Query about FAT12 format

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Query about FAT12 format

Postby Andyhhp » Fri Oct 26, 2007 10:52 am

Hi,

In your tutorial OSDev5.html, about half way down, when describing the two bpb values 'bpbReservedSectors and bpbNumberOfFATs', you make the comment

bpbNumberOfFATs rpresents the number of File Allocation Tables (FATs) on the disk. The FAT12 File System always has 2, 1 in the second sector, and 1 in the third sector of the disk.


Through expermenting with reading images from formatted floppies and foppies with data on, I cant find a situation where the 2nd FAT starts in sector 3. What I have found is this:

Sector 1 - Bootsector with bpb.
Sectors 2 - 10 FAT 1 (assuming no hidden sectors)
Sectors 11-19 FAT 2
Sectors 19+ Root Dir.

I am just wondering if this is me doing something stupid or not as I am very new to writing bootloaders and operating systems.

As for the rest of the tutorial set - its brilliant. All I have ever been able to find online before are tutorials saying "if your new, DONT try writing a bootloader" or "Here is a hello world bootloader that does nothing".

10 out of 10!

Thanks,

Andrew
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Postby Mike » Sat Oct 27, 2007 4:33 am

Hello,

Thank you very much for your positive feedback on the series :)

Through expermenting with reading images from formatted floppies and foppies with data on, I cant find a situation where the 2nd FAT starts in sector 3.

Each FAT is 9 sectors in size. As such, you are indeed correct.

We are currently in the process of uploading the newest version of the series. We will fix that little error as well.

Please remember, however, that most of the time only the first FAT is used. Because of this, you may never see any changes in the second FAT.
Lead Programmer for BrokenThorn Entertainment, Co.
Website: http://www.brokenthorn.com
Email: webmaster@brokenthorn.com
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