can some one help me on understanding FAT12 ?

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05-26-2010
Masterx

can some one help me on understanding FAT12 ?

hello all , recently we were told to make a simple os, therefore we were again told to go and study FAT12 file system . so far so good.
but the problem is , there is sth that i cant understand , i mean i couldnt figure out the reason why this is like this .
to be clear, im talking about File allocation table in FAT12 system file.
as you can see in the picture i provided there are two entries which are reserved !! 0 and 1 .
well the question is , what are these two entries? and why are they reserved ?
thats all i want to know now .
Thank you in advance .
05-26-2010
Sebastiani

Quote:

Originally Posted by Masterx

hello all , recently we were told to make a simple os, therefore we were again told to go and study FAT12 file system . so far so good.
but the problem is , there is sth that i cant understand , i mean i couldnt figure out the reason why this is like this .
to be clear, im talking about File allocation table in FAT12 system file.
as you can see in the picture i provided there are two entries which are reserved !! 0 and 1 .
well the question is , what are these two entries? and why are they reserved ?
thats all i want to know now .
Thank you in advance .

"Reserved" basically just means that you should be careful not to write data there, but needn't worry about what it's for. Usually set aside to allow for future extensibility, or to store data used internally by the OS (and thus meaningless to you).
05-26-2010
phantomotap

The first sector is reserved for the "Parameter Block".

The next sectors may be reserved for any reason by the software formatting the disc. You should actually examine the parameters to determine if you can use the second sector. (You need to look at the contents to know where the first "FAT" is stored.)

Soma
05-26-2010
Masterx

Quote:

Originally Posted by Sebastiani

"Reserved" basically just means that you should be careful not to write data there, but needn't worry about what it's for. Usually set aside to allow for future extensibility, or to store data used internally by the OS (and thus meaningless to you).

thank you very much for you response , but the case here is , to find out what these are for.it seems no body knows about it , i nearly search everywhere i knew to find out what actually resides there , or atleast what they created it for!

Quote:

Originally Posted by phantomotap

The first sector is reserved for the "Parameter Block".

The next sectors may be reserved for any reason by the software formatting the disc. You should actually examine the parameters to determine if you can use the second sector. (You need to look at the contents to know where the first "FAT" is stored.)

Soma

thank you dear soma ,
i dont know whether i understood you right , but if you meant BPB , by mentioning Parameter Block i think the BPB ( bios parameter block resides in boot sector , which in our case is not true ! we are in FAT table .and boot sector is before this section (bootsector,FAT copies,root directory and data area)

(your idea about formatting the disk looks interesting , it can be , but i need to make sure what they are for .)
05-26-2010
Salem

OSRC: File Systems
Several documents there for reading ;)
05-26-2010
Masterx

Quote:

Originally Posted by Salem

OSRC: File Systems
Several documents there for reading ;)

thank you verymuch for that .
i have another question ( might be silly but here it is )
why do we load bootloader at address "0x7C00" ? why cant we load it some where else?
edited :
found the answer to my second question :

Quote:

We do not load it to 0x7c00, the BIOS does. There is no specific reason why 0x7c00, it has became a standard loading address for most, if not all, BIOS software.
05-26-2010
Masterx

i have finally discovered what those two were reserved for .

Quote:

A DOS 1.X boot sector didn't use to provide any details about the disk, there were only 2 types of disks supported so it wasn't a problem. To identify which disk type it was the first two entries in the FAT were used, this has stuck and has become known as the FAT signature.

source : FOXy2K: FAT System Guide
Thanks a billion Salem , i found a vague article on FAT with the help of the link you provided, ( i found media descriptor or FAT ID and then searched that and hooray i made it :) )