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Using a struct for the FAT Bios Parameter Block
Posted: Mon Mar 31, 2008 7:12 am
by Stevo14
Upon opening and reading the official FAT manual from (the one and only) Microsoft, and seeing their nicely formatted table that explains the Bios Parameter Block, I thought it would be nice to encase it in a structure for easy access. Basically what I wanted was syntax like:
So first I read the bytes from the disk (at the start of the first partition):
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ata_read_bytes(drive, bs, partition_base_sector, 0, 512);
And then I allocate a new BPB and memcpy the data straight into it:
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fat_BS_t *fat_boot = (fat_BS_t*)kmalloc(sizeof(fat_BS_t));
memcpy(fat_boot,bs,sizeof(fat_BS_t));
However, when I try to access part of the structure, for instance the signature:
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write_hex(fat_boot->boot_signature);
It spews out the wrong value, and I can see no consistent pattern as to which value it spews (except that it spews the same wrong value each time).
Here is how I declare the structure:
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typedef struct fat_BS
{
unsigned char bootjmp[3];
unsigned char oem_name[8];
unsigned short bytes_per_sector;
unsigned char sectors_per_cluster;
unsigned short reserved_sector_count;
unsigned char table_count;
unsigned short root_entry_count;
unsigned short total_sectors_16;
unsigned char media_type;
unsigned short table_size_16;
unsigned short sectors_per_track;
unsigned short head_side_count;
unsigned int hidden_sector_count;
unsigned int total_sectors_32;
//extended fat32 stuff
unsigned int table_size_32;
unsigned short extended_flags;
unsigned short fat_version;
unsigned int root_cluster;
unsigned short fat_info;
unsigned short backup_BS_sector;
unsigned char reserved_0[12];
unsigned char drive_number;
unsigned char reserved_1;
unsigned char boot_signature;
unsigned int volume_id;
unsigned char volume_label[11];
unsigned char fat_type_label[8];
}fat_BS_t;
I based this directly off of the manual, so there
shouldn't be anything wrong with it.
Posted: Mon Mar 31, 2008 7:52 am
by AJ
Hi,
You need to declare the struct as:
(put this after the closing curly brace). This will mean that the compiler does not try to pad out chars and short integers to the architecture word size.
Cheers,
Adam
Posted: Mon Mar 31, 2008 9:22 am
by Stevo14
Thanks AJ. It works now! Ive seen that syntax used many other places and I have always wondered exactly why it was used.
It seems though that fixing this problem has brought up another one (side note: does anyone else find this common?). When I read the BPB, fat_boot->root_cluster reports 2 (which translates to offset of 0x8200), which is normal according to the manual, but it seems that the root directory, in the fat image that I'm using for testing, is actually located at offset 0xA6600 (I found it with a hex editor) which, should be cluster number 1331! I must be missing something because the manual says:
The first cluster of the root directory on a FAT32 volume is stored in BPB_RootClus.
and yet the only values that I can find at cluster number 2 (on the first partition) are all zeros.
Posted: Tue Apr 01, 2008 5:00 am
by Ready4Dis
Well, the way I figure out where the Root directory of my FAT32 partition starts is by doing this:
RootStart = ptrFat32->FatStart + (ptrFat32->FatCount*FatSectors);
Then I start reading the root directory. I honestly don't even use RootCluster, except in my Fat32 formating utility, I set it to 2 and forget about it. Possibly I'm doing something wrong, but my code reads/writes Fat32 drives partitioned with my utility or under windows, and windows chkdisk finds no errors in my parttition after formating, so until I find issue's, it's good to go I guess.
Posted: Tue Apr 01, 2008 6:21 am
by Stevo14
RootStart = ptrFat32->FatStart + (ptrFat32->FatCount*FatSectors);
I actually figured that out earlier today. Thanks for the reply though!
I honestly don't even use RootCluster, except in my Fat32 formating utility, I set it to 2 and forget about it.
It is set to 2 on my test disk as well. There could be an outside chance that it points to the first
data cluster of the root directory (as it does on my test disk), but I think that this is probably just a coincidence.
Posted: Tue Apr 01, 2008 6:32 am
by Ready4Dis
Well, glad you had that part, wasn't positive what you were using the RootCluster for. I wrote my Fat code over a year ago, so most of it isn't very fresh in my head
. I browsed my code, and the only place I saw RootCluster (besides the structure definition) was setting it to 2 in my format function. I am not positive what it points to, or what it's supposed to be used for, but I ignore it completely with no negative side effects. If you are having problems with something, let me know and i'll dig up my code and hopefully get you going in the right direction.
Posted: Tue Apr 01, 2008 11:31 am
by Stevo14
Ready4Dis wrote:If you are having problems with something, let me know and i'll dig up my code and hopefully get you going in the right direction.
If I have any problems, I will be sure to post them here in the forums and maybe PM you if that is the appropriate thing to do.
Again, thanks for the help.
Posted: Tue Apr 01, 2008 9:34 pm
by Ready4Dis
Yes, feel free to PM me if you want, or post on the forums, either or both is fine. I've got working Fat code, so I should be able to help you out if you need it.