If you boot from an USB device using such a BIOS you can read this device (and only this one) using INT 13h, AH=2 or AH=42h. The second problem you are facing is that most BIOSs do not manage USB storage devices. What is done when "formatting an USB drive" is nothing but simply writing sectors (INT 13h, AH=3 or AH=43h) to the disk that does not contain file system information, yet, or overwriting existing file system information. The INT 13h functions for formatting a hard disk will typically not work with IDE, SCSI or SATA hard disks. Modern hard disks (everything that came after ESDI technology in 1980s) are formatted in the factory and there is no standardized or documented way how to do re-format these disks at home. This kind of formatting is called low-level formatting (INT 13h, AH=5 or AH=6).įlash devices are never formatted this way because they do not have an unstructured "magnetic" surface. Floppy disks had to be formatted to create sectors and tracks on an unstructured "magnetic" surface. Flash drives and hard disks are typically not formatted in the way floppy disks are (were) formatted.
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