INT 21,51 - Get Current Process ID (Undocumented DOS 2.x)

	AH = 51h

	on return:
	BX = process ID

	- The process ID  number is actually the segment address of program's
	  PSP.	 This in conjunction with INT 21,50 is useful for TSR's to
	  access their own file handle table in their respective PSP.
	- this function cannot be called while in an INT 28 handler in
	  DOS 2.x unless the critical error flag is set or stack corruption
	  will occur
	- INT 21,62 is highly recommended for DOS 3.x due to a possible bug
	  when activated from a TSR.  DOS may switch to the wrong internal
	  stack which may cause a problems with TSR's if called during an
	  INT 28.
	- see INT 21,62 (Get PSP segment) for DOS 3.x applications