Boot Sector on Flash Sometimes Detecting as HDD and not FDD

[kemosparc]

Hi,

I have followed the baby step 2 at http://wiki.osdev.org/Babystep2 listed on the tutorial page of osdev.

I compiled the code using :

Code: Select all

nasm boot.asm -f bin -o boot.bin

and then wrote it to a USB device using the dd command:

Code: Select all

dd if=boot.bin of=/dev/sdb

When I use the USB to boot, sometimes the BIOS recognizes the USB as FDD and some times as HDD. When the USB is recognized as HDD it does not boot. I tried this on a lenovo thinkpad x220 which has both USB-FDD and USB-HDD in its boot list and the USB-FDD has a higher priority, when I press F12 the USB shows up as USB-HDD.

Thanks,
Karim.

[Antti]

If you look at the binary layout of the first 512 bytes, there is neither a BPB nor a partition table (MBR).

The BIOS does not know how it should handle it.

[kemosparc]

Hi,

So you mean it is up to the BIOS, and that is why some BIOS will detect it and some others will not.

So to make sure I understand, if I add the FAT12 BPB to my boot sector and copied it to a flash it should work and detect the FLASH as USB-FDD, right?

Thanks,
Karim.

[Antti]

if I add the FAT12 BPB to my boot sector and copied it to a flash it should work and detect the FLASH as USB-FDD, right?

There is no warranty. It depends on the BIOS implementation. Having a valid "floppy-like" structure just makes it more likely.

[digo_rp]

This in some cases can be changed in the bios usb device emulation. Like fdd hdd optical, etc. Try to take a look at bios and find it.
i hope this helps.

Good luck.

[kemosparc]

Hi,

Actually the only think that I can do is to add them to the boot list. But the USB type isdetected at boot time and the type is selected and displayed accordingly.

Thanks
Karim.

[Muazzam]

If there is BIOS Parameter Block in first sector of USB drive, BIOS usually detects it as Floppy otherwise as a Hard disk.