I'm not aware of any way to 'list' the available bios drives in real mode.
However, there are some things that might help:
- The number of hard drives is stored at
40:75in the the BIOS Data Area. While I don't 'know' that drive numbers are assigned sequentially, I'd expect them to be. If so, the list of hard drives would run from 0x80 to 0x80 + (40:75 - 1) (although conceivably there could be 0 hard drives, so 0x80 might not be valid either). - There is also
int 21h function 0E(select default drive), which returns "the highest drive present." Without checking, I'm guessing this is related to LastDrive, but it might be useful. - And finally there's
int 13h function 08(read drive parameters) and incrementing the drive number until you get an error. I'm pretty sure DOS maxed out at 26 drives (including hard disks, cds, floppies, etc), so this loop wouldn't run very long.
Note that for bios functions, hard drives typically have the most significant bit set. So hard drive #0 is referred to as 0x80, #1 is 0x81, etc. Floppy drives don't have this bit set and so count from 0.
By the time USB drives came along, I had stopped using DOS, so I'm not sure whether they appear as hard drives or floppies. Since they are removable, I wouldn't expect them to be listed as hard drives, but it may depend on the driver you use.
If you're booting from USB, then you're almost certainly doing it via the BIOS's floppy drive emulation mode. If you're booted from some other drive and running under DOS, then it will entirely depend upon the driver.
DOS function 0Eh deals with logical drives, many of which could be on the same physical drive. The BIOS drive numbers all indicate physical drives.
On my true DOS computer, the USB drive is treated as a hard disk by DOS and BIOS alike.