Compatibility: 4.0+
Expects: AX    3305H
Returns: DL    boot drive (1=A, 2=B, etc.)
Info: Returns with DL containing the number of the drive which was used
to boot the system.

You can use this to help locate CONFIG.SYS or AUTOEXEC.BAT or
DBLSPACE.BIN, etc. (files that live in the root of the boot

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DOS Fn 3305H: Get Boot Drive