INT 2F - DBLSPACE.BIN - "GetDriveMapping" - GET DRIVE MAPPING
        AX = 4A11h
        BX = 0001h
        DL = drive number (0=A:)
Return: AX = status (see also below)
            0000h successful
                BL = host drive (bit 7 set if specified drive is compressed)
                BH = DoubleSpace sequence number
            other error code (0101h) (see below)
Note:   the compressed volume file for the specified compressed drive is
          host:\DBLSPACE.sequence
SeeAlso: AX=4A11h/BX=0000h

Values for status:
 0000h  successful
 0100h  bad function
 0101h  invalid drive
 0102h  not a compressed drive
 0103h  drive already swapped
 0104h  drive not swapped

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