Hi Guys,
Well, here's my first project where I have done some real FAT jumping...
In case anyone is interested, below is a screen shot of reading a Logical Disk sector from the virtual disk in a file.
The file is Disk-10.txt which is loaded with the 2002 records of J's and the record number (big endian).
I call for sector 7D0 (2000), and the first Seek needs to read the FAT, which it does, FAT table starts at sector 0x000000F7.
on the next jump, the seek takes it deeper into the FAT and requires to read the next FAT sector at ...F8
the rest of the seeks just jump around that same sector of the FAT until it finds the right data cluster.
The next "hop" is not shown because it's reached it, and I would guess that would be cluster 13A.
2 Clusters are reserved, so subtract 2. 0x13A = 0x139. There are 0x20 sectors per cluster, so multiply
0x139 * 0x20 = 0x2700. Data sectors start at 0x303 (for this SD card), so add that in....
0x2700 + 0x303 = 0x2A03, which is close enough to 0x2A17 to find the data...
Just kidding, the sector within that cluster is found by 0x7D0 (logical sector desired) divided by 4 and MOD 0x20.
That comes out to 0x14...
0x2A03 + 0x14 = 0x2A17 ... and there's the right data!
Actually, it loads the 4 logical sectors of 128 bytes in each 512 sector, then it picks the right quadrant based on 0x7D0 MOD 4
And Bob's your uncle!
Regards,
Josh Bensadon
Well, here's my first project where I have done some real FAT jumping...
In case anyone is interested, below is a screen shot of reading a Logical Disk sector from the virtual disk in a file.
The file is Disk-10.txt which is loaded with the 2002 records of J's and the record number (big endian).
I call for sector 7D0 (2000), and the first Seek needs to read the FAT, which it does, FAT table starts at sector 0x000000F7.
on the next jump, the seek takes it deeper into the FAT and requires to read the next FAT sector at ...F8
the rest of the seeks just jump around that same sector of the FAT until it finds the right data cluster.
The next "hop" is not shown because it's reached it, and I would guess that would be cluster 13A.
2 Clusters are reserved, so subtract 2. 0x13A = 0x139. There are 0x20 sectors per cluster, so multiply
0x139 * 0x20 = 0x2700. Data sectors start at 0x303 (for this SD card), so add that in....
0x2700 + 0x303 = 0x2A03, which is close enough to 0x2A17 to find the data...
Just kidding, the sector within that cluster is found by 0x7D0 (logical sector desired) divided by 4 and MOD 0x20.
That comes out to 0x14...
0x2A03 + 0x14 = 0x2A17 ... and there's the right data!
Actually, it loads the 4 logical sectors of 128 bytes in each 512 sector, then it picks the right quadrant based on 0x7D0 MOD 4
And Bob's your uncle!
Regards,
Josh Bensadon