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I'm trying to implement a FAT 12 file system. The disk is formatted as follows:

Boot Sector (1 sector) | Reserved (1 sector) | FAT 1 (9 sectors) | FAT 2 (9 sectors) | Root ....

My Root directory starts at sector 21. Each cylinder has 18 sectors on the disk. So the starting root sector is at the second cylander at sector 3. I used the following code to try to load that sector into memory:

MOV ax, 0x1000
MOV es, ax ;es - extra segment. Pointer to extra data. es = 0x1000
MOV bx, 0 ;bx - Base register. Used as a pointer to data. bx = 0

MOV al, 1 ;Load just 1 disk sector
MOV ch, 1 ;Cylander = 1
MOV cl, 3 ;Sector = 3
MOV dh, 0 ;Head = 0
MOV dl, 0 ;Drive = 0 (A drive)

MOV ah, 2
INT 0x13

The above code causes bochs to enter an endless loop that keeps on printing out the following message:

prefetch: EIP [00010000] > CS.limit [0000ffff]

Any ideas as to what is wrong?

Also, I haven't had any luck reading any sector that was not in the first cylinder.

Thanks!

are you jumping to the loaded sector to run it? i dont really know about files systems but i know bootprog does this. i looked at it, and somewhere there is a jmp 0x1000? i think thats it

Yes. After the sector is loaded to memory I jump to the code in memory. The jump code is:

Stavros wrote:My Root directory starts at sector 21. Each cylinder has 18 sectors on the disk. So the starting root sector is at the second cylander at sector 3.

I think sector twenty-one is at sector three, head one, cylinder zero actually... Floppies have two sides.

The Bochs error usually indicates that you are running at a bogus memory address (like above 1M in real mode).

Stavros wrote:Also, I haven't had any luck reading any sector that was not in the first cylinder.

This may be related to the head issue.

Yes. That worked. Thanks! So I guess floppies only have 1 cylinder per head?

Stavros wrote:Yes. That worked. Thanks! So I guess floppies only have 1 cylinder per head?

Lol. No, there's 80 of them per head, 2 heads.

check wikipedia on different floppy disks and then you can set your bootlaoder to use, say you want to make one for a different size, i would check for future references