Int 21 Fn 64  U - Os/2 V2.1 Virtual Dos Machine - Get Lastdrive            [O]

   AH = 64h
   DX = 0003h (function number)
   CX = 636Ch (magic value, "cl")
   BX = 0000h (indicates special request)

Return: AL = highest drive supported

Notes: if CX is not 636Ch on entry, INT 21/AH=6Ch is invoked, because a bug
     in OS/2 1.x FAPI erroneously called AH=64h instead of AH=6Ch
   used by WinOS2
   not supported by OS/2 Warp 3.0, check list of lists instead (see #1313)

See Also: AH=52h