Int 21 Fn 51  - Dos 2+ Internal - Get Current Process Id (get Psp Address  [D]

   AH = 51h

Return: BX = segment of PSP for current process

Notes: DOS uses the current PSP 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 undocumented prior to the release of DOS 5.0

See Also: AH=26h,AH=50h,AH=62h