long
RootDir (starting is sector 25) is 512 entries * 32 bytes/entry = 32 sectors
long
Data starts in sector 57 (cluster 2) and goes up to cluster 3913 with 8
sectors/cluster. Therefore free space is (3913-1)*8sectors/cluster * 512
bytes/sector= 16,023,552 bytes.  Note that 3913 clusters is less than 4085
so it is FAT12.

The spec says there can be 1,2,4,...128 sectors/cluster. (I'm using 8 right
now) Bytes/cluster should not exceed 32K (I have 8*512= 4K bytes).

The largest mmc card that is available is 128Meg.  To see if it will fit in
a FAT12 format, maybe we can work backwards.  If we have 64 sectors/cluster,
ie 32K bytes/cluster (max. allowed) then we would have roughly 128 Meg/32K
or 4K clusters (4096 clusters).  This doesn't include any room for Boot,
FAT1, FAT2 and RootDir, so with maybe a small amount of wastage, the < 4085
cluster limit for FAT12 can be met.

Does anyone have a large mmc card and can tell me what format is being used?

I guess if I could reduce the number of sectors/cluster to 1,2 or 4, this
would push me above the 4085 threshold and mean that a FAT16 format could be
used.  I guess the FAT1 and FAT2 tables would have to be larger.  W2K
doesn't seem to support FAT16 format.  Any recommendations?

Interestingly, on my 16Meg mmc only 16052736 bytes of the 16,056,320 bytes
are used.  The 3584 bytes are surplus and wasted.  I guess that has occurred
because of the numbers chosen for FAT1 and FAT2 size, RootDir size,
SectorsPerCluster etc.

Regards,

Gordon Williams


----- Original Message -----
From: "Herbert Graf" <mailinglist@FARCITE.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Wednesday, November 26, 2003 11:45 AM
Subject: Re: [PIC:] PIC connected to Multimedia Card


> > Gordon Williams wrote:
> >
> > >Thanks Brent,  that wiped the mmc clean with all zeros.  I was
> > then able to
> > >reformat it using W2K.
> > >
> > >The size of the FAT decreased from 16 to 12 sectors, number of
> > sectors per
> > >track and number of heads are also different than what the card
> > came with.
> > >
> > >Anyone know if this is important?
> > >
> > >
> >
> > Isn't that the old FAT12 (floppy drive) format vs FAT16?
> > David...
>
>         I don't believe so, FAT12 is not supposed to be supported for a
volume that
> big. I believe the op is referring to the size of the FAT in sectors. It
> sounds like he's using a larger cluster size, which results in a smaller
> number of FAT sectors. TTYL
>
> ----------------------------------
> Herbert's PIC Stuff:
> http://repatch.dyndns.org:8383/pic_stuff/
>
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