INT 2F - DBLSPACE.BIN - "ActivateDrive" - MOUNT COMPRESSED DRIVE
        AX = 4A11h
        BX = 0005h
        DL = drive number (0=A:) to assign to new drive
        ES:SI -> activation record (see below)
Return: status returned in activation record (see below)
SeeAlso: AX=4A11h/BX=0000h,AX=4A11h/BX=0006h

Format of activation record:
Offset  Size    Description
 00h  2 BYTEs   signature "MD" (4Dh 44h)
 02h    BYTE    4Dh ('M') mount command
 03h    BYTE    error code (set to FFh before calling)
                00h successful
                01h drive letter not available for DoubleSpace
                02h drive letter already in use
                03h no more disk units (increase MaxRemovableDrives in .INI)
                09h CVF too fragmented
 04h    BYTE    host drive number (0=A:)
 05h    ???     DISK_UNIT structure (not documented)

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