INT 21 U - OS/2 v2.1+ Virtual DOS MachineA special type of virtual machine provided by OS/2 version 2.0, in which a copy of MS-DOS or an MS-DOS compatible operating system (or even an incompatible 8086 operating system) is run and appears to have full control of the system.  See also Virtual Machine, INT 21h Function 64h. - GET SIZE OF PTDA JFTsee Job File Table
	AH = 64h
	DX = 0004h (function number)
	CX = 636Ch (magic value, "cl")
	BX = 0000h (indicates special request)
Return: AX = number of entries in OS/2 JFTsee Job File Table for VDMsee Virtual DOS Machine
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 VDMsee Virtual DOS Machine, the DOS Job File TableThe Job File Table (also called Open File Table) stored in a program's PSP which translates handles into SFT numbers.  See also INT 21h Function 26h. in the PSPsee Program Segment Prefix contains an index
	  into the OS/2 JFTsee Job File Table in the Per-Task Data Area rather than an SFTsee System File Table index
	  because the OS/2 SFTsee System File Table can contain more than 255 entries