INT 21 - DOS 2+ internal - SET CURRENT PROCESS ID (SET PSPsee Program Segment Prefix ADDRESS)
	AH = 50h
	BX = segment of PSPsee Program Segment Prefix for new 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 MS-DOS 3.0+ and DR DOS 3.41+, 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
	some Microsoft applications such as Quick C 2.51 use segments of 0000h
	  and FFFFh and direct access to the SDAsee Swappable Data Area (see #01687 at AX=5D06h) to
	  test whether they are running under MS-DOS rather than a compatible
	  OS; although one should only call this function with valid PSPsee Program Segment Prefix
	  addresses, any program hooking it should be prepared to handle
	  invalid addresses
	this function is supported by the OS/2 compatibility box
	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=51h,AH=62h