Int 21 Fn 64  U - OS/2 V2.1+ Virtual DOS Machine - Get Size Of Ptda Jft    [O]

   AH = 64h
   DX = 0004h (function number)
   CX = 636Ch (magic value, "cl")
   BX = 0000h (indicates special request)

Return: AX = number of entries in OS/2 JFT for VDM

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
   in an OS/2 VDM, the DOS Job File Table in the PSP contains an index
     into the OS/2 JFT in the Per-Task Data Area rather than an SFT index
     because the OS/2 SFT can contain more than 255 entries