INT 21 - DOS 2+ internal - GET CURRENT PROCESS ID (GET PSPsee Program Segment Prefix ADDRESS)
	AH = 51h
Return: BX = segment of PSPsee Program Segment Prefix for current process
Notes:	DOS uses the current PSPsee Program Segment Prefix address to determine which processes own files
	  and memory; it corresponds to process identifiers used by other OSs
	under DOS 2.x, this function cannot be invoked inside an INT 28h
	  handler without setting the Critical Error flag
	under DOS 3.0+, this function does not use any of the DOS-internal
	  stacks and may thus be called at any time, even during another
	  INT 21h call
	supported by OS/2 compatibility box
	identical to the documented AH=62h
	this call was undocumentedInformation about a product which is not publicly available from the manufacturer, and must be determined by reverse-engineering (disassembly, trial-and-error, etc.).	 Undocumented information tends to change -- often dramatically -- between successive revisions of a product, since the manufacturer has no obligation to maintain compatibility in behavior which is not explicitly stated. prior to the release of DOS 5.0
SeeAlso: AH=26h,AH=50h,AH=62h